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Air pollutionand asthma: molecular mechanisms
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Air pollution and asthma: molecular mechanisms ~' Air pollution, in particular that generated by road '~' traffic, is a matter of rising public concern and has been implicated in the worsening of asthma. In this article, the evidence that air pollutants (particularly i sulphur dioxide, ozone and nitrogen dioxide) can i ! affect the airways of asthmatic patients is reviewed, i and the possible molecular mechanisms that may 'i~ link air pollution to increased inflammation in the airways are discussed. Airway epithelial cells may Frrespond to oxidant pollutants by the activation • of transcription factors, such as nuclear factor ,~ , kappa B, resulting in increased transcription of ~!'i, i i flenes for certain cytokines, such as interleukin 8 ~ , '? 'and inflammatory enzymes, such as inducible ~,~ nitric oxide synthase and cyclo-oxygenase. P
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T is one of the commonest diseases in industrialized countries, .fiffecting over 10% of children and 5-10% of adults (Box r). There is ~nvineing evidence for a worldwide increase in asthma prevalence /id morbidity. The question of whether air pollution contributes to ~ statistics has become a matter of increasing public concern. While ~s~.no convincing evidence that the worldwide increase in the evalence of asthma is due to air pollution, there is increasing evidence theft air pollution may contribute to an increase in asthma morbidity. Indoor air pollution includes tobacco smoke (passive smoking), indoor allergens (particularly house dust mite and cat), volatile organic comunds, and nitrogen dioxide (NO~) generated by gas appliances. For example, there is some evidence that gas cookers are associated with increased symptoms in asthmatic patients t. However, most concern has focused on whether outdoor air pollution, particularly that generated by ~ r o a d traffic, is increasing asthma symptoms and exacerbations 2.
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he constituents of air pollution ~Several pollutants are present in outdoor air (Table 1). Vehicle exhaust rues contain particulates (various respirable particles with an m
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• Asthma is associated with chronic airway inflammation, characterized by Allergens activation of mast cells and T cells and infiltration of activated eosinophils Air pollutants: 03, NO2 ('eosinophilic bronchitis'). The severity Virus infections of asthma is related to the degree of air inflammation. • Inflammatory and structural cells of the airway release multiple inflammatory Inflammation mediators, including lipid mediators Airway 'Chronic eosinophilic (leukotrienes, prostaglandins, plateletHyper-responsiveness bronchitis' activating factor) and cytokines. • Airway epithelial cells may be an important source of inflammatorymediators, Triggers which may be released in response to Allergens environmental stimuli, such as allergens Exercise and air pollutants. Cold air • Cytokines play a critical role in orCough Wheeze SO2 chestrating the inflammatory response Chest tightness Dyspnoea Particulates in asthma by recruiting, activating and allowing the persistence of inflammatory cells. Mechanismsof asthma. • Acute inflammatory responses in the airwaysresult in bronchoconstriction,hyperaemia, plasma exudation and oedema, mucus hypersecretion and activation of sensory nerve endings. • Airway inflammationunderlies airwayhyper-responsiveness,which is an increased sensitivityof the airways to many triggers, includingexercise, cold air and irritants such as SO2. • Chronic effects of airway inflammation include increased airway smooth muscle thickening, fibrosis, new vessel formation and hyperplasia of mucussecreting cells. These changes may result in irreversible narrowing of the airways. • Asthma symptoms of wheeze and dyspnoea arise from narrowing of the airways. Cough and chest tightness arise from activation of sensitized sensory nerve endings (Fig. 1). Symptomsmay increase because of increased inflammation(exposure to allergens, 03, NO+.,virus infections) or through the effect of various triggers (exercise, cold air, allergens that activate airway mast cells, and irritants such as SO.+that activate airway sensory nerves).
aerodynamic diameter of <10 ~m, described as PMI0), volatile organic compounds, oxides of nitrogen (NOx, mainly NO+_)and sulphur dioxide (SO+). Diesel particulates may be of particular concern, as the small particles) <2.5 ~m) may reach the alveoli and initiate alveolar, as well as airway, inflammation. Ozone (03) is a secondary pollutant, which is formed by the action of sunlight on oxides of nitrogen in the presence of hydrocarbons. The information of 03 is favoured by warm temperatures, sunlight and low winds and, once formed, O 3 may be carried long distances by prevailing winds. 03 is broken down in the presence of NO+, so that its concentration in cities may be less than in rural areas. There has been a change in the nature of air pollution in industrialized countries, with a decline in the levels of SO+. and smoke created by the burning of fossil fuels, but there has been an increase in the levels of NO+_,03 and diesel particulates, associated with increases in the density of road traffic. The establishment of a firm link between traffic pollution and asthma has proved difficult. It is possible that air pollution may cause asthma by increasing the risk of developing asthma in an atopic individual, or by increasing the risks of sensitization, thus leading to an increase in asthma prevalence. However, it is more likely that air pollution may increase symptoms in asthmatic individuals, thus increasing asthma morbidity. Although there has been considerable interest in this question, evidence l
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to support it is conflicting5'6. This is partly because of the difficulty in establishing a clear association between air pollution and asthma symptoms, as there are several conflicting factors. However, two approaches have been used. The first is an epidemiological approach, which attempts to link asthma symptoms and exacerbations with episodes of air pollution, either in whole populations, some members of which may be asthmatic, or in defined groups of patients with asthma. The second approach is controlled exposure to ambient concentrations of pollutants, either singly or in combination. Epidemiological studies There are several well-documented episodes of major air pollution, including the infamous London smog of 1952, that were associated with cold weather and little wind, so that polluted air became trapped, resulting in high concentrations of SO2 [>1000 parts per billion (ppb)], sulphuric acid and particulates. During these episodes, there was evidence of increased symptoms and an increase in hospital admissions in patients with asthma. Population studies of respiratory symptoms and their connection with air pollution are difficult to interpret because of socioeconomic factors, the effect of coexistent aero-allergens, the influence of cigarette smoking and respiratory infections, climatic changes, and the difficulty
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Table 1. The composition of air pollution a Pollutant
Source
WHO guidelinesb
DOE air quality~
1h Average
24 h Average
Maximum I h average Poor Very poor
SO2 (ppb)
Burning of fossil fuels: power stations coal burning
122
-
125-399
>_300
NO2 (ppb)
Vehicle exhaust Burning fossil fuels Gas appliances
210
80
100-299
_>400
03 (ppb)
Photochemical conversion of O2 + NO2 (temperature, sunlight) related to vehicle exhaust
76-100
50-60 (8 h average)
90-179
_>180
Particulates (PM10: I~g m3)
Diesel exhaust Burning fossil fuels
150a
"Abbreviations:ppb, parts per billion; DOE,UK Departmentof the Environment. bDatafrom Ref.3. 'Data from Ref.4. ~USstandard.
in diagnosing asthma. Furthermore, it is not easy to estimate the exposure of any one individual to air pollutants. In a study of six US cities, there was an association between the level of suspended particles in the air and the incidence of cough and bronchitis, but no association was found between levels of particulates, SO~.or NO2 in the air and the incidence of patients with persistent wheeze or asthma 7. A large survey in Switzerland showed no relationship between the levels of air pollutants and respiratory symptoms in children, apart from an association with particulates, although there was no evidence for increased symptoms in asthmatic, compared with non-asthmatic, children s. In another study of US children, there was an association between PM10 levels and respiratory symptoms, such as cough and wheeze, which was more marked in children with asthma 9. 'There are relatively few studies that have monitored the effects of air pollution on known asthmatic patients. Some studies have demonstrated small effects of levels of SO2 on asthma symptoms. In one study in Los Angeles, there was a small effect of 03 concentrations of >300 ppb on asthma symptoms in only a proportion of the 45 asthmatic patients studied I°. In a study of Austrian school children, there was a small increase in airway responsiveness in areas of high O3 (120 ppb), compared with low 03 (<60 ppb), but there was no difference in respiratory symptoms, suggesting that these small changes in reactivity are unlikely to be clinically relevant". A study of Dutch children found a small association between ambient 03 level and lung function, but there was no difference between asthmatic and non-asthmatic children 12. Studies linking admission or emergency visits to hospital with asthma and levels of air pollution have produced conflicting results, with some studies demonstrating associations with various pollutants, whereas others have shown no obvious links5'6.It is possible that pollutants and allergens may have a greater effect in combination. There was a significant association between hospital admissions for asthma in Toronto (Canada) and levels of
03 and hydrogen ion (H÷) levels (acid haze) in the air in three consecutive summers ~3,and an association between emergency visits to hospital for asthma and PM10 has also been reported 14.Proximity to a major traffic artery and levels of SO2 and smoke have been associated with an increase in hospital emergency visits with asthma in Birmingham (UK) (Ref. 15). There is no convincing evidence that air pollution is related to the prevalence of asthma, as evidenced by the fact that asthma is no more common in urban than in rural communities in industrialized countries where levels of air pollution would be expected to be higher. For example, there is no evidence that the prevalence.of asthma in Los.Angeles, which has high levels of air pollution, is higher than that in rural areas of the USA. Similarly, a recent study found no evidence for an increase in the prevalence of asthma in polluted areas of Sydney (Australia) compared with non-polluted areas of the city 16. The reunification of Germany has provided data on two populations exposed to very different types of air pollution I7. East German industrialized towns, such as Leipzig and Erfurt, have high concentrations of SO2 and particulates in the air, whereas West German cities, such as Munich and Hamburg, have low levels of SO2 and particulates, but
Glossary Bronchoconstriction - Contraction of airway smooth muscle. Hyperaemia - Vasodilation of bronchial blood vessels. Dyspnoea - Shortness of breath.. Bronchoalveolar lavage - Washings removed from the airways by bronchoscopy after instillation of saline.
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Box 2. Possible effects of air pollutants on asthmaa • The pollutants may act as inciters or triggers ('irritants') when the airways are hyper-responsive,resulting in transient narrowing of the airway (SO2, particulates). • The pollutant may act as an inducer to increase airway inflammation and thereforeairway hyper-responsiveness,which may persist beyond the exposure time (03 , NO,?, particulates?). • The pollutant may have a direct toxic effect on the airways, leading to asthma symptoms in normal individuals (SO,-?). • The pollutant may affect the immune system, resulting in sensitization or increased allergic responses in the airway (particulates?).
Effects of individual pollutants SO,
Bronchoconstriction at >250 ppb Respiratory symptoms (cough, chest tightness) Increased bronchoconstrictorresponse to exercise and breathing cold or dry air Potentiatedby 03, NO,Probablyno effect on airway inflammation May increase mucus secretion NO,- Potentiates effects of 03, allergen, SO_, Increased inflammation 03 Increased symptoms (chest tightness) Increased airway hyper-responsiveness Increased airway inflammation (neutrophils, eosinophils, cytokines) Increased airway epithelial permeability Interindividual variability in response PM10 Increased symptoms (chest tightness) Bronchoconstriction?
IDatafrom Refs 18-20.
higher levels of N O 2. The prevalence of asthma and allergic diseases is reported to be higher in Munich than in Leipzig, but the prevalence of bronchitis is higher in Leipzig. Similar differences have been reported based on questionnaires, and have revealed a much higher frequency of reported asthma attacks in Hamburg than in Erfurt. There is also a higher prevalence of sensitization to the indoor allergens, house dust mite and cats, in school children from East Germany, although there was no difference in sensitivity to outdoor allergens such as grass pollens. This evidence suggests that indoor exposure to allergens is a much more important determinant of asthma than exposure to outdoor air pollutants such as SO 2 and particulates.
Exposure studies Studies of exposure to air pollutants provide more controlled information than population studies, but have the disadvantage that the exposure time is limited, and that exposure to single agents may not reflect the role of an individual pollutant in combination with other agents. These studies have established that air pollutants, in relevant concentrations, may have several effects on the airways of individuals with asthma (Box 2). Most of the studies have measured the effects of controlled exposure to a single agent on airway function or airway responsiveness. These studies have usually been carried out in subjects m
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with mild asthma and may therefore underestimate any effect of air pollutants in individuals with more severe asthma.
Sulphur dioxide SO,- is an irritant, and inhalation causes chest tightness and bronchoconstriction, but the concentration of this chemical required to cause these effects depends, to some extent, on the degree of airway hyperresponsiveness. For normal subjects, concentrations of >5000 ppb SO_, are usually required to provoke bronchoconstriction whereas, in asthmatics, bronchoconstriction develops at concentrations >1000 ppb (Ref. 21). With moderate exercise, bronchoconstriction occurs in patients with asthma at concentrations of 250 ppb SO: when the subjects breathe through a mouthpiece, but concentrations of >400 ppb are needed when subjects breath through the nose, as some SO z is removed in the upper respiratory tract. The incremental effect of these low concentrations of SO 2 on exercise-induced asthma appears to be similar, irrespective of the severity of asthma :~. As the highest concentrations of SO: in the atmosphere are likely to be of the order of 250 ppb, this suggests that SO: may induce symptoms in asthmatic individuals only with exercise, which increases the delivery of the irritant to the lower airways. The bronchoconstrictor response to SO 2 in individuals with asthma is rapid in onset and recovers rapidly once exposure ceases; there is no evidence that SO_, increases airway responsiveness at these concentrations. SO,- probably acts as a bronchial irritant via activation of sensory nerves, resulting in cough, and causes wheezing by activation of a neural reflex that results in bronchoconstriction. Interactions with other pollutants have been examined. Prior exposure to 03 (120 ppb for 45 min) increased the sensitivity to SO,- in individuals with mild asthma, so that concentrations as low as 100 ppb SO: cause bronchoconstriction during moderate exercise2'-.However, the effects are small and of doubtful clinical significance. Prior inhalation of NO, had no effect on the airway response to SO,. inhalation in asthmatic patients ~-a, but increased the airway response to inhaled allergens :4.
Nitrogen dioxide There has been great interest in the effects of NO,_on airway function in control and asthmatic individuals, as the concentrations of this pollutant have increased with traffic density ~. Although exposure to high concentrations of NO z have caused small and inconsistent effects, exposure to concentrations of < 1000 ppb have not had any significant effect~-5. Although some studies have demonstrated small effects on airway responsiveness in asthmatic patients of 100-300 ppb for 1 h; however, most have failed to show any effect at concentrations up to 4000 ppb in conjunction with exercise in individuals with asthma. Long-term exposure of patients with asthma (including patients with severe asthma) to NO 2 at concentrations of 300 and 600 ppb, and to ambient air in Los Angeles with an NO, content of 90 ppb, had no effect on airway function, even in patients with very severe asthma. Thus, even at concentrations higher than those achieved in the atmosphere at peak periods, NO., has little or no effect on airway function in patients with asthma. However, NO_, may amplify the effects of other pollutants. Thus, NO_, (600 ppb for 2 h) potentiated the effect of 03 on lung function and airway hyper-responsiveness :6. A recent study demonstrated that NO 2 (400 ppb for 1 h) increased the bronchoconstrictor response to allergen _,7,although this was not seen in another study unless SO_, was also inhaled 24.
Ozone The effects of ozone on respiratory function have been investigated extensively 28. Early studies in animals and humans used high concentrations (1000-5000 ppb), which had tissue-damaging effects
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that are unlikely to be relevant to ambient exposure. Using ~80-labelled 03, an exposure of 400 ppb for 2 h during exercise gave significant tsO incorporation into cells lavaged from the lower respiratory tract 29. Most studies have reported a small increase in airway responsiveness at concentrations of 100-300ppb, and there do not appear to be any differences in the responses of normal and asthmatic subjects 3°. There appear to be large variations in responsiveness to ozone between individuals; it is possible that this may reflect differences in anti-oxidant defences of the respiratory Wact3t. On heavy exercise, a concentration of 200 ppb over a period of I h may increase airway responsiveness and cause tracheal irritation. With longer exposure, effects at lower concentrations of 03 may be observed. Thus, exposure to 80-120 ppb for over 6 h has been reported to increase airway responsiveness in normal individuals. 03 may also increase airway responsiveness to asthma Iriggers. Exposure of individuals with asthma to 120ppb 03 for 1 h caused a small increase in airway response to inhaled allergen 32, although there was no increase in exerciseinduced bronchoconstriction 33. This suggests that a positive interaction may be seen between 03 and inflammatory stimuli (such as allergen exposure, which increases airway inflammation), but not with non-inflammatory stimuli (such as exercise, which may stimulate degranulation of mast cells, but does not increase airway inflammation).
Otherpolhttants Sulphuric acid and hydroxymethane sulphonic acid are components of acid fog, but have no reported short-term effects on airway function in normal or asthmatic individuals. Acidity in the air may be measured as H÷ ion (proton) concentration, and there is some evidence for a link between H+concentrations and hospital admissions of patients with asthma rE. Carbon monoxide, formaldehyde, secondary aldehydes and mixtures of short-lived radicals are present in low concentrations, but there has been little research into their effects at ambient concentrations on normal or asthmatic airways.
Cellular and molecular mechanisms As there is controversy about the effects of short-term exposure to air pollutants on airway function, it is likely that any effects are relatively small. This may be in part because measurements of lung function and airway responsiveness may not reflect inflammatory changes in the airways. While
Ozone
NO 2
NF-Y,B
Nucleu~
. t
I n ~
Enzymes
Ct~okine~
iNOS COX-2 cPLA 2
11_-8
RANTES
• NO
• PGs • PGs, LTs, PAF
Figure 1. Effect of ozone and nitrogen dioxide (NOz) on airway epithelial cells. Exposure to oxidants results in the activation of the transcription factor nuclear factor kappa B (NF-kB), which then binds to the promoter sequence of several inducible genes resulting in increased gene expression. These genes include interleukin-8 (IL-8) and RANTES (Released from Activated Normal T cells, Expressed and Secreted), and the enzymes inducible nitric oxide synthase (iNOS), which forms nitric oxide (NO), inducible cyclooxygenase (COX-2), which forms prostaglandins (PGs) and inducible cytosolic phospholipase A2 (cPL.A2),which forms pPGs, leukotrienes (LTs) and platelet-activating factor (PAF).
Macrophage
Mucus secretion =,===._
SO 2 PM10
. NO2
~
~ I IL-I~ TNF-c¢
03
Airway epithelium
Cough Reflex
Neutrophils
bronchoconstriction mucus secretion
(Bronchitis)
Eosinophils
Vasodilation
1" Asthmatic inflammation
Figure 2. Effects of air pollutants on the airways, Sulphur dioxide (SO2) activates sensory nerves in the airways, resulting in symptoms (cough) and reflex effects. Ozone (0 3) and nitrggen dioxide (NO=) release inflammatory mediators from airway epithelial cells directly, or via activation of macrophages. These mediators include the cytokines interleukin 8 (IL-8) and granulocyte--macrophage colony-stimulating factor (GM-CSF), resulting in amplification of asthmatic inflammation. Release of nitric oxide (NO) and prostaglandins (PGs) also amplify inflammation via a vasodilator action.
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ambient concentrations of SO., may cause bronchoconstriction as a result of a neural reflex triggered by activation of sensory nerves in the airways, there is evidence that 0 3 and NO,. may induce inflammatory changes. Exposure of control subjects to 03 increases the numbers of neutrophils in bronchoalveolar lavage, suggesting that ozone provokes an inflammatory response29'34J-~.Similar changes have been observed with inhaled NO v although the concentrations needed for inflammatory effects are rather higher (>2000 ppb), as NO 2 is a weaker oxidant than 0 3 (Ref. 36). However, NO 2 and O 3 may have additive or synergistic effects on airway inflammation. The inflammatory response is accompanied by increased concentrations of the cytokines interleukin 8 (IL-8) and granulocytemacrophage colony-stimulating factor (GM-CSF) in bronchoalveolar lavage fluid 35. This inflammatory response is still present 18h after exposure to 80ppb 0 3 for 6h (Ref. 37). 03 at a concentration of 400 ppb for 4 h increased the eosinophilic inflammatory response to intranasally applied allergen in atopic subjects 38, and concentrations as low as 120ppb for 90min in conjunction with moderate exercise caused an inflammatory response in the nose of asthmatic, but not normal, subjects, with an increase in the concentration of neutrophils and IL-8 (Ref. 39). These studies suggest that 0 3, at concentrations which may be present during the summer months, may increase in airway inflammation. Asthmatic patients may be more susceptible to the inflammatory effects of ozone than normal individuals because of an existing eosinophilic inflammation. The mechanisms by which O 3 and NO 2 increase airway inflammation are not yet understood, but recent studies in cultured airway epithelial cells suggest that relevant levels of these pollutants may increase the expression of cytokines, such as IL-6, IL-8, GM-CSF and tumor necrosis factor ct (TNF-ot) (Ref. 40). The mechanism may involve oxidative activation of the transcription factor nuclear factor KB(NF-KB) (Ref. 41). Exposure of human epithelial cells to oxidative stress results in the activation of NF-~d3 and increased transcription of several inducible genes that may amplify inflammation4'-. These include inducible NO synthase 42, inducible cyclo-oxygenase (COX-2) (Ref. 43), inducible cytosolic phospholipase A,_ and various chemoattractant cytokines (chemokines), such as IL-8, which attracts neutrophils and RANTES (Released from Activated Normal T cells, Expressed and Secreted), which attracts eosinophils 44'45(Fig. 1). The release of IL-8 from airway epithelial cells may result in neutrophilic inflammation whereas, in patients with asthma, IL-8 might result in an eosinophilic inflammation, as IL-8 becomes chemoattractant for eosinophils in the presence of cytokines present in asthmatic airways, including IL-5 and GM-CSF (Ref. 46). Chronic exposure of epithelial cells to these pollutants may therefore amplify eosinophilic inflammation in asthmatic airways, thus increasing asthma symptoms (Fig. 2). This would also amplify the inflammatory response to inhaled allergens, resulting in enhanced bronchoconstrictor responses. By contrast, SO 2 and particulates may act as irritants, stimulating sensory nerves in the airways to cause cough, reflex bronchoconstriction in asthmatic patients and increased mucus secretion. In normal subjects, a combination of SO 2, 03 and NO 2 would result in increased secretion of mucus and this may be amplified by neutrophilic inflammation, leading to chronic bronchitis (Fig. 2). In asthmatic patients, the same pollutants may amplify allergic infammafion (particularly in the presence of allergens) and activate reflex bronchoconstriction. F u t u r e directions
Urban air pollution is increasing worldwide as road traffic increases, and there is already compelling evidence that this may increase the m
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morbidity of asthma. Many important questions about air pollution and asthma remain to be answered. It is possible that some individuals may be more susceptible 1o air pollution because of reduced anti-oxidant defences. Normally, oxidant stress is counteracted by endogenous antioxidant mechanisms (superoxide dismutase, glutathione peroxidase and catalase) and by dietary anti-oxidants vitamins C and E). Susceptibility to the effects of air pollutants may therefore vary between patients and may be dependent on dietary intake of anti-oxidants. More studies of individual risk factors are needed once the effects of air pollutants can be quantified. There is an urgent need to measure individual exposure to different pollutants more accurately. It now appears that combinations of air pollutants may have additive or even synergistic effects and the mechanisms of interaction need to be studied in more detail. The molecular mechanisms whereby air pollutants may increase airway inflammation are also emerging. Better understanding of the molecular pathways involved is important if logical therapeutic strategies are to be developed. If tbe proposed mechanism involving NF-KB is important, then glucocorticoids, which directly inhibit NF-KB activation, would be expected to inhibit this mechanism and inhaled glucocorticoids should be protective. In the future, more specific NF-KB inhibitors may be developed, or effective oral or inhaled anti-oxidant therapies used. The obvious solution to air pollution is to control road traffic through legislative means or to use alternative fuels that do not result in harmful exhaust products.
The outstanding questions • What are the key target cells in the airways for the effects of air pollutants? (Macrophages, epithelial cells, sensory nerves, flbroblasts?) • How do air pollutants increase airway inflammation? (Activation of transcription factors, effects on enzyme activity, direct effects on inflammatory cells?) • Does asthma (and airway inflammation) increase the susceptibility to the inflammatory effects of air pollutants? (By increasing or changing the nature of the inflammatory response?) • Do air pollutants increase the inflammatory response to allergens or viruses in patients with asthma? • Is there interindividual susceptibility to the inflammatory effects of air pollution? (Related to endogenous anti-oxidant defences, dietary anti-oxidants, genetic factors?)
References
10stro, B.D. et al. (1994) Indoor air pollution and asthma, Ant. J. Respir. Crit. Care Med. 149, 1400-1405 2 WorldHealthOrganization(1992) Urban Air Polhaion in Megacities of the World, Blackwell 3 WHO Regional office for Europe (1987) Air Quality Guidelines for Europe, Copenhagen: WHO Regional Publications, Et~ropeanSeries No. 23 4 Department of the Environment (1990) This Common Inheritance. Britain's Eavironmental Strategy, HMSO 5 Wardlaw,AJ. (1993)The roleof air pollutionin asthma,Clin. Exp. Allergy 23, 81-96 6 Barnes,P.J.(1994) Air pollutionand asthma, Postgrad. Med. J. 70, 319-325 7 Dockery,D.W.eta/. (1993)An associationbetween air pollutionand mortalityin six US cities,New EngL J. Med. 329, 1753-1759 8 Braun-Fahrl~inder,C. eta/. (1992) Air pollution and respiratory symptoms in preschool children, Am. Rev. Respir. Dis. 145,42--47
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9 Pope, C.A. and Dockery. D.W. (1992) Acute effects of PM10 pollution on symptomalic and asymplomatic children. Ant. Rev. Respir. Dis. 145, 1123-1128 10 Kurata, J.H.. Grovsky. M.M., Newcomb, R.L. and Easlon, J.C. (1976) Multifactorial study of patients with asthma. 2: Air pollution, animal dander and asthma symptoms./tun. AIh't:~y 37, 398-409 11 Zwick H. et al. ( 1991 ) Effects of ozone on the respiratory health, allergen sensitization and cellular immune system in children, Ant. Rev. Respir. Dis. 144, 1075-1079 12 Hock, G. et al. (1993) Acute effects of ambient ozone on pulmonary function of children in The Netherlands, Ant. Rev. Respir Dis. 147, I ll-117 13 Thurston.G.D. et al. (1994) Respiratory hospital admissions and summer time haze air pollution in Toronto, Ontario: consideration of the role of acid aerosols, Euvirou. Res. 65, 271-290 14 Schwartz, J. eta/. (1993) Particulate air pollution and hospital emergency room visits for asthma in Seattle, Am. Rev. Respir. Dis. 147. 826-83 I 15 Edwards, J., Waiters, S. and Griffiths, R.K. (1994) Hospital admissions for asthma in preschool children: relationship to major roads in Birmingham, United Kingdom, Arch. Euviron. Heohh 49, 223-227 16 Peat, J.K., Salome, C.M. and Woolcock, A..I. (1992) Factors associated with bronchial hyperresponsiveness in Australian adults and children, Eur Respir. J. 5, 921-929 17 Magnussen, H., J6rres, R. and Nowak, D. (1993) Effect of air pollution on the prevalence of asthma and allergy: lessons from the German reunification, Thor~z~" 48,879-881 18 Advisory Group of the Medical Aspects of Air Pollution Episodes (1991) First Report: O=oue, p. 90, London, Department of Health, HMSO 19 Advisory Group of the Medical Aspects of Air Pollution Episodes (1992) Second Report: Sulphur Dioxide. AcM Aerosols attd Particulates, p. 131, London, Department of Health, HMSO 20 Advisory Group of the Medical Aspects of Air Pollution Episodes (1993) Third Report: Oxides of Nitrogen. London. Department of Health, HMSO 21 Linn,W.S. et al. (1987) Replicated dose-response study of sulfur dioxide effects in normal, atopic and asthmatic volunteers, Ant. Rev. Respir. Dis. 126, 1127-1134 22 Koenig, J.Q., Covert, D.S., Hanley,G.B. and Pierson, W.E. (1990) Prior exposure to ozone potentiates subsequent responses to sulfur dioxide in adolescent asthmatic subjects, Ant. Rev. Respir. Dis. 141,377-380 23 Rubenstein,I., Bigby,B.G., Reiss,T.E and Boushey,H.A. (1990) Short-term exposure to 0.3 ppm nitrogen dioxide does not potentiate airway responsiveness to sulphur dioxide in asthmatic subject, Ant. Rev. Respir. Dis. 141, 381-385 24 Devalia, J.L. et al. (1994) Effect of nitrogen dioxide and sulphur dioxide on airway responses of mild asthmatic patients to allergen inhalation, Lancet 344, 1668-1171 25 Samet, J.M. and Utell, M.J. (1990) The risk of nitrogen dioxide: what have we learned from epidemiological and clinical studies? Toxieol. Ind. Health 6, 247-262 26 Hazucha, M.J., Folinsbee, L.J., Seal, E. and Bromberg, P.A. (1994) Lung function response of healthy women after sequential exposures to NO,. and O~, Ant. J. Respir Crit. Care Med. 150, 642--647 27 Tunnicliffe,W.S.,Burge, P.S.and Ayres,J.G. (1994) Effect of domestic concentrations of nitrogen dioxide on airway responses to inhaled allergen in asthmatic patients, Lancet 344, 1733-1736 28 Lippman, M. (1993) Health effects of ozone. A critical review, J. Air Pollut. Control Assoc. 39, 672-695 29 Hatch, G.E. et al. (1994) Ozone dose and effect in humans and rats, Ant. J. Respir. Crit. Care. Med. 150, 676-683 30 Koenig, J.Q. et al. (1987) The effects of ozone and nitrogen dioxide on pulmonary function in healthy and in asthmatic adolescents, Am. Rev. Respir. Dis. 136, 1152-1157 31 Menzel, D.B. (1993) Antioxidant vitamins and prevention of lung disease, Ann. NY Acad. Sci. 669, 141-145 32 Molfino,N.A. et al. ( 1991) Effect of inw concentration of ozone on inhaled allergen responses in asthmatic subjects, Lancet 338, 199-203
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33 Weymer, A.R., Gong, H., Lyness, A. and Linn, W.S. (1994) Pre-exposure to ozone does not enhance or produce exercise-induced asthma, Am. J. Respir. Crit. Care Med. 149. 1413-1419 34 Korcn. H.S. et al. (1989) Ozone and induced inflammation in the lower airways of human subjects, Am. Rev. Respir Dis. 139, 407-415 35 Ark, R.M. et al. (1993) Ozone-induced airway inflammation in human subjects as determined by airway lavage and biopsy, Am. Rev. Respir. Dis. 148, 1363-1372 36 Sandstr6m, T. et al. ( 1991) Inflammatory cell response in bronchoaiveolar lavage fluid after nitrogen dioxide exposure in healthy subjects: a dase-response study, Era: Respir. J. 4, 332-339 37 Devlin, R.B. et aL (1991) Exposure of humans to ambient levels of ozone for 6.6 hours causes cellular and biochemical changes in the lung, Am. J. Respir Cell Mol. Biol. 4. 72-81 38 Bascom, R. et al. (1990) Effect of ozone inhalation on the response to nasal challenge with antigen in allergic subjects, Am. Rev. Respir. Dis. 142, 594-601 39 McBride. D.E. et al. (1994) Inflammatory effects of ozone in the upper airways of subjects with asthma, Am. J. Respir. Crit. Care Med. 149, 1192-1197 40 Devalia, J.L. et al. (1993) Effect of nitrogen dioxide on synthesis of inflammatory cytokines expressed by human bronchial epithelial cells, Ant. J. Respir. Cell Mol. Biol. 9, 271-278 41 Schreck, R., Rieber, P. and Baeurle, P.A. (1991) Reactive oxygen intermediates as apparently widely used messengers in the activation of the NF-kappa B transcription factor and HIV-I, EMBO J. 10, 2247-2258 42 Adcock, IM., Brown, C.R., Kwon, O.J. and Barnes, PJ. (1994) Oxidative stress induces NF-Id3 DNA binding and inducible NOS mRNA in human epithelial cells, Biochem. Biophys. Res. Commun. 199, 1518-1524 43 Newton, R., Adcock, I.M. and Barnes, PJ. (1995) Stimulation of COX-2 message by cytokines or phorbol ester is preceded by a massive and rapid induction of NF-r,B binding activity, Ant. J. Respir. Crit. Care Med. 151, A165 44 Mukaido, N. et ul. (1994) Novel mechanisms of glucocorticoid-mediated gene repression: NF-kB is target for glucocorticoid-mediated IL-8 gene expression, J. Biol. Chem. 269. 13289-13295 45 Nelson, PJ. et al. (1993) Genomic organization and transcriptional regulation of the RANTES chemokine gene, J. hnmunol. 151, 2601-2612 46 Warringa,R.AJ. et al. (1992) Modulation of easinophil chemotaxis by interleukin 5, Ant. J. Respir. Cell Mol. Biol. 7, 631-636
Next month's Round up in MMT The meetings: • Advances in Genetic Diagnostics for Infectious Diseases • Etiology and Pathogenesis of Infectious Diseases
The books • The Oncogene Factsbook • The Oncogene Handbook • CellularCancer Markers
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Air pollution and asthma: molecular mechanisms ~' Air pollution, in particular that generated by road '~' traffic, is a matter of rising public concern and has been implicated in the worsening of asthma. In this article, the evidence that air pollutants (particularly i sulphur dioxide, ozone and nitrogen dioxide) can i ! affect the airways of asthmatic patients is reviewed, i and the possible molecular mechanisms that may 'i~ link air pollution to increased inflammation in the airways are discussed. Airway epithelial cells may Frrespond to oxidant pollutants by the activation • of transcription factors, such as nuclear factor ,~ , kappa B, resulting in increased transcription of ~!'i, i i flenes for certain cytokines, such as interleukin 8 ~ , '? 'and inflammatory enzymes, such as inducible ~,~ nitric oxide synthase and cyclo-oxygenase. P
,i
T is one of the commonest diseases in industrialized countries, .fiffecting over 10% of children and 5-10% of adults (Box r). There is ~nvineing evidence for a worldwide increase in asthma prevalence /id morbidity. The question of whether air pollution contributes to ~ statistics has become a matter of increasing public concern. While ~s~.no convincing evidence that the worldwide increase in the evalence of asthma is due to air pollution, there is increasing evidence theft air pollution may contribute to an increase in asthma morbidity. Indoor air pollution includes tobacco smoke (passive smoking), indoor allergens (particularly house dust mite and cat), volatile organic comunds, and nitrogen dioxide (NO~) generated by gas appliances. For example, there is some evidence that gas cookers are associated with increased symptoms in asthmatic patients t. However, most concern has focused on whether outdoor air pollution, particularly that generated by ~ r o a d traffic, is increasing asthma symptoms and exacerbations 2.
Ipo
he constituents of air pollution ~Several pollutants are present in outdoor air (Table 1). Vehicle exhaust rues contain particulates (various respirable particles with an m
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• Asthma is associated with chronic airway inflammation, characterized by Allergens activation of mast cells and T cells and infiltration of activated eosinophils Air pollutants: 03, NO2 ('eosinophilic bronchitis'). The severity Virus infections of asthma is related to the degree of air inflammation. • Inflammatory and structural cells of the airway release multiple inflammatory Inflammation mediators, including lipid mediators Airway 'Chronic eosinophilic (leukotrienes, prostaglandins, plateletHyper-responsiveness bronchitis' activating factor) and cytokines. • Airway epithelial cells may be an important source of inflammatorymediators, Triggers which may be released in response to Allergens environmental stimuli, such as allergens Exercise and air pollutants. Cold air • Cytokines play a critical role in orCough Wheeze SO2 chestrating the inflammatory response Chest tightness Dyspnoea Particulates in asthma by recruiting, activating and allowing the persistence of inflammatory cells. Mechanismsof asthma. • Acute inflammatory responses in the airwaysresult in bronchoconstriction,hyperaemia, plasma exudation and oedema, mucus hypersecretion and activation of sensory nerve endings. • Airway inflammationunderlies airwayhyper-responsiveness,which is an increased sensitivityof the airways to many triggers, includingexercise, cold air and irritants such as SO2. • Chronic effects of airway inflammation include increased airway smooth muscle thickening, fibrosis, new vessel formation and hyperplasia of mucussecreting cells. These changes may result in irreversible narrowing of the airways. • Asthma symptoms of wheeze and dyspnoea arise from narrowing of the airways. Cough and chest tightness arise from activation of sensitized sensory nerve endings (Fig. 1). Symptomsmay increase because of increased inflammation(exposure to allergens, 03, NO+.,virus infections) or through the effect of various triggers (exercise, cold air, allergens that activate airway mast cells, and irritants such as SO.+that activate airway sensory nerves).
aerodynamic diameter of <10 ~m, described as PMI0), volatile organic compounds, oxides of nitrogen (NOx, mainly NO+_)and sulphur dioxide (SO+). Diesel particulates may be of particular concern, as the small particles) <2.5 ~m) may reach the alveoli and initiate alveolar, as well as airway, inflammation. Ozone (03) is a secondary pollutant, which is formed by the action of sunlight on oxides of nitrogen in the presence of hydrocarbons. The information of 03 is favoured by warm temperatures, sunlight and low winds and, once formed, O 3 may be carried long distances by prevailing winds. 03 is broken down in the presence of NO+, so that its concentration in cities may be less than in rural areas. There has been a change in the nature of air pollution in industrialized countries, with a decline in the levels of SO+. and smoke created by the burning of fossil fuels, but there has been an increase in the levels of NO+_,03 and diesel particulates, associated with increases in the density of road traffic. The establishment of a firm link between traffic pollution and asthma has proved difficult. It is possible that air pollution may cause asthma by increasing the risk of developing asthma in an atopic individual, or by increasing the risks of sensitization, thus leading to an increase in asthma prevalence. However, it is more likely that air pollution may increase symptoms in asthmatic individuals, thus increasing asthma morbidity. Although there has been considerable interest in this question, evidence l
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to support it is conflicting5'6. This is partly because of the difficulty in establishing a clear association between air pollution and asthma symptoms, as there are several conflicting factors. However, two approaches have been used. The first is an epidemiological approach, which attempts to link asthma symptoms and exacerbations with episodes of air pollution, either in whole populations, some members of which may be asthmatic, or in defined groups of patients with asthma. The second approach is controlled exposure to ambient concentrations of pollutants, either singly or in combination. Epidemiological studies There are several well-documented episodes of major air pollution, including the infamous London smog of 1952, that were associated with cold weather and little wind, so that polluted air became trapped, resulting in high concentrations of SO2 [>1000 parts per billion (ppb)], sulphuric acid and particulates. During these episodes, there was evidence of increased symptoms and an increase in hospital admissions in patients with asthma. Population studies of respiratory symptoms and their connection with air pollution are difficult to interpret because of socioeconomic factors, the effect of coexistent aero-allergens, the influence of cigarette smoking and respiratory infections, climatic changes, and the difficulty
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Table 1. The composition of air pollution a Pollutant
Source
WHO guidelinesb
DOE air quality~
1h Average
24 h Average
Maximum I h average Poor Very poor
SO2 (ppb)
Burning of fossil fuels: power stations coal burning
122
-
125-399
>_300
NO2 (ppb)
Vehicle exhaust Burning fossil fuels Gas appliances
210
80
100-299
_>400
03 (ppb)
Photochemical conversion of O2 + NO2 (temperature, sunlight) related to vehicle exhaust
76-100
50-60 (8 h average)
90-179
_>180
Particulates (PM10: I~g m3)
Diesel exhaust Burning fossil fuels
150a
"Abbreviations:ppb, parts per billion; DOE,UK Departmentof the Environment. bDatafrom Ref.3. 'Data from Ref.4. ~USstandard.
in diagnosing asthma. Furthermore, it is not easy to estimate the exposure of any one individual to air pollutants. In a study of six US cities, there was an association between the level of suspended particles in the air and the incidence of cough and bronchitis, but no association was found between levels of particulates, SO~.or NO2 in the air and the incidence of patients with persistent wheeze or asthma 7. A large survey in Switzerland showed no relationship between the levels of air pollutants and respiratory symptoms in children, apart from an association with particulates, although there was no evidence for increased symptoms in asthmatic, compared with non-asthmatic, children s. In another study of US children, there was an association between PM10 levels and respiratory symptoms, such as cough and wheeze, which was more marked in children with asthma 9. 'There are relatively few studies that have monitored the effects of air pollution on known asthmatic patients. Some studies have demonstrated small effects of levels of SO2 on asthma symptoms. In one study in Los Angeles, there was a small effect of 03 concentrations of >300 ppb on asthma symptoms in only a proportion of the 45 asthmatic patients studied I°. In a study of Austrian school children, there was a small increase in airway responsiveness in areas of high O3 (120 ppb), compared with low 03 (<60 ppb), but there was no difference in respiratory symptoms, suggesting that these small changes in reactivity are unlikely to be clinically relevant". A study of Dutch children found a small association between ambient 03 level and lung function, but there was no difference between asthmatic and non-asthmatic children 12. Studies linking admission or emergency visits to hospital with asthma and levels of air pollution have produced conflicting results, with some studies demonstrating associations with various pollutants, whereas others have shown no obvious links5'6.It is possible that pollutants and allergens may have a greater effect in combination. There was a significant association between hospital admissions for asthma in Toronto (Canada) and levels of
03 and hydrogen ion (H÷) levels (acid haze) in the air in three consecutive summers ~3,and an association between emergency visits to hospital for asthma and PM10 has also been reported 14.Proximity to a major traffic artery and levels of SO2 and smoke have been associated with an increase in hospital emergency visits with asthma in Birmingham (UK) (Ref. 15). There is no convincing evidence that air pollution is related to the prevalence of asthma, as evidenced by the fact that asthma is no more common in urban than in rural communities in industrialized countries where levels of air pollution would be expected to be higher. For example, there is no evidence that the prevalence.of asthma in Los.Angeles, which has high levels of air pollution, is higher than that in rural areas of the USA. Similarly, a recent study found no evidence for an increase in the prevalence of asthma in polluted areas of Sydney (Australia) compared with non-polluted areas of the city 16. The reunification of Germany has provided data on two populations exposed to very different types of air pollution I7. East German industrialized towns, such as Leipzig and Erfurt, have high concentrations of SO2 and particulates in the air, whereas West German cities, such as Munich and Hamburg, have low levels of SO2 and particulates, but
Glossary Bronchoconstriction - Contraction of airway smooth muscle. Hyperaemia - Vasodilation of bronchial blood vessels. Dyspnoea - Shortness of breath.. Bronchoalveolar lavage - Washings removed from the airways by bronchoscopy after instillation of saline.
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Box 2. Possible effects of air pollutants on asthmaa • The pollutants may act as inciters or triggers ('irritants') when the airways are hyper-responsive,resulting in transient narrowing of the airway (SO2, particulates). • The pollutant may act as an inducer to increase airway inflammation and thereforeairway hyper-responsiveness,which may persist beyond the exposure time (03 , NO,?, particulates?). • The pollutant may have a direct toxic effect on the airways, leading to asthma symptoms in normal individuals (SO,-?). • The pollutant may affect the immune system, resulting in sensitization or increased allergic responses in the airway (particulates?).
Effects of individual pollutants SO,
Bronchoconstriction at >250 ppb Respiratory symptoms (cough, chest tightness) Increased bronchoconstrictorresponse to exercise and breathing cold or dry air Potentiatedby 03, NO,Probablyno effect on airway inflammation May increase mucus secretion NO,- Potentiates effects of 03, allergen, SO_, Increased inflammation 03 Increased symptoms (chest tightness) Increased airway hyper-responsiveness Increased airway inflammation (neutrophils, eosinophils, cytokines) Increased airway epithelial permeability Interindividual variability in response PM10 Increased symptoms (chest tightness) Bronchoconstriction?
IDatafrom Refs 18-20.
higher levels of N O 2. The prevalence of asthma and allergic diseases is reported to be higher in Munich than in Leipzig, but the prevalence of bronchitis is higher in Leipzig. Similar differences have been reported based on questionnaires, and have revealed a much higher frequency of reported asthma attacks in Hamburg than in Erfurt. There is also a higher prevalence of sensitization to the indoor allergens, house dust mite and cats, in school children from East Germany, although there was no difference in sensitivity to outdoor allergens such as grass pollens. This evidence suggests that indoor exposure to allergens is a much more important determinant of asthma than exposure to outdoor air pollutants such as SO 2 and particulates.
Exposure studies Studies of exposure to air pollutants provide more controlled information than population studies, but have the disadvantage that the exposure time is limited, and that exposure to single agents may not reflect the role of an individual pollutant in combination with other agents. These studies have established that air pollutants, in relevant concentrations, may have several effects on the airways of individuals with asthma (Box 2). Most of the studies have measured the effects of controlled exposure to a single agent on airway function or airway responsiveness. These studies have usually been carried out in subjects m
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with mild asthma and may therefore underestimate any effect of air pollutants in individuals with more severe asthma.
Sulphur dioxide SO,- is an irritant, and inhalation causes chest tightness and bronchoconstriction, but the concentration of this chemical required to cause these effects depends, to some extent, on the degree of airway hyperresponsiveness. For normal subjects, concentrations of >5000 ppb SO_, are usually required to provoke bronchoconstriction whereas, in asthmatics, bronchoconstriction develops at concentrations >1000 ppb (Ref. 21). With moderate exercise, bronchoconstriction occurs in patients with asthma at concentrations of 250 ppb SO: when the subjects breathe through a mouthpiece, but concentrations of >400 ppb are needed when subjects breath through the nose, as some SO z is removed in the upper respiratory tract. The incremental effect of these low concentrations of SO 2 on exercise-induced asthma appears to be similar, irrespective of the severity of asthma :~. As the highest concentrations of SO: in the atmosphere are likely to be of the order of 250 ppb, this suggests that SO: may induce symptoms in asthmatic individuals only with exercise, which increases the delivery of the irritant to the lower airways. The bronchoconstrictor response to SO 2 in individuals with asthma is rapid in onset and recovers rapidly once exposure ceases; there is no evidence that SO_, increases airway responsiveness at these concentrations. SO,- probably acts as a bronchial irritant via activation of sensory nerves, resulting in cough, and causes wheezing by activation of a neural reflex that results in bronchoconstriction. Interactions with other pollutants have been examined. Prior exposure to 03 (120 ppb for 45 min) increased the sensitivity to SO,- in individuals with mild asthma, so that concentrations as low as 100 ppb SO: cause bronchoconstriction during moderate exercise2'-.However, the effects are small and of doubtful clinical significance. Prior inhalation of NO, had no effect on the airway response to SO,. inhalation in asthmatic patients ~-a, but increased the airway response to inhaled allergens :4.
Nitrogen dioxide There has been great interest in the effects of NO,_on airway function in control and asthmatic individuals, as the concentrations of this pollutant have increased with traffic density ~. Although exposure to high concentrations of NO z have caused small and inconsistent effects, exposure to concentrations of < 1000 ppb have not had any significant effect~-5. Although some studies have demonstrated small effects on airway responsiveness in asthmatic patients of 100-300 ppb for 1 h; however, most have failed to show any effect at concentrations up to 4000 ppb in conjunction with exercise in individuals with asthma. Long-term exposure of patients with asthma (including patients with severe asthma) to NO 2 at concentrations of 300 and 600 ppb, and to ambient air in Los Angeles with an NO, content of 90 ppb, had no effect on airway function, even in patients with very severe asthma. Thus, even at concentrations higher than those achieved in the atmosphere at peak periods, NO., has little or no effect on airway function in patients with asthma. However, NO_, may amplify the effects of other pollutants. Thus, NO_, (600 ppb for 2 h) potentiated the effect of 03 on lung function and airway hyper-responsiveness :6. A recent study demonstrated that NO 2 (400 ppb for 1 h) increased the bronchoconstrictor response to allergen _,7,although this was not seen in another study unless SO_, was also inhaled 24.
Ozone The effects of ozone on respiratory function have been investigated extensively 28. Early studies in animals and humans used high concentrations (1000-5000 ppb), which had tissue-damaging effects
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that are unlikely to be relevant to ambient exposure. Using ~80-labelled 03, an exposure of 400 ppb for 2 h during exercise gave significant tsO incorporation into cells lavaged from the lower respiratory tract 29. Most studies have reported a small increase in airway responsiveness at concentrations of 100-300ppb, and there do not appear to be any differences in the responses of normal and asthmatic subjects 3°. There appear to be large variations in responsiveness to ozone between individuals; it is possible that this may reflect differences in anti-oxidant defences of the respiratory Wact3t. On heavy exercise, a concentration of 200 ppb over a period of I h may increase airway responsiveness and cause tracheal irritation. With longer exposure, effects at lower concentrations of 03 may be observed. Thus, exposure to 80-120 ppb for over 6 h has been reported to increase airway responsiveness in normal individuals. 03 may also increase airway responsiveness to asthma Iriggers. Exposure of individuals with asthma to 120ppb 03 for 1 h caused a small increase in airway response to inhaled allergen 32, although there was no increase in exerciseinduced bronchoconstriction 33. This suggests that a positive interaction may be seen between 03 and inflammatory stimuli (such as allergen exposure, which increases airway inflammation), but not with non-inflammatory stimuli (such as exercise, which may stimulate degranulation of mast cells, but does not increase airway inflammation).
Otherpolhttants Sulphuric acid and hydroxymethane sulphonic acid are components of acid fog, but have no reported short-term effects on airway function in normal or asthmatic individuals. Acidity in the air may be measured as H÷ ion (proton) concentration, and there is some evidence for a link between H+concentrations and hospital admissions of patients with asthma rE. Carbon monoxide, formaldehyde, secondary aldehydes and mixtures of short-lived radicals are present in low concentrations, but there has been little research into their effects at ambient concentrations on normal or asthmatic airways.
Cellular and molecular mechanisms As there is controversy about the effects of short-term exposure to air pollutants on airway function, it is likely that any effects are relatively small. This may be in part because measurements of lung function and airway responsiveness may not reflect inflammatory changes in the airways. While
Ozone
NO 2
NF-Y,B
Nucleu~
. t
I n ~
Enzymes
Ct~okine~
iNOS COX-2 cPLA 2
11_-8
RANTES
• NO
• PGs • PGs, LTs, PAF
Figure 1. Effect of ozone and nitrogen dioxide (NOz) on airway epithelial cells. Exposure to oxidants results in the activation of the transcription factor nuclear factor kappa B (NF-kB), which then binds to the promoter sequence of several inducible genes resulting in increased gene expression. These genes include interleukin-8 (IL-8) and RANTES (Released from Activated Normal T cells, Expressed and Secreted), and the enzymes inducible nitric oxide synthase (iNOS), which forms nitric oxide (NO), inducible cyclooxygenase (COX-2), which forms prostaglandins (PGs) and inducible cytosolic phospholipase A2 (cPL.A2),which forms pPGs, leukotrienes (LTs) and platelet-activating factor (PAF).
Macrophage
Mucus secretion =,===._
SO 2 PM10
. NO2
~
~ I IL-I~ TNF-c¢
03
Airway epithelium
Cough Reflex
Neutrophils
bronchoconstriction mucus secretion
(Bronchitis)
Eosinophils
Vasodilation
1" Asthmatic inflammation
Figure 2. Effects of air pollutants on the airways, Sulphur dioxide (SO2) activates sensory nerves in the airways, resulting in symptoms (cough) and reflex effects. Ozone (0 3) and nitrggen dioxide (NO=) release inflammatory mediators from airway epithelial cells directly, or via activation of macrophages. These mediators include the cytokines interleukin 8 (IL-8) and granulocyte--macrophage colony-stimulating factor (GM-CSF), resulting in amplification of asthmatic inflammation. Release of nitric oxide (NO) and prostaglandins (PGs) also amplify inflammation via a vasodilator action.
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ambient concentrations of SO., may cause bronchoconstriction as a result of a neural reflex triggered by activation of sensory nerves in the airways, there is evidence that 0 3 and NO,. may induce inflammatory changes. Exposure of control subjects to 03 increases the numbers of neutrophils in bronchoalveolar lavage, suggesting that ozone provokes an inflammatory response29'34J-~.Similar changes have been observed with inhaled NO v although the concentrations needed for inflammatory effects are rather higher (>2000 ppb), as NO 2 is a weaker oxidant than 0 3 (Ref. 36). However, NO 2 and O 3 may have additive or synergistic effects on airway inflammation. The inflammatory response is accompanied by increased concentrations of the cytokines interleukin 8 (IL-8) and granulocytemacrophage colony-stimulating factor (GM-CSF) in bronchoalveolar lavage fluid 35. This inflammatory response is still present 18h after exposure to 80ppb 0 3 for 6h (Ref. 37). 03 at a concentration of 400 ppb for 4 h increased the eosinophilic inflammatory response to intranasally applied allergen in atopic subjects 38, and concentrations as low as 120ppb for 90min in conjunction with moderate exercise caused an inflammatory response in the nose of asthmatic, but not normal, subjects, with an increase in the concentration of neutrophils and IL-8 (Ref. 39). These studies suggest that 0 3, at concentrations which may be present during the summer months, may increase in airway inflammation. Asthmatic patients may be more susceptible to the inflammatory effects of ozone than normal individuals because of an existing eosinophilic inflammation. The mechanisms by which O 3 and NO 2 increase airway inflammation are not yet understood, but recent studies in cultured airway epithelial cells suggest that relevant levels of these pollutants may increase the expression of cytokines, such as IL-6, IL-8, GM-CSF and tumor necrosis factor ct (TNF-ot) (Ref. 40). The mechanism may involve oxidative activation of the transcription factor nuclear factor KB(NF-KB) (Ref. 41). Exposure of human epithelial cells to oxidative stress results in the activation of NF-~d3 and increased transcription of several inducible genes that may amplify inflammation4'-. These include inducible NO synthase 42, inducible cyclo-oxygenase (COX-2) (Ref. 43), inducible cytosolic phospholipase A,_ and various chemoattractant cytokines (chemokines), such as IL-8, which attracts neutrophils and RANTES (Released from Activated Normal T cells, Expressed and Secreted), which attracts eosinophils 44'45(Fig. 1). The release of IL-8 from airway epithelial cells may result in neutrophilic inflammation whereas, in patients with asthma, IL-8 might result in an eosinophilic inflammation, as IL-8 becomes chemoattractant for eosinophils in the presence of cytokines present in asthmatic airways, including IL-5 and GM-CSF (Ref. 46). Chronic exposure of epithelial cells to these pollutants may therefore amplify eosinophilic inflammation in asthmatic airways, thus increasing asthma symptoms (Fig. 2). This would also amplify the inflammatory response to inhaled allergens, resulting in enhanced bronchoconstrictor responses. By contrast, SO 2 and particulates may act as irritants, stimulating sensory nerves in the airways to cause cough, reflex bronchoconstriction in asthmatic patients and increased mucus secretion. In normal subjects, a combination of SO 2, 03 and NO 2 would result in increased secretion of mucus and this may be amplified by neutrophilic inflammation, leading to chronic bronchitis (Fig. 2). In asthmatic patients, the same pollutants may amplify allergic infammafion (particularly in the presence of allergens) and activate reflex bronchoconstriction. F u t u r e directions
Urban air pollution is increasing worldwide as road traffic increases, and there is already compelling evidence that this may increase the m
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morbidity of asthma. Many important questions about air pollution and asthma remain to be answered. It is possible that some individuals may be more susceptible 1o air pollution because of reduced anti-oxidant defences. Normally, oxidant stress is counteracted by endogenous antioxidant mechanisms (superoxide dismutase, glutathione peroxidase and catalase) and by dietary anti-oxidants vitamins C and E). Susceptibility to the effects of air pollutants may therefore vary between patients and may be dependent on dietary intake of anti-oxidants. More studies of individual risk factors are needed once the effects of air pollutants can be quantified. There is an urgent need to measure individual exposure to different pollutants more accurately. It now appears that combinations of air pollutants may have additive or even synergistic effects and the mechanisms of interaction need to be studied in more detail. The molecular mechanisms whereby air pollutants may increase airway inflammation are also emerging. Better understanding of the molecular pathways involved is important if logical therapeutic strategies are to be developed. If tbe proposed mechanism involving NF-KB is important, then glucocorticoids, which directly inhibit NF-KB activation, would be expected to inhibit this mechanism and inhaled glucocorticoids should be protective. In the future, more specific NF-KB inhibitors may be developed, or effective oral or inhaled anti-oxidant therapies used. The obvious solution to air pollution is to control road traffic through legislative means or to use alternative fuels that do not result in harmful exhaust products.
The outstanding questions • What are the key target cells in the airways for the effects of air pollutants? (Macrophages, epithelial cells, sensory nerves, flbroblasts?) • How do air pollutants increase airway inflammation? (Activation of transcription factors, effects on enzyme activity, direct effects on inflammatory cells?) • Does asthma (and airway inflammation) increase the susceptibility to the inflammatory effects of air pollutants? (By increasing or changing the nature of the inflammatory response?) • Do air pollutants increase the inflammatory response to allergens or viruses in patients with asthma? • Is there interindividual susceptibility to the inflammatory effects of air pollution? (Related to endogenous anti-oxidant defences, dietary anti-oxidants, genetic factors?)
References
10stro, B.D. et al. (1994) Indoor air pollution and asthma, Ant. J. Respir. Crit. Care Med. 149, 1400-1405 2 WorldHealthOrganization(1992) Urban Air Polhaion in Megacities of the World, Blackwell 3 WHO Regional office for Europe (1987) Air Quality Guidelines for Europe, Copenhagen: WHO Regional Publications, Et~ropeanSeries No. 23 4 Department of the Environment (1990) This Common Inheritance. Britain's Eavironmental Strategy, HMSO 5 Wardlaw,AJ. (1993)The roleof air pollutionin asthma,Clin. Exp. Allergy 23, 81-96 6 Barnes,P.J.(1994) Air pollutionand asthma, Postgrad. Med. J. 70, 319-325 7 Dockery,D.W.eta/. (1993)An associationbetween air pollutionand mortalityin six US cities,New EngL J. Med. 329, 1753-1759 8 Braun-Fahrl~inder,C. eta/. (1992) Air pollution and respiratory symptoms in preschool children, Am. Rev. Respir. Dis. 145,42--47
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9 Pope, C.A. and Dockery. D.W. (1992) Acute effects of PM10 pollution on symptomalic and asymplomatic children. Ant. Rev. Respir. Dis. 145, 1123-1128 10 Kurata, J.H.. Grovsky. M.M., Newcomb, R.L. and Easlon, J.C. (1976) Multifactorial study of patients with asthma. 2: Air pollution, animal dander and asthma symptoms./tun. AIh't:~y 37, 398-409 11 Zwick H. et al. ( 1991 ) Effects of ozone on the respiratory health, allergen sensitization and cellular immune system in children, Ant. Rev. Respir. Dis. 144, 1075-1079 12 Hock, G. et al. (1993) Acute effects of ambient ozone on pulmonary function of children in The Netherlands, Ant. Rev. Respir Dis. 147, I ll-117 13 Thurston.G.D. et al. (1994) Respiratory hospital admissions and summer time haze air pollution in Toronto, Ontario: consideration of the role of acid aerosols, Euvirou. Res. 65, 271-290 14 Schwartz, J. eta/. (1993) Particulate air pollution and hospital emergency room visits for asthma in Seattle, Am. Rev. Respir. Dis. 147. 826-83 I 15 Edwards, J., Waiters, S. and Griffiths, R.K. (1994) Hospital admissions for asthma in preschool children: relationship to major roads in Birmingham, United Kingdom, Arch. Euviron. Heohh 49, 223-227 16 Peat, J.K., Salome, C.M. and Woolcock, A..I. (1992) Factors associated with bronchial hyperresponsiveness in Australian adults and children, Eur Respir. J. 5, 921-929 17 Magnussen, H., J6rres, R. and Nowak, D. (1993) Effect of air pollution on the prevalence of asthma and allergy: lessons from the German reunification, Thor~z~" 48,879-881 18 Advisory Group of the Medical Aspects of Air Pollution Episodes (1991) First Report: O=oue, p. 90, London, Department of Health, HMSO 19 Advisory Group of the Medical Aspects of Air Pollution Episodes (1992) Second Report: Sulphur Dioxide. AcM Aerosols attd Particulates, p. 131, London, Department of Health, HMSO 20 Advisory Group of the Medical Aspects of Air Pollution Episodes (1993) Third Report: Oxides of Nitrogen. London. Department of Health, HMSO 21 Linn,W.S. et al. (1987) Replicated dose-response study of sulfur dioxide effects in normal, atopic and asthmatic volunteers, Ant. Rev. Respir. Dis. 126, 1127-1134 22 Koenig, J.Q., Covert, D.S., Hanley,G.B. and Pierson, W.E. (1990) Prior exposure to ozone potentiates subsequent responses to sulfur dioxide in adolescent asthmatic subjects, Ant. Rev. Respir. Dis. 141,377-380 23 Rubenstein,I., Bigby,B.G., Reiss,T.E and Boushey,H.A. (1990) Short-term exposure to 0.3 ppm nitrogen dioxide does not potentiate airway responsiveness to sulphur dioxide in asthmatic subject, Ant. Rev. Respir. Dis. 141, 381-385 24 Devalia, J.L. et al. (1994) Effect of nitrogen dioxide and sulphur dioxide on airway responses of mild asthmatic patients to allergen inhalation, Lancet 344, 1668-1171 25 Samet, J.M. and Utell, M.J. (1990) The risk of nitrogen dioxide: what have we learned from epidemiological and clinical studies? Toxieol. Ind. Health 6, 247-262 26 Hazucha, M.J., Folinsbee, L.J., Seal, E. and Bromberg, P.A. (1994) Lung function response of healthy women after sequential exposures to NO,. and O~, Ant. J. Respir Crit. Care Med. 150, 642--647 27 Tunnicliffe,W.S.,Burge, P.S.and Ayres,J.G. (1994) Effect of domestic concentrations of nitrogen dioxide on airway responses to inhaled allergen in asthmatic patients, Lancet 344, 1733-1736 28 Lippman, M. (1993) Health effects of ozone. A critical review, J. Air Pollut. Control Assoc. 39, 672-695 29 Hatch, G.E. et al. (1994) Ozone dose and effect in humans and rats, Ant. J. Respir. Crit. Care. Med. 150, 676-683 30 Koenig, J.Q. et al. (1987) The effects of ozone and nitrogen dioxide on pulmonary function in healthy and in asthmatic adolescents, Am. Rev. Respir. Dis. 136, 1152-1157 31 Menzel, D.B. (1993) Antioxidant vitamins and prevention of lung disease, Ann. NY Acad. Sci. 669, 141-145 32 Molfino,N.A. et al. ( 1991) Effect of inw concentration of ozone on inhaled allergen responses in asthmatic subjects, Lancet 338, 199-203
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Next month's Round up in MMT The meetings: • Advances in Genetic Diagnostics for Infectious Diseases • Etiology and Pathogenesis of Infectious Diseases
The books • The Oncogene Factsbook • The Oncogene Handbook • CellularCancer Markers
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