- Email: [email protected]
Database for the toxicological evaluation of mixtures in occupational atmospheres
Environmental Toxicology and Pharmacology 18 (2004) 235–242
Database for the toxicological evaluation of mixtures in occupational atmospheres Adolf Vyskocila,∗ , Daniel Droletb , Claude Viaua , Jules Brodeura , Robert Tardifa , Michel G´erina , Marc Barilb , Ginette Truchonb , Gilles Lapointec a
D´epartement de sant´e environnementale et sant´e au travail, Universit´e de Montr´eal, C.P. 6128, succ. Centre-ville, Montr´eal, Qu´e., Canada H3C 3J7 b Institut de recherche Robert-Sauv´ e en sant´e et en s´ecurit´e du travail, Montr´eal, Qu´e., Canada c Le Service du R´ epertoire toxicologique, Commission de la sant´e et de la s´ecurit´e du travail, Montr´eal, Qu´e., Canada Received 10 December 2002; accepted 18 November 2003 Available online 6 October 2004
Abstract Workers are regularly simultaneously exposed to multiple chemical substances. As in the ACGIH (American Conference of Governmental Industrial Hygienists) approach, the Qu´ebec Regulation prescribes that when two or more hazardous substances are present in workplaces and have similar effects on the same organs of the human body, their effects should be considered additive, unless established otherwise. This project was undertaken to develop a user-friendly toxicological database aid in identification of possible interactive effects of mixtures present in the work environment. In the first phase of the project, standard general literature references were used to compile critical data, such as target organs, effects on the target organs, mechanisms of action, and toxicokinetic characteristics of each of the 668 chemical substances appearing in the regulation. Each substance was assigned to one or more of 32 classes of biological effects retained by a group of toxicologists. The resulting database allows the user to find if there is potential additivity among components of a mixture. © 2004 Elsevier B.V. All rights reserved. Keywords: Mixtures; Occupational atmosphere; Database; Interactions
1. Introduction A majority of workers are exposed simultaneously to many chemical contaminants (Burgess, 1995). The health criteria relating to these exposures in workers generally do not take into account the possibility of interactions between these contaminants, which may result in a change in toxicity. However, priority should be given to this factor (NIOSH, 1996). Regarding general environment, both the U.S. Environmental Protection Agency (2000), and the Agency for Toxic Substances and Disease Registry (2001) published guidance documents on this topic in recent years. The central question involves the nature of the possible interactions between these contaminants and their conse∗
Corresponding author. Tel.: +1 514 343 6146; fax: +1 514 343 2200. E-mail address: [email protected] (A. Vyskocil).
1382-6689/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.etap.2003.11.010
quences on the toxicity of a mixture. Is the toxicity simply the sum of the toxic effects of the individual substances at the same level of exposure? Or do the substances in the mixture interact supra-additively, thus producing an effect greater than the sum of the effects of the individual substances? Or does the interaction lead to a reduction in the effects of each of the components in the mixture, producing an antagonistic situation (infra-additivity)? Regulations and common industrial practices address the question of interactions by hypothesizing, by default, that toxic effects are additive. In some cases, this hypothesis may lead to an underestimation or an overestimation of the actual risk (Yang, 1994). As in the ACGIH approach (American Conference of Governmental Industrial Hygienists, 1999), the Qu´ebec “Regulation Respecting the Quality of the Work Environ´ ment” (RRQWE) (Editeur officiel du Qu´ebec, 1999) prescribes that when two or more hazardous substances are
236
A. Vyskocil et al. / Environmental Toxicology and Pharmacology 18 (2004) 235–242
present in the workplace and have similar effects on the same organs of the human body, their effects be considered additive, unless established otherwise. In both cases, the mechanism of toxic action is not taken in account. The exposure index to the substances in a mixture is calculated as follows: Rm =
C1 C2 Cn + + ··· + T1 T2 Tn
where Rm is the sum of the C/T ratios from the individual components of the mixture, C the measured concentration of a substance at a work location, T the permissible timeweighted average exposure value (TWAEV) in accordance with the RRQWE, 1, 2, . . ., n the indices designating the various individual substances in the mixture. If Rm is greater than 1, the TWA permissible value of the mixture of the substances is exceeded. The objectives of this research were (a) to document the biological effects of the chemical contaminants listed in the RRQWE on their target organs; (b) to define what constitutes “similar toxicological effects” and (c) to organize this information in a structured database that can be used in a user-friendly computer tool that indicates potential additivity among components of a mixture and calculates the Rm -value of a given mixture.
2. Methodology This project is conducted in two phases. In the first phase, which is the subject of this paper information on toxicokinetics and target organs involved in the toxicity of all of the chemical substances listed in the RRQWE (current regulation during the conduct of this project) was obtained. The types of effects on these organs and the mechanisms of toxicity were also determined when specified in secondary sources. The target organs were identified in relation to the exposure concentration of the substances in the air, when this information was available. Therefore, in humans, the target organs and effects were determined only for realistic exposure concentrations up to the STEV (where the “Short-Term Exposure Value” is the maximum concentration to which workers can be exposed for a period of 15 min), up to the ceiling value (CV), or when no STEV is prescribed, up to five times the TWAEV (which represents the average concentration of a given chemical to which workers can be exposed for normal 8-h workdays, 5 days a week). This last factor was chosen because, according to the RRQWE, “none of the excursions in exposure levels may exceed five times the time-weighted average exposure value during any length of time whatsoever”. Animal data were used when no human data were available. In this case, the target organs and the effects were determined only for exposure concentrations up to 100 times the TWAEV or the CV [factor of 10 for ex-
trapolation of the LOAEL (lowest observed adverse effect level) towards the NOAEL (no observed adverse effect level) and 10 for the differences between the species]. In some cases, the ACGIH TLV® Committee uses these uncertainty factors to establish the “Threshold Limit Value” (TLV® ) from animal data, when there are no satisfactory human data available, and Quebec regulation is largely based on ACGIH recommendations. The only excursion from this “up to 100 times” rule was applied for the carcinogenic effects, as animal cancer bioasssays often involve exposures to concentrations exceeding 100 times the TWAEV. When available in the secondary literature sources examined, additional information on toxicokinetic, including formation of metabolites (with their CAS number), was included. Currently, toxicokinetic interactions per se are not used in assessing potential additivity. Once the effects of all the substances in the RRQWE had been identified, we determined classes of effects, regrouping effects considered as similar. This concept is necessary, because it constitutes the basis for the additivity rule in calculating the Rm . Information was taken mainly from secondary references. We therefore, consulted the ACGIH’s CD-ROM version of “TLV® and other occupational exposure values, 1999” (American Conference of Governmental Industrial Hygienists, 1999). It contains pertinent toxicological information for establishing threshold limit values (TLV® ), with supporting bibliographical references. We also consulted OSHA’s “Chemical information manual” (Occupational Safety and Health Administration, 1991), as well as “Proctor and Hughes’ chemical hazards of the workplace” (Hathaway et al., 1991), “Toxicologie industrielle et intoxications professionnelles” by Lauwerys (1991), NIOSH’s “Pocket guide to chemical hazards” (Institute National of Occupational Safety and Health, 1997), and the “International Chemical Safety Cards” database (WHO/IPCS/ILO, 1999) on CD. To evaluate carcinogenic properties, we also used data from IARC (International Agency for Research on Cancer, 1987–1999), and from the DFG (German regulation) (Deutsche Forschungsgemeinschaft, 1998). We also consulted the CSST’s Service du r´epertoire toxicologique, a toxicological on line database (http://www.reptox.csst.qc.ca) developed and maintained by the Quebec Occupational Health and Safety Commission (Commission de la sant´e et de la s´ecurit´e du travail, 1999). A Computer program was developed for creating work sheets as well as data sheets for the user. The software used was Microsoft Access version ’97. The database has several tables: names of substances in the RRQWE, their CAS number, and their toxicokinetic characteristics; effects of these substances on the body, target organs or systems; as well as intermediate tables that link the main tables. Once the information was entered in the database, model queries were developed for searching the database.
A. Vyskocil et al. / Environmental Toxicology and Pharmacology 18 (2004) 235–242 Table 1 List of organs and systems
Table 2 List of classes
Autonomic nervous system Bones Cardiac system Central nervous system Embryo or fetus Endocrine system except for the thyroid Entire human body: organs and systems Eyes Female reproductive system Gastrointestinal tract Hematopoietic tissue Immune system Kidney Liver Lower respiratory tract Lymphatic system Male reproductive system Mammary glands Peripheral nervous system Prostate Skin Spleen Teeth Thyroid Upper respiratory tract Urinary tract: other than the kidney Vascular system
Class no.
n
Effect (organ/system)
1
395
Cataracts (eyes) Corneal necrosis (eyes) Eye damage (eyes) Eye irritation (eyes)
2
393
Upper respiratory tract irritation (upper respiratory tract)
3
198
Asthma (lower respiratory tract) Berylliosis (lower respiratory tract) Bronchitis (lower respiratory tract) Bronchopneumonia (lower respiratory tract) Lower respiratory tract irritation (lower respiratory tract) Lung damage (lower respiratory tract) Metal fume fever (lower respiratory tract) Pneumoconiosis (lower respiratory tract) Pulmonary edema (lower respiratory tract) Pulmonary emphysema (lower respiratory tract) Pulmonary fibrosis (lower respiratory tract)
4
71
Anemia (hematopoietic system] Carboxyhemoglobinemia (hematopoietic system) Cytochrome oxydase inhibition (entire human body: organs and systems) Hemolysis (hematopoietic system) Inhibition of heme synthesis (hematopoietic system) Methemoglobinemia (hematopoietic system) Nitrosylhemoglobin formation (hematopoietic system) Reduction in the number of red blood cells (hematopoietic system) Simple asphyxia (entire human body: organs and systems)
5
4
Coagulation problems (hematopoietic system) Increase in the number of platelets (hematopoietic system)
6
7
Leucopenia (hematopoietic system)
7
1
Metabolic acidosis (entire human body: organs and systems)
8
2
Stimulation of basal metabolism (entire human body: organs and systems)
9
4
Antithyroid effect (thyroid)
10
1
Immune system impairment (immune system)
11
114
Hepatic necrosis (liver) Liver damage (liver)
12
1
Spleen damage (spleen)
13
84
14
3
15
26
16
7
Vascular system impairment (vascular system) Vasoconstriction (vascular system)
17
12
Vascular system impairment (vascular system) Vasodilation (vascular system)
By choosing the database as well as its structure as a tool, easy export is possible in an internet-adaptable format. 3. Results and discussion 3.1. Determining the non-carcinogenic effects It is known that for many substances, human data are very limited. It is therefore, not surprising that the mechanism of toxicity for many substances is rarely described in the secondary references. Very often, we found only general information, of the type “liver damage” and “CNS impairment”. Since one of our specific objectives was to determine the classes of similar effects from the list of selected effects, more general effects (e.g., damage to the lower respiratory tract) had to be combined with more specific effects (e.g., pulmonary edema), since we could not disregard any possibility of interaction between two substances for the simple reason that we do not currently know a given substance’s specific effect on an organ. We retained the general effects when the unique, too specific or rather rare specific effect rendered the information of little use for the purpose of this work. However, this detailed information, when available, was included in the data sheets of substances as justification for the effect retained. There are two effects, where we found sufficient information to be able to use a more detailed classification: effects on the respiratory tract and on the hematopoeitic system. Tables 1 and 2 present the lists of organs and effects.
237
Glomerular damage (kidney) Kidney damage (kidney) Tubular damage (kidney) Gastrointestinal damage (gastrointestinal system) Cardiac system impairment (cardiac system)
238
A. Vyskocil et al. / Environmental Toxicology and Pharmacology 18 (2004) 235–242
3.2. Determining the carcinogenic effects
Table 2 (Continued ) Class no.
n
Effect (organ/system)
18
37
Autonomic nervous system impairment (autonomic nervous system) Cholinesterase inhibition (autonomic nervous system)
19
130
Central nervous system convulsion (central nervous system) Central nervous system depression (central nervous system) Central nervous system impairment (central nervous system)
20
19
Peripheral nervous system impairment (peripheral nervous system) Peripheral neuropathy (peripheral nervous system)
21
2
Auditory nerve impairment (central nervous system) Cochlear impairment (central nervous system) Vestibular impairment (central nervous system)
22
1
Muscular stimulation (entire human body: organs and systems)
23
9
Bone damage (bones)
24
1
Dental fluorosis (teeth)
25
2
Dental erosion (teeth)
26
2
Argyria (skin)
27
274
28
26
Damage to the male reproductive system (male reproductive system) Testicular damage (male reproductive system)
29
6
Damage to the female reproductive system (excluding teratogenic effects and embryonic and fetal damage (female reproductive system)
30
7
Embryonic and fetal damage (embryo or fetus)
31
16
32
191
Chloracne (skin) Contact dermatitis (skin) Skin damage (skin) Skin irritation (skin) Skin sensitization (skin)
Teratogenic effect (embryo or fetus) Bladder cancer (urinary tract: other than kidney) Cancer (entire human body: organs and systems) Cancer nasal (upper respiratory tract) Lung cancer (lower respiratory tract) Cancer of the larynx (upper respiratory tract) Cancer of the upper respiratory tract (upper respiratory tract) Leukemia (hematopoietic system) Liver cancer (liver) Mutagenic effect (entire human body: organs and systems) Prostate cancer (prostate) Sino-nasal cancer (upper respiratory tract) Skin cancer (skin) Testicular cancer (male reproductive system)
n: number of substances in the class.
As a basis for carcinogen classification, we used information from the RRQWE, ACGIH and IARC. The RRQWE was chosen because, this database applies to the Qu´ebec standards. The ACGIH’s recommendations were chosen because, this organization is recognized as reliable and is respected by occupational health practitioners around the world. IARC’s classifications were also retained, because it is a renowned international organization. In the majority of cases, ACGIH and IARC have similar classifications for carcinogenic effects. Recognition of a substance as carcinogenic in humans and animals by any of these three organizations is sufficient for it to be considered as such in this work. 3.3. Determining classes of similar effects As seen in Table 2, the effects retained do not all have the same degree of specificity, and the list contains some rather specific effects as well as some rather general ones. The general effects often encompass effects that are more specific. As a precaution, the definition of the similarity of effects in the organs must, therefore be broad enough to cover, e.g., a possible interaction between a substance for which detailed human toxicological data are available, and another substance for which even animal toxicological data are scarce and for which the TLV® was estimated by ACGIH by physicochemical analogy with another substance (American Conference of Governmental Industrial Hygienists, 1999). The limitations of this latter approach must be recognized and the Documentation on TLV® should be carefully examined to verify how the similarity of the physicochemical characteristics was established. This is why, in many cases, we have defined all effects on an organ or a system as being similar, irrespective of the degree of specificity of this effect. We therefore, determined 32 classes of similar effects for the substances in the RRQWE (Table 2). Among these 32 classes, 16 list only one and often very general effect (e.g., “Embryonic and fetal damage”). Class no. 2 includes chemical and physical irritants. Alarie et al. (2000) proposed a principle of additivity for sensory irritations. Respiratory mucosa exposed to an initial irritant will likely suffer more damage if they are also exposed to a second irritant, regardless of the nature of the irritation. We came to the following conclusions about the additivity of effects caused by sensitizers on the lung (class no. 3): all sensitizers should not be considered as additive. For asthma resulting from a sensitization mechanism, we in fact, do not know whether being exposed to two or three sensitizing substances at the same time (e.g., an automobile painter exposed to isocyanates, acrylates and amines) increases the risk of sensitization to any one of them. In the case of these allergic sensitization mechanisms, the specificity of the immune mechanisms involved generally suggests that such additivity is unlikely in most cases. However, the effects of irritants and sensitizers should be considered as potentially additive. There
A. Vyskocil et al. / Environmental Toxicology and Pharmacology 18 (2004) 235–242
is a potentiating effect of irritating substances on the reaction to a sensitizing agent. For example, exposure to ozone potentiates the reactivity to a sensitizer, such as pollen or mold (Krieger et al., 1998). Class no. 4 includes effects that disrupt oxygen transport from the lungs to the tissues. These effects include the reduction in the oxygen concentration in the air (simple asphyxiation), the reduction in the number of red blood cells transporting oxygen in the blood (anemia, the reduction in the number of red blood cells, the inhibition of heme synthesis, hemolysis), the disruption of oxygen transport by red blood cells (carboxyhemoglobinemia, the formation of nitrosylhemoglobin, methemoglobinemia), and the use of oxygen in the tissues (inhibition of cytochrome oxydase). The conclusion is that none of the effects in this class is independent of the other effects in it. Two effects on the vascular system, vasoconstriction and vasodilatation, are antagonistic. This is why they were divided into two different classes (class nos. 16 and 17). However, we classified them individually with the general effect “Vascular system impairment”, because it is impossible to conclude that each of the effects in this class is independent of the other effects that make up the class. From a purely mechanistic standpoint, convulsive effects and depressor effects are antagonistic. However, from a clinical standpoint, a given substance may have a sequence of effects that include convulsion and depression (Lauwerys, 1991). It is impossible to conclude that each of the effects in class no. 19 is independent of the other effects that make up this class. Similar to the lung sensitizers, skin sensitizers (class no. 27) should not be considered as additive. In fact, sensitizers act through specific immune reactions that should not be considered as additive. However, if the chemical structures of two sensitizers are very similar, cross-reaction may occur in which, e.g., lymphocytes sensitized to one allergen also react with the second. Furthermore, it has been noted empirically that some allergens often pair in an unexpected way, e.g., nickel and cobalt (Lauwerys, 1991). The possibility of interactions between sensitizers in some given mixtures must be dealt with individually. Skin irritants and sensitizers should be considered as additive. In fact, skin on which an immune reaction has occurred will show characteristics of inflammation. If an irritant that acts with an inflammatory mechanism is added to the same area, there will be additivity. Conversely, skin where the integrity of the horny layer has been disrupted by an irritant will allow a sensitizer better penetration. Class no. 32 contains all the carcinogenic and mutagenic effects. This is because, for the majority of carcinogens, information is lacking on the human organs affected, since data on their carcinogenicity come from animal studies. Human cancer sites are well established for only 23 substances in the RRQWE. It is difficult to predict which organs may be affected in humans based on results of animal studies. In the majority of cases, tumors are observed in multiple sites in
239
animals. When multiple sites exist, we simply identified the generic effects as cancer. Further attention must be paid to the case of carcinogens. First, there is a general principle whereby exposure to a carcinogen must be kept to a minimum (often referred to as the “As Low As Reasonably Achievable” (ALARA) principle). Article 5.2 of the RRQWE specifies, in fact that “such exposure shall be reduced to a minimum, even when it remains within the standards provided under this schedule”. Deductively, it therefore appears contrary to the spirit of the regulation to calculate an Rm for a mixture of carcinogenic substances, since the principle that must prevail is that of the lowest exposure possible. Furthermore, when a substance is a known human carcinogen, the site of carcinogenic action has not always been clearly identified. Finally, a known carcinogen for a given anatomic site in animals may be carcinogenic for a different site in humans. The additional problem that is raised, involves additivity between the carcinogenic and non-carcinogenic effects on a given organ or system. The calculation of an Rm in this case could suggest that the concentrations of any of the substances could be reduced in the workplace in order to reach an Rm value of less than 1, and thus comply with the regulation. Specific efforts must, however, be made to reduce exposure to carcinogens. For all these reasons, we do not recommend including carcinogenic substances in the calculation of Rm . 3.4. Substances with similar effects Table 2 presents the number of substances in each class having similar effects. We observe that nine classes contain only one or two substances. However, class no. 27 contains 207 for skin irritation, class no. 1 contains 395 for eye irritation, and class no. 2 contains 393 for upper respiratory tract irritation. The majority of the ACGIH’s TLV® s (American Conference of Governmental Industrial Hygienists, 1999) are based on irritant effects (for 401 of the 679 substances). We had to take into consideration all of these irritant effects, even in cases where information on the exposure level was missing. This means that for many mixtures, the possibility of interaction due to irritant effects is very real. 3.5. Toxicity data sheets on substances in the RRQWE and computer application The information entered in the different tables in the database was organized to create a separate data sheet for each substance. Fig. 1 gives an example of the data sheet for toluene. The database contains the 668 data sheets that correspond to all of the entries in the current RRQWE. It shows the pertinence of developing a computer application for searching for substances in a drop-down list and particularly the cross-analysis of each of the substances queried to help determine whether the additivity rule should apply or not.
240
A. Vyskocil et al. / Environmental Toxicology and Pharmacology 18 (2004) 235–242
Fig. 1. Data sheets of toluene and xylene.
The application, which was developed in Java Script language, essentially consists of a series of HTML pages. The main page is the search form, which is presented in Fig. 2. There are ten drop-down zones on this page, and from each, a substance in the RRQWE can be selected. Once a selection has been made, the application shows the TWA level (or the ceiling value if applicable) as well as the list of classes for each of the substances. After receiving comments from potential field-users, we added an input zone for entering the exposure value measured in the workplace. In this way, the application not only determines, whether the additivity rule should apply or not, but also calculates all the possible combinations of Rm in relation to the chosen substances. Hyperlinks also provide very easy access to the list of
classes as well as to the individual toxicity data sheets for the substances. For illustration, three members of volatile organic chemicals, which can be found in occupational chemical mixtures, were selected: toluene, xylene, and n-hexane. We supposed that their measured air concentrations were 200, 250, and 100 mg/m3 , respectively. Fig. 2 shows that for this mixture, application indicates 3 common classes for toluene and xylene (class no. 1: eye impairment, class no. 2: upper respiratory tract irritation, and class no. 19: central nervous system impairment). Data sheets for both toluene and xylene (Fig. 1) show that eye irritation is involved in class 1 and central nervous system depression in class 19. n-hexane has no common class with either toluene or xylene (for the effects involved in
A. Vyskocil et al. / Environmental Toxicology and Pharmacology 18 (2004) 235–242
241
Fig. 2. Search page.
each class; Table 2). So, Rm formula is used only for toluene and xylene. In our case, the Rm index greater than 100% indicates that the TWA permissible value of the mixture of toluene and xylene is exceeded.
4. Conclusion Since, relevant human toxicological data are rare, it is not surprising that the procedures adopted by different countries to define the permissible exposure values (PEV) differ. This is why, we took the toxicological data from several secondary references, despite the fact that the majority of Qu´ebec PEVs are based on the ACGIH’s TLV® s. In this first phase of the project, it is impossible in most cases to anticipate the type of interaction (additivity, supraadditivity, and infra-additivity) for mixtures of substances
causing similar effects, either because these effects are not well-enough known or because the toxic action mechanisms remain unknown. In this phase, the thousands of primary literature sources available for almost 700 regulated substances were not scrutinized. This is why, using the precautionary principle recommended by the ACGIH (“in the absence of information to the contrary, the effects on the different hazards should be considered as additive”), we are considering all the effects found in the classes of similar effects as additive. The computer utility from this first phase should serve as an initial approach in estimating the possibility of addivity or interaction, which, in the case of similar effects, should be further investigated. In the second phase of this project, we will specify the type of interaction for mixtures most likely to be found in the occupational environment and for which primary literature data are avail-
242
A. Vyskocil et al. / Environmental Toxicology and Pharmacology 18 (2004) 235–242
able. Computer utility will include the results from the two phases of the project. Presently, the database is available in French version only on the web and can be accessed from http://www.irsst.qc.ca/en/ut intertox index.htm. The English version will be available shortly. Acknowledgements This study was supported by the Institut de recherche Robert-Sauv´e en sant´e et en s´ecurit´e du travail (Qu´ebec, Canada). References Agency for Toxic Substances and Disease Registry, 2001. Guidance Manual for the Assessment of Joint Toxic Action of Chemical Mixtures. Division of Toxicology, ATSDR, Atlanta, GA. Alarie, Y., Nielsen, G.D., Schaper, M., 2000. Animal bioassays for evaluation of indoor air quality. In: Spengler, J.D., Samet, J.M., McCarthy, J.F. (Eds.), Indoor Air Quality Handbook. McGraw-Hill, New York. American Conference of Governmental Industrial Hygienists, 1999. TLV® s and Other Occupational Exposure Values. CD ROM, ACGIH, Cincinnati. Burgess, W.A., 1995. Recognition of health hazards in industry. In: A Review of Materials and Processes, second ed. Wiley, New York. Commission de la sant´e et de la s´ecurit´e du travail, 1999. Fiches de Renseignements. Service du R´epertoire Toxicologique. CSST, Montr´eal, Canada.
Deutsche Forschungsgemeinschaft, 1998. MAK- and BAT-Values 1998. Weinheim, Deutsche Forschungsgemeinschaft. ´ Editeur officiel du Qu´ebec, 1999. R`eglement sur la Qualit´e du Milieu de Travail. Qu´ebec. Hathaway, G.J., Proctor, N.H., Hughes, J.P., et al., 1991. Proctor and Hughes’ Chemical Hazards of the Workplace, third ed. Van Nostrand Reinhold, New York. International Agency for Research on Cancer, 1987–1999. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, vol. 1–71, IARC, Lyon. Krieger, P., Deblay, F., Pauli, G., et al., 1998. Asthma and domestic chemical pollutants (excluding tobacco). Rev. Mal. Respir. 15, 11. Lauwerys, R., 1991. Toxicologie Industrielle et Intoxications Professionnelles, third ed. Masson, Paris. National Institute for Occupational Safety and Health, 1997. Pocket Guide to Chemical Hazards. NIOSH, Cincinnati, OH. National Institute of Occupational Safety and Health, 1996. National Occupational Research Agenda, Mixed Exposures. NIOSH, Cincinnati, OH. (http://www.cdc.gov/niosh/worken.html#mixeds). Occupational Safety and Health Administration, 1991 Chemical information manual. OSHA Instruction CPL 2-2.43A. OSHA, Department of Labor, Washington, DC. U.S. Environmental Protection Agency, 2000. Supplementary Guidance for Conducting Health Risk Assessment of Chemical Mixtures. Risk Assessment Forum. EPA/630/R-00/002, EPA, Washington, DC. World Health Organization/International Programme on Chemical Safety, 1999. International Chemistry Safety Cards Database System, version 1.13, PrettyBit Software Ltd., Tampere. Yang, R.S.H., 1994. Toxicology of Chemical Mixtures: Case Studies, Mechanisms and Novel Approaches. Academic Press, San Diego.
Database for the toxicological evaluation of mixtures in occupational atmospheres Adolf Vyskocila,∗ , Daniel Droletb , Claude Viaua , Jules Brodeura , Robert Tardifa , Michel G´erina , Marc Barilb , Ginette Truchonb , Gilles Lapointec a
D´epartement de sant´e environnementale et sant´e au travail, Universit´e de Montr´eal, C.P. 6128, succ. Centre-ville, Montr´eal, Qu´e., Canada H3C 3J7 b Institut de recherche Robert-Sauv´ e en sant´e et en s´ecurit´e du travail, Montr´eal, Qu´e., Canada c Le Service du R´ epertoire toxicologique, Commission de la sant´e et de la s´ecurit´e du travail, Montr´eal, Qu´e., Canada Received 10 December 2002; accepted 18 November 2003 Available online 6 October 2004
Abstract Workers are regularly simultaneously exposed to multiple chemical substances. As in the ACGIH (American Conference of Governmental Industrial Hygienists) approach, the Qu´ebec Regulation prescribes that when two or more hazardous substances are present in workplaces and have similar effects on the same organs of the human body, their effects should be considered additive, unless established otherwise. This project was undertaken to develop a user-friendly toxicological database aid in identification of possible interactive effects of mixtures present in the work environment. In the first phase of the project, standard general literature references were used to compile critical data, such as target organs, effects on the target organs, mechanisms of action, and toxicokinetic characteristics of each of the 668 chemical substances appearing in the regulation. Each substance was assigned to one or more of 32 classes of biological effects retained by a group of toxicologists. The resulting database allows the user to find if there is potential additivity among components of a mixture. © 2004 Elsevier B.V. All rights reserved. Keywords: Mixtures; Occupational atmosphere; Database; Interactions
1. Introduction A majority of workers are exposed simultaneously to many chemical contaminants (Burgess, 1995). The health criteria relating to these exposures in workers generally do not take into account the possibility of interactions between these contaminants, which may result in a change in toxicity. However, priority should be given to this factor (NIOSH, 1996). Regarding general environment, both the U.S. Environmental Protection Agency (2000), and the Agency for Toxic Substances and Disease Registry (2001) published guidance documents on this topic in recent years. The central question involves the nature of the possible interactions between these contaminants and their conse∗
Corresponding author. Tel.: +1 514 343 6146; fax: +1 514 343 2200. E-mail address: [email protected] (A. Vyskocil).
1382-6689/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.etap.2003.11.010
quences on the toxicity of a mixture. Is the toxicity simply the sum of the toxic effects of the individual substances at the same level of exposure? Or do the substances in the mixture interact supra-additively, thus producing an effect greater than the sum of the effects of the individual substances? Or does the interaction lead to a reduction in the effects of each of the components in the mixture, producing an antagonistic situation (infra-additivity)? Regulations and common industrial practices address the question of interactions by hypothesizing, by default, that toxic effects are additive. In some cases, this hypothesis may lead to an underestimation or an overestimation of the actual risk (Yang, 1994). As in the ACGIH approach (American Conference of Governmental Industrial Hygienists, 1999), the Qu´ebec “Regulation Respecting the Quality of the Work Environ´ ment” (RRQWE) (Editeur officiel du Qu´ebec, 1999) prescribes that when two or more hazardous substances are
236
A. Vyskocil et al. / Environmental Toxicology and Pharmacology 18 (2004) 235–242
present in the workplace and have similar effects on the same organs of the human body, their effects be considered additive, unless established otherwise. In both cases, the mechanism of toxic action is not taken in account. The exposure index to the substances in a mixture is calculated as follows: Rm =
C1 C2 Cn + + ··· + T1 T2 Tn
where Rm is the sum of the C/T ratios from the individual components of the mixture, C the measured concentration of a substance at a work location, T the permissible timeweighted average exposure value (TWAEV) in accordance with the RRQWE, 1, 2, . . ., n the indices designating the various individual substances in the mixture. If Rm is greater than 1, the TWA permissible value of the mixture of the substances is exceeded. The objectives of this research were (a) to document the biological effects of the chemical contaminants listed in the RRQWE on their target organs; (b) to define what constitutes “similar toxicological effects” and (c) to organize this information in a structured database that can be used in a user-friendly computer tool that indicates potential additivity among components of a mixture and calculates the Rm -value of a given mixture.
2. Methodology This project is conducted in two phases. In the first phase, which is the subject of this paper information on toxicokinetics and target organs involved in the toxicity of all of the chemical substances listed in the RRQWE (current regulation during the conduct of this project) was obtained. The types of effects on these organs and the mechanisms of toxicity were also determined when specified in secondary sources. The target organs were identified in relation to the exposure concentration of the substances in the air, when this information was available. Therefore, in humans, the target organs and effects were determined only for realistic exposure concentrations up to the STEV (where the “Short-Term Exposure Value” is the maximum concentration to which workers can be exposed for a period of 15 min), up to the ceiling value (CV), or when no STEV is prescribed, up to five times the TWAEV (which represents the average concentration of a given chemical to which workers can be exposed for normal 8-h workdays, 5 days a week). This last factor was chosen because, according to the RRQWE, “none of the excursions in exposure levels may exceed five times the time-weighted average exposure value during any length of time whatsoever”. Animal data were used when no human data were available. In this case, the target organs and the effects were determined only for exposure concentrations up to 100 times the TWAEV or the CV [factor of 10 for ex-
trapolation of the LOAEL (lowest observed adverse effect level) towards the NOAEL (no observed adverse effect level) and 10 for the differences between the species]. In some cases, the ACGIH TLV® Committee uses these uncertainty factors to establish the “Threshold Limit Value” (TLV® ) from animal data, when there are no satisfactory human data available, and Quebec regulation is largely based on ACGIH recommendations. The only excursion from this “up to 100 times” rule was applied for the carcinogenic effects, as animal cancer bioasssays often involve exposures to concentrations exceeding 100 times the TWAEV. When available in the secondary literature sources examined, additional information on toxicokinetic, including formation of metabolites (with their CAS number), was included. Currently, toxicokinetic interactions per se are not used in assessing potential additivity. Once the effects of all the substances in the RRQWE had been identified, we determined classes of effects, regrouping effects considered as similar. This concept is necessary, because it constitutes the basis for the additivity rule in calculating the Rm . Information was taken mainly from secondary references. We therefore, consulted the ACGIH’s CD-ROM version of “TLV® and other occupational exposure values, 1999” (American Conference of Governmental Industrial Hygienists, 1999). It contains pertinent toxicological information for establishing threshold limit values (TLV® ), with supporting bibliographical references. We also consulted OSHA’s “Chemical information manual” (Occupational Safety and Health Administration, 1991), as well as “Proctor and Hughes’ chemical hazards of the workplace” (Hathaway et al., 1991), “Toxicologie industrielle et intoxications professionnelles” by Lauwerys (1991), NIOSH’s “Pocket guide to chemical hazards” (Institute National of Occupational Safety and Health, 1997), and the “International Chemical Safety Cards” database (WHO/IPCS/ILO, 1999) on CD. To evaluate carcinogenic properties, we also used data from IARC (International Agency for Research on Cancer, 1987–1999), and from the DFG (German regulation) (Deutsche Forschungsgemeinschaft, 1998). We also consulted the CSST’s Service du r´epertoire toxicologique, a toxicological on line database (http://www.reptox.csst.qc.ca) developed and maintained by the Quebec Occupational Health and Safety Commission (Commission de la sant´e et de la s´ecurit´e du travail, 1999). A Computer program was developed for creating work sheets as well as data sheets for the user. The software used was Microsoft Access version ’97. The database has several tables: names of substances in the RRQWE, their CAS number, and their toxicokinetic characteristics; effects of these substances on the body, target organs or systems; as well as intermediate tables that link the main tables. Once the information was entered in the database, model queries were developed for searching the database.
A. Vyskocil et al. / Environmental Toxicology and Pharmacology 18 (2004) 235–242 Table 1 List of organs and systems
Table 2 List of classes
Autonomic nervous system Bones Cardiac system Central nervous system Embryo or fetus Endocrine system except for the thyroid Entire human body: organs and systems Eyes Female reproductive system Gastrointestinal tract Hematopoietic tissue Immune system Kidney Liver Lower respiratory tract Lymphatic system Male reproductive system Mammary glands Peripheral nervous system Prostate Skin Spleen Teeth Thyroid Upper respiratory tract Urinary tract: other than the kidney Vascular system
Class no.
n
Effect (organ/system)
1
395
Cataracts (eyes) Corneal necrosis (eyes) Eye damage (eyes) Eye irritation (eyes)
2
393
Upper respiratory tract irritation (upper respiratory tract)
3
198
Asthma (lower respiratory tract) Berylliosis (lower respiratory tract) Bronchitis (lower respiratory tract) Bronchopneumonia (lower respiratory tract) Lower respiratory tract irritation (lower respiratory tract) Lung damage (lower respiratory tract) Metal fume fever (lower respiratory tract) Pneumoconiosis (lower respiratory tract) Pulmonary edema (lower respiratory tract) Pulmonary emphysema (lower respiratory tract) Pulmonary fibrosis (lower respiratory tract)
4
71
Anemia (hematopoietic system] Carboxyhemoglobinemia (hematopoietic system) Cytochrome oxydase inhibition (entire human body: organs and systems) Hemolysis (hematopoietic system) Inhibition of heme synthesis (hematopoietic system) Methemoglobinemia (hematopoietic system) Nitrosylhemoglobin formation (hematopoietic system) Reduction in the number of red blood cells (hematopoietic system) Simple asphyxia (entire human body: organs and systems)
5
4
Coagulation problems (hematopoietic system) Increase in the number of platelets (hematopoietic system)
6
7
Leucopenia (hematopoietic system)
7
1
Metabolic acidosis (entire human body: organs and systems)
8
2
Stimulation of basal metabolism (entire human body: organs and systems)
9
4
Antithyroid effect (thyroid)
10
1
Immune system impairment (immune system)
11
114
Hepatic necrosis (liver) Liver damage (liver)
12
1
Spleen damage (spleen)
13
84
14
3
15
26
16
7
Vascular system impairment (vascular system) Vasoconstriction (vascular system)
17
12
Vascular system impairment (vascular system) Vasodilation (vascular system)
By choosing the database as well as its structure as a tool, easy export is possible in an internet-adaptable format. 3. Results and discussion 3.1. Determining the non-carcinogenic effects It is known that for many substances, human data are very limited. It is therefore, not surprising that the mechanism of toxicity for many substances is rarely described in the secondary references. Very often, we found only general information, of the type “liver damage” and “CNS impairment”. Since one of our specific objectives was to determine the classes of similar effects from the list of selected effects, more general effects (e.g., damage to the lower respiratory tract) had to be combined with more specific effects (e.g., pulmonary edema), since we could not disregard any possibility of interaction between two substances for the simple reason that we do not currently know a given substance’s specific effect on an organ. We retained the general effects when the unique, too specific or rather rare specific effect rendered the information of little use for the purpose of this work. However, this detailed information, when available, was included in the data sheets of substances as justification for the effect retained. There are two effects, where we found sufficient information to be able to use a more detailed classification: effects on the respiratory tract and on the hematopoeitic system. Tables 1 and 2 present the lists of organs and effects.
237
Glomerular damage (kidney) Kidney damage (kidney) Tubular damage (kidney) Gastrointestinal damage (gastrointestinal system) Cardiac system impairment (cardiac system)
238
A. Vyskocil et al. / Environmental Toxicology and Pharmacology 18 (2004) 235–242
3.2. Determining the carcinogenic effects
Table 2 (Continued ) Class no.
n
Effect (organ/system)
18
37
Autonomic nervous system impairment (autonomic nervous system) Cholinesterase inhibition (autonomic nervous system)
19
130
Central nervous system convulsion (central nervous system) Central nervous system depression (central nervous system) Central nervous system impairment (central nervous system)
20
19
Peripheral nervous system impairment (peripheral nervous system) Peripheral neuropathy (peripheral nervous system)
21
2
Auditory nerve impairment (central nervous system) Cochlear impairment (central nervous system) Vestibular impairment (central nervous system)
22
1
Muscular stimulation (entire human body: organs and systems)
23
9
Bone damage (bones)
24
1
Dental fluorosis (teeth)
25
2
Dental erosion (teeth)
26
2
Argyria (skin)
27
274
28
26
Damage to the male reproductive system (male reproductive system) Testicular damage (male reproductive system)
29
6
Damage to the female reproductive system (excluding teratogenic effects and embryonic and fetal damage (female reproductive system)
30
7
Embryonic and fetal damage (embryo or fetus)
31
16
32
191
Chloracne (skin) Contact dermatitis (skin) Skin damage (skin) Skin irritation (skin) Skin sensitization (skin)
Teratogenic effect (embryo or fetus) Bladder cancer (urinary tract: other than kidney) Cancer (entire human body: organs and systems) Cancer nasal (upper respiratory tract) Lung cancer (lower respiratory tract) Cancer of the larynx (upper respiratory tract) Cancer of the upper respiratory tract (upper respiratory tract) Leukemia (hematopoietic system) Liver cancer (liver) Mutagenic effect (entire human body: organs and systems) Prostate cancer (prostate) Sino-nasal cancer (upper respiratory tract) Skin cancer (skin) Testicular cancer (male reproductive system)
n: number of substances in the class.
As a basis for carcinogen classification, we used information from the RRQWE, ACGIH and IARC. The RRQWE was chosen because, this database applies to the Qu´ebec standards. The ACGIH’s recommendations were chosen because, this organization is recognized as reliable and is respected by occupational health practitioners around the world. IARC’s classifications were also retained, because it is a renowned international organization. In the majority of cases, ACGIH and IARC have similar classifications for carcinogenic effects. Recognition of a substance as carcinogenic in humans and animals by any of these three organizations is sufficient for it to be considered as such in this work. 3.3. Determining classes of similar effects As seen in Table 2, the effects retained do not all have the same degree of specificity, and the list contains some rather specific effects as well as some rather general ones. The general effects often encompass effects that are more specific. As a precaution, the definition of the similarity of effects in the organs must, therefore be broad enough to cover, e.g., a possible interaction between a substance for which detailed human toxicological data are available, and another substance for which even animal toxicological data are scarce and for which the TLV® was estimated by ACGIH by physicochemical analogy with another substance (American Conference of Governmental Industrial Hygienists, 1999). The limitations of this latter approach must be recognized and the Documentation on TLV® should be carefully examined to verify how the similarity of the physicochemical characteristics was established. This is why, in many cases, we have defined all effects on an organ or a system as being similar, irrespective of the degree of specificity of this effect. We therefore, determined 32 classes of similar effects for the substances in the RRQWE (Table 2). Among these 32 classes, 16 list only one and often very general effect (e.g., “Embryonic and fetal damage”). Class no. 2 includes chemical and physical irritants. Alarie et al. (2000) proposed a principle of additivity for sensory irritations. Respiratory mucosa exposed to an initial irritant will likely suffer more damage if they are also exposed to a second irritant, regardless of the nature of the irritation. We came to the following conclusions about the additivity of effects caused by sensitizers on the lung (class no. 3): all sensitizers should not be considered as additive. For asthma resulting from a sensitization mechanism, we in fact, do not know whether being exposed to two or three sensitizing substances at the same time (e.g., an automobile painter exposed to isocyanates, acrylates and amines) increases the risk of sensitization to any one of them. In the case of these allergic sensitization mechanisms, the specificity of the immune mechanisms involved generally suggests that such additivity is unlikely in most cases. However, the effects of irritants and sensitizers should be considered as potentially additive. There
A. Vyskocil et al. / Environmental Toxicology and Pharmacology 18 (2004) 235–242
is a potentiating effect of irritating substances on the reaction to a sensitizing agent. For example, exposure to ozone potentiates the reactivity to a sensitizer, such as pollen or mold (Krieger et al., 1998). Class no. 4 includes effects that disrupt oxygen transport from the lungs to the tissues. These effects include the reduction in the oxygen concentration in the air (simple asphyxiation), the reduction in the number of red blood cells transporting oxygen in the blood (anemia, the reduction in the number of red blood cells, the inhibition of heme synthesis, hemolysis), the disruption of oxygen transport by red blood cells (carboxyhemoglobinemia, the formation of nitrosylhemoglobin, methemoglobinemia), and the use of oxygen in the tissues (inhibition of cytochrome oxydase). The conclusion is that none of the effects in this class is independent of the other effects in it. Two effects on the vascular system, vasoconstriction and vasodilatation, are antagonistic. This is why they were divided into two different classes (class nos. 16 and 17). However, we classified them individually with the general effect “Vascular system impairment”, because it is impossible to conclude that each of the effects in this class is independent of the other effects that make up the class. From a purely mechanistic standpoint, convulsive effects and depressor effects are antagonistic. However, from a clinical standpoint, a given substance may have a sequence of effects that include convulsion and depression (Lauwerys, 1991). It is impossible to conclude that each of the effects in class no. 19 is independent of the other effects that make up this class. Similar to the lung sensitizers, skin sensitizers (class no. 27) should not be considered as additive. In fact, sensitizers act through specific immune reactions that should not be considered as additive. However, if the chemical structures of two sensitizers are very similar, cross-reaction may occur in which, e.g., lymphocytes sensitized to one allergen also react with the second. Furthermore, it has been noted empirically that some allergens often pair in an unexpected way, e.g., nickel and cobalt (Lauwerys, 1991). The possibility of interactions between sensitizers in some given mixtures must be dealt with individually. Skin irritants and sensitizers should be considered as additive. In fact, skin on which an immune reaction has occurred will show characteristics of inflammation. If an irritant that acts with an inflammatory mechanism is added to the same area, there will be additivity. Conversely, skin where the integrity of the horny layer has been disrupted by an irritant will allow a sensitizer better penetration. Class no. 32 contains all the carcinogenic and mutagenic effects. This is because, for the majority of carcinogens, information is lacking on the human organs affected, since data on their carcinogenicity come from animal studies. Human cancer sites are well established for only 23 substances in the RRQWE. It is difficult to predict which organs may be affected in humans based on results of animal studies. In the majority of cases, tumors are observed in multiple sites in
239
animals. When multiple sites exist, we simply identified the generic effects as cancer. Further attention must be paid to the case of carcinogens. First, there is a general principle whereby exposure to a carcinogen must be kept to a minimum (often referred to as the “As Low As Reasonably Achievable” (ALARA) principle). Article 5.2 of the RRQWE specifies, in fact that “such exposure shall be reduced to a minimum, even when it remains within the standards provided under this schedule”. Deductively, it therefore appears contrary to the spirit of the regulation to calculate an Rm for a mixture of carcinogenic substances, since the principle that must prevail is that of the lowest exposure possible. Furthermore, when a substance is a known human carcinogen, the site of carcinogenic action has not always been clearly identified. Finally, a known carcinogen for a given anatomic site in animals may be carcinogenic for a different site in humans. The additional problem that is raised, involves additivity between the carcinogenic and non-carcinogenic effects on a given organ or system. The calculation of an Rm in this case could suggest that the concentrations of any of the substances could be reduced in the workplace in order to reach an Rm value of less than 1, and thus comply with the regulation. Specific efforts must, however, be made to reduce exposure to carcinogens. For all these reasons, we do not recommend including carcinogenic substances in the calculation of Rm . 3.4. Substances with similar effects Table 2 presents the number of substances in each class having similar effects. We observe that nine classes contain only one or two substances. However, class no. 27 contains 207 for skin irritation, class no. 1 contains 395 for eye irritation, and class no. 2 contains 393 for upper respiratory tract irritation. The majority of the ACGIH’s TLV® s (American Conference of Governmental Industrial Hygienists, 1999) are based on irritant effects (for 401 of the 679 substances). We had to take into consideration all of these irritant effects, even in cases where information on the exposure level was missing. This means that for many mixtures, the possibility of interaction due to irritant effects is very real. 3.5. Toxicity data sheets on substances in the RRQWE and computer application The information entered in the different tables in the database was organized to create a separate data sheet for each substance. Fig. 1 gives an example of the data sheet for toluene. The database contains the 668 data sheets that correspond to all of the entries in the current RRQWE. It shows the pertinence of developing a computer application for searching for substances in a drop-down list and particularly the cross-analysis of each of the substances queried to help determine whether the additivity rule should apply or not.
240
A. Vyskocil et al. / Environmental Toxicology and Pharmacology 18 (2004) 235–242
Fig. 1. Data sheets of toluene and xylene.
The application, which was developed in Java Script language, essentially consists of a series of HTML pages. The main page is the search form, which is presented in Fig. 2. There are ten drop-down zones on this page, and from each, a substance in the RRQWE can be selected. Once a selection has been made, the application shows the TWA level (or the ceiling value if applicable) as well as the list of classes for each of the substances. After receiving comments from potential field-users, we added an input zone for entering the exposure value measured in the workplace. In this way, the application not only determines, whether the additivity rule should apply or not, but also calculates all the possible combinations of Rm in relation to the chosen substances. Hyperlinks also provide very easy access to the list of
classes as well as to the individual toxicity data sheets for the substances. For illustration, three members of volatile organic chemicals, which can be found in occupational chemical mixtures, were selected: toluene, xylene, and n-hexane. We supposed that their measured air concentrations were 200, 250, and 100 mg/m3 , respectively. Fig. 2 shows that for this mixture, application indicates 3 common classes for toluene and xylene (class no. 1: eye impairment, class no. 2: upper respiratory tract irritation, and class no. 19: central nervous system impairment). Data sheets for both toluene and xylene (Fig. 1) show that eye irritation is involved in class 1 and central nervous system depression in class 19. n-hexane has no common class with either toluene or xylene (for the effects involved in
A. Vyskocil et al. / Environmental Toxicology and Pharmacology 18 (2004) 235–242
241
Fig. 2. Search page.
each class; Table 2). So, Rm formula is used only for toluene and xylene. In our case, the Rm index greater than 100% indicates that the TWA permissible value of the mixture of toluene and xylene is exceeded.
4. Conclusion Since, relevant human toxicological data are rare, it is not surprising that the procedures adopted by different countries to define the permissible exposure values (PEV) differ. This is why, we took the toxicological data from several secondary references, despite the fact that the majority of Qu´ebec PEVs are based on the ACGIH’s TLV® s. In this first phase of the project, it is impossible in most cases to anticipate the type of interaction (additivity, supraadditivity, and infra-additivity) for mixtures of substances
causing similar effects, either because these effects are not well-enough known or because the toxic action mechanisms remain unknown. In this phase, the thousands of primary literature sources available for almost 700 regulated substances were not scrutinized. This is why, using the precautionary principle recommended by the ACGIH (“in the absence of information to the contrary, the effects on the different hazards should be considered as additive”), we are considering all the effects found in the classes of similar effects as additive. The computer utility from this first phase should serve as an initial approach in estimating the possibility of addivity or interaction, which, in the case of similar effects, should be further investigated. In the second phase of this project, we will specify the type of interaction for mixtures most likely to be found in the occupational environment and for which primary literature data are avail-
242
A. Vyskocil et al. / Environmental Toxicology and Pharmacology 18 (2004) 235–242
able. Computer utility will include the results from the two phases of the project. Presently, the database is available in French version only on the web and can be accessed from http://www.irsst.qc.ca/en/ut intertox index.htm. The English version will be available shortly. Acknowledgements This study was supported by the Institut de recherche Robert-Sauv´e en sant´e et en s´ecurit´e du travail (Qu´ebec, Canada). References Agency for Toxic Substances and Disease Registry, 2001. Guidance Manual for the Assessment of Joint Toxic Action of Chemical Mixtures. Division of Toxicology, ATSDR, Atlanta, GA. Alarie, Y., Nielsen, G.D., Schaper, M., 2000. Animal bioassays for evaluation of indoor air quality. In: Spengler, J.D., Samet, J.M., McCarthy, J.F. (Eds.), Indoor Air Quality Handbook. McGraw-Hill, New York. American Conference of Governmental Industrial Hygienists, 1999. TLV® s and Other Occupational Exposure Values. CD ROM, ACGIH, Cincinnati. Burgess, W.A., 1995. Recognition of health hazards in industry. In: A Review of Materials and Processes, second ed. Wiley, New York. Commission de la sant´e et de la s´ecurit´e du travail, 1999. Fiches de Renseignements. Service du R´epertoire Toxicologique. CSST, Montr´eal, Canada.
Deutsche Forschungsgemeinschaft, 1998. MAK- and BAT-Values 1998. Weinheim, Deutsche Forschungsgemeinschaft. ´ Editeur officiel du Qu´ebec, 1999. R`eglement sur la Qualit´e du Milieu de Travail. Qu´ebec. Hathaway, G.J., Proctor, N.H., Hughes, J.P., et al., 1991. Proctor and Hughes’ Chemical Hazards of the Workplace, third ed. Van Nostrand Reinhold, New York. International Agency for Research on Cancer, 1987–1999. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, vol. 1–71, IARC, Lyon. Krieger, P., Deblay, F., Pauli, G., et al., 1998. Asthma and domestic chemical pollutants (excluding tobacco). Rev. Mal. Respir. 15, 11. Lauwerys, R., 1991. Toxicologie Industrielle et Intoxications Professionnelles, third ed. Masson, Paris. National Institute for Occupational Safety and Health, 1997. Pocket Guide to Chemical Hazards. NIOSH, Cincinnati, OH. National Institute of Occupational Safety and Health, 1996. National Occupational Research Agenda, Mixed Exposures. NIOSH, Cincinnati, OH. (http://www.cdc.gov/niosh/worken.html#mixeds). Occupational Safety and Health Administration, 1991 Chemical information manual. OSHA Instruction CPL 2-2.43A. OSHA, Department of Labor, Washington, DC. U.S. Environmental Protection Agency, 2000. Supplementary Guidance for Conducting Health Risk Assessment of Chemical Mixtures. Risk Assessment Forum. EPA/630/R-00/002, EPA, Washington, DC. World Health Organization/International Programme on Chemical Safety, 1999. International Chemistry Safety Cards Database System, version 1.13, PrettyBit Software Ltd., Tampere. Yang, R.S.H., 1994. Toxicology of Chemical Mixtures: Case Studies, Mechanisms and Novel Approaches. Academic Press, San Diego.