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Emissions of Aerial Pollutants in Livestock Buildings in Northern Europe: Overview of a Multinational Project
J. agric. Engng Res. (1998) 70, 3—9
Emissions of Aerial Pollutants in Livestock Buildings in Northern Europe: Overview of a Multinational Project C. M. Wathes1; V. R. Phillips1; M. R. Holden1; R. W. Sneath1; J. L. Short1; R. P. White1; J. Hartung2; J. Seedorf2; M. Schro¨der2; K. H. Linkert2; S. Pedersen3; H. Takai3; J. O. Johnsen3; P. W. G. Groot Koerkamp4; G. H. Uenk4; J. H. M. Metz4; T. Hinz5; V. Caspary5; S. Linke5 1 Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK; 2 Tiera¨rztliche Hochschule Hannover, Institut fu¨r Tierhygiene und Tierschutz, Bu¨nteweg 17 p, 30559 Hannover, Germany; 3 Danish Institute of Animal Science, Research Centre Bygholm, Dept. of Agricultural Engineering, PO Box 536, DK-8700 Horsens, Denmark; 4 Instituut voor Milieu- en Agritechniek, Postbus 43, 6700 AA Wageningen, The Netherlands; 5 Institut fu¨r Biosystemtechnik, Bundesallee 50, 38116 Braunschweig, Germany (Received 1 April 1997; accepted in revised form 12 January 1998)
multi-factorial environmental diseases, such as atrophic rhinitis in pigs. Third, livestock production systems are major sources of gaseous pollutants, such as ammonia and methane, which are involved in soil acidification and global warming. There are, however, few international field surveys of aerial pollutants in livestock buildings that have used common methods and thereby allow objective comparisons between production systems and animal species across national boundaries. Numerical models and supporting experimental studies of the dynamics of aerial pollutants in livestock buildings are also needed to complement the broad brush approach of field surveys. The overall aim of the work reported in the papers in this issue was to advance our scientific understanding of the emissions of aerial pollutants within and from livestock buildings in Northern Europe. This, in turn, will lead to improvements in the health and welfare of housed farm animals as well as of stockmen, through a reduction in the burden of aerial pollutants in livestock buildings, and to a greater protection of the countryside through reduced emissions of important environmental pollutants, such as ammonia. The main object of the project was to undertake a field survey of the emissions of aerial pollutants within and from a large number of livestock buildings in the partner countries, comprising Denmark, Germany, the Netherlands and England. This paper describes the overall aims and rationale behind the project, outlines the common methods used in the field survey, considers the likely implications for human and animal health and environmental impact, and makes some recommendations for measures to control and abate pollutant emissions within livestock buildings.
The principal object of this project was to undertake a large survey of the emissions of aerial pollutants within and from livestock buildings in Northern Europe. Measurements of aerial pollutants were made using common methods which were developed in the first year of the project. This allowed reliable comparisons to be made between results from different countries, species and types of building. The survey covered pigs, cattle and poultry and the major housing types. A total of 329 livestock buildings were surveyed. All the results are described in the accompanying papers in this issue. High concentrations of ammonia, airborne dust, endotoxin and microorganisms were measured in poultry and some pig houses: air quality was acceptable in cattle houses in terms of animal health. These burdens of aerial pollutants cause some concern because of their potentially harmful effects on human and animal health. Atmospheric ammonia is an important pollutant of the atmosphere and its rates of emission per animal housed were highest from cattle buildings. Measures based on husbandry and engineering techniques should be taken to reduce these emissions. ( 1998 Silsoe Research Institute
1. Introduction The air within many livestock buildings seethes with a dense miasma of microorganisms, dust particles and gases.1 These aerial pollutants give cause for concern for several reasons. First, primary and opportunist microbial pathogens may cause directly infectious and allergic diseases in humans and farm animals. Second, chronic exposure to some types of aerial pollutants may exacerbate 0021-8634/98/050003#07 $25.00/0/ag980278
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( 1998 Silsoe Research Institute
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C. M . W AT H ES E¹ A¸ .
2. The tasks 2.1. ¼ork plan The project started in March 1992 and was completed in February 1996. The research was carried out by Silsoe Research Institute, UK; Tiera¨rztliche Hochschule Hannover, Germany; Instituut voor Milieuen Agritechniek, the Netherlands; and the Danish Institute of Animal Science, Denmark. It comprised two main tasks: (A) the development of a common suite of sampling methods for use by the four partners involved in the field survey of aerial pollutant emissions; and (B) a field survey of aerial pollutant emissions within and from 329 livestock buildings in the four countries. Research towards a second object was undertaken by the Institut fu¨r Biosystemtechnik and concerned a more comprehensive study of aerial pollutant emissions (task C).
2.2 ¹ask A: Development of sampling methods for aerial pollutants in livestock buildings The development of a set of sampling methods for aerial pollutants in livestock buildings was carried out jointly by the four partners involved in the field survey. This task was essential if comparisons between countries were to be made. Each partner was given the responsibility of developing a particular technique on behalf of the partnership. Techniques were devised to measure the main environmental variables (Table 1). Each measurement technique involved technical development, supply and calibration of instruments to all partners, staff training, quality control of the data and overall technical advice. A full description of the techniques is given by Phillips et al.2
Table 1 Responsibilities for technique development Temperature, humidity and weather Aerial ammonia Airborne microorganisms and endotoxin Airborne dust Ventilation rate by CO 2 mass balance Ventilation rate by fan-wheel anemometry Data logging and statistical analysis
Silsoe Research Institute Instituut voor Milieu- en Agritechniek Tiera¨rztliche Hochschule Hannover Danish Institute of Animal Science Danish Institute of Animal Science and Instituut voor Milieu- en Agritechniek Institut fu¨r Biosystemtechnik Silsoe Research Institute
The primary aerial pollutants of interest were ammonia, dust, bacteria, fungal propagules and endotoxin. Each of these pollutants may affect adversely animal and human health,1 while ammonia is also an environmental pollutant that contributes to acid rain and other effects. The concentration of carbon dioxide was also measured not only because of concern over its potential hazard but also for calculations of ventilation rate.2 Temperature and humidity were measured to indicate the effectiveness of the environmental control system, as well as to ascertain any dependent association with emission of a pollutant.
2.3. ¹ask B: A field survey of aerial pollutant emissions in livestock buildings The field survey of the emissions of aerial pollutants within and from livestock buildings started at the beginning of the second year of the project, after the common sampling techniques had been developed. The survey covered the major types of livestock housing for cattle, pigs and poultry that were in common use in the four countries (Table 2). Usually, four replicates of each type of building were surveyed in each country. Where possible, according to husbandry practice, measurements were made in the same building once in the winter and once in the summer, defined arbitrarily as from November to April and from
Table 2 Numbers and types of livestock buildings across all four countries included in the field survey Species
Manure system
Dairy cows
Tie stalls or litter Cubicles Litter Slats with slurry Littter Slats with slurry Subtotal 118 Litter Slats or mesh floor with slurry Slats or mesh floor with slurry Litter or flushing systems Slats with slurry Subtotal 130 Aviary or perchery Cages with belts or deep pit Litter Subtotal 81 Grand total
Beef cattle Calves
Sows Weaner pigs Fattening pigs
Laying hens Broiler chickens
Number in survey 24 24 10 20 24 16 16 32 32 16 34 22 26 33 329
E MI S SI O NS O F AE RI AL P O LLU T AN TS IN LIVE S TO CK B UIL DI N G S
May to October, respectively. Each set of measurements was made over 24 h, starting at 06 : 00 h. The measurement programme in any one building took about a week and included erection of the equipment, measurement, removal, cleaning and disinfection of the equipment, preliminary data analysis and transfer to the next farm. A high standard of hygiene was adopted to avoid the spread of disease between farms. In each building, measurements were made in a vertical cross-section at seven locations which were representative of the location of the animals, the stockman and the exhaust of the ventilation system. An eighth location was sited outside the building so that the ambient conditions could be recorded. The ventilation rate of the building was estimated from a mass balance of the CO 2 emissions. The emission rate of other pollutants was calculated from the product of the ventilation rate and the outlet concentration or, for ammonia only, the difference in concentration between the ambient and outlet. The data were analysed usually by analysis of variance, after transformation, where appropriate. The effects of species, building type, season and time of sampling (day or night) were determined.
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balance of the production and clearance of the metabolic CO emitted in the expired air of the animals. Second, the 2 concentration of airborne dust was measured with modified personal samplers which separated the dust into the inhalable and respirable size fractions. Third, the concentration of airborne microorganisms was determined using a new design5 of bioaerosol sampler which permitted independent, remote sampling. Fourth, the ammonia analyser was a combination of a chemiluminescent nitric oxide (NO) analyser and an upstream thermal NH con3 verter. Samples of gases were presented to this analyser and an infra-red analyser for CO using a multi-point gas 2 handling system. Fifth, signals from the sensors for air temperature, air humidity, wind speed and direction were transmitted by radiotelemetry to avoid the encumbrance and inconvenience of cables which would have required a total length of at least 1)5 km. Finally, an automated data logging system was developed to control the gas analysis, gas handling system, etc., and to record all the data in a pre-processed form. The four sampling systems were in full time operation in 329 livestock buildings for over 2)5 yr in each country and worked satisfactorily apart from some minor breakdowns due to failure of various components such as gas pumps.
2.4. ¹ask C: A comprehensive study of aerial pollutant emissions from livestock buildings Full-scale measurements were made in two livestock buildings, a fattening piggery and a Louisiana style broiler house.3,4 The measurements of aerial pollutant emissions were made with similar methods to those used in Task B but at more locations and over a longer period than 24 h. An additional method of measuring ventilation rate with fan-wheel anemometers was used in the piggery, to supplement the findings of the CO mass balance. The 2 main focus of the measurements was the physical environment within the livestock building, though some limited observations of animal activity were also made.
3. Results 3.1. ¹ask A: Development of sampling methods for aerial pollutants Table 3 lists the measurement techniques which were developed in the first year of the project. The techniques were chosen according to their reliability, reproducibility and capital and running costs. A complete set of apparatus was provided to each of the four partners and was installed in a mobile laboratory. Several features of the sampling methods were novel. First, the ventilation rate was estimated from a mass
3.2. ¹ask B: A field survey of aerial pollutant emissions in livestock buildings Three balance methods6 for calculating ventilation flow rates were compared. These were based on the output of heat, moisture or carbon dioxide from the livestock within the buildings. A correction had to be made in the first two balances for the evaporation of water from sources other than the animals themselves. Partitioning of the metabolic heat production into sensible and latent components was also necessary. Over 24 h, the three methods gave comparable results except when there were small differences between indoor and outdoor air temperature, absolute humidity or concentration of CO . Estimates of the ventilation rate over 2 periods shorter than 24 h were also unreliable, unless they were corrected for diurnal changes in the production of metabolic heat, moisture and carbon dioxide. The measurements7 of ventilation rates in the 329 livestock buildings surveyed showed that the mean ventilation rates over 24 h were usually in agreement with recommended values. There was some evidence for insufficient maximum capacity in certain types of housing, e.g. sows and weaners on slats, caged laying hens and broiler chickens. This would have an important bearing on the risk of heat stress in summer, since removal of metabolic heat is primarily by the ventilation system.
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Table 3 Measurement techniques for aerial pollutants in livestock buildings Variable Ammonia concentration
Carbon dioxide concentration Ventilation rate Airborne dust concentration Airborne endotoxin concentration Air temperature Relative humidity Airborne microorganisms concentration Wind speed Wind direction
Technique Chemiluminescence NO x analyzer Photo-accoustic analyzer* Wet chemistry* Infra-red analyzer Photo-accoustic analyzer* Indirect mass balance of CO 2 Flow anemometer* Gravimetric filtration Mass oscillator* Gravimetric filtration
Location
Frequency
3]animal height 3]human height 1]outlet 1]ambient As aboves
Hourly
Hourly
As above
Hourly 12 h
Platinum resistance Capacitance sensor Impaction
As above but not ambient As above but samples pooled As above As above Single
6 min 6 min 12 h
Cup anemometer Wind vane
1]ambient 1]ambient
6 min 6 min
12 h
* Used in task C at various frequencies and locations s The same locations as used for measurements of ammonia concentration with exceptions listed for airborne dust and endotoxin concentration.
Mean air temperatures and humidities8 were also in broad agreement with recommended values. However, high air temperatures were recorded on occasions in the summer and were due to insufficient ventilation (vide supra). Furthermore, the temperature control system in some pig and poultry houses failed to maintain the target temperature once the ambient air temperature had reached approximately 17°C. The mean concentration9 of both inhalable and respirable dust was highest in poultry houses (3)60 and 0)45 mg/m3) and was lowest in cattle buildings (0)38 and 0)07 mg/m3). There was a diurnal rhythm of concentration in most building types due to changes in animal activity. Inhalable dust emission rates from pig and poultry buildings were about 40% higher in summer than winter. Dust emissions from cattle buildings and respirable dust emissions from pig and poultry buildings were not affected greatly by the season. The highest mean concentration10 of ammonia was recorded in poultry houses, with a range from 5 to 30 p.p.m. In pig houses, the ammonia concentration ranged between 5 and 18 p.p.m while in cattle buildings the concentration was less than 8 p.p.m. The emission rates of ammonia were highest from poultry and least in cattle, when expressed per liveweight unit, equivalent to 500 kg. A significant burden of airborne endotoxins11 was recorded in both pig and poultry houses (52—187 and
339—860 ng/m3, respectively for the inhalable fraction) but the concentration in cattle buildings was low. There was no correlation between the concentration of airborne endotoxin and Enterobacteriaceae. Concentrations of total bacteria ranged from 4)3 to 6)4 log (colony forming 10 units/m3) in cattle and poultry houses, respectively, and were between 10 and 100 times those of fungal propagules.
3.3. ¹ask C: A comprehensive study of aerial pollutant emissions from livestock buildings The results of the full-scale measurements in the Louisiana-type broiler house and the fattening piggery are given in Hinz and Linke.3,4 Comparison of the two main methods, e.g. fan-wheel anemometers and a CO 2 mass balance, for measuring ventilation rate in a mechanically ventilated building showed an acceptable agreement to within 6% (SD$11)7%) for measurements over 24 h. The correlation was poorer for a shorter measurement period. The measurements of the concentrations of airborne dust, endotoxin and ammonia gave similar overall results to those found in the field survey. There was little effect of sampling height, and measurements made in the central cross-section, as in the field survey, were representative of the building as a whole, which justified the choice of sampling positions in the field
E MI S SI O NS O F AE RI AL P O LLU T AN TS IN LIVE S TO CK B UIL DI N G S
survey. There was a clear influence of animal activity upon dust concentration in the piggery but not the broiler house, where dust concentration was dominated by the operation of the sidewall ventilation curtains.
4. Discussion This project achieved its object. The field survey of the concentrations and emission of aerial pollutants in livestock buildings is by far the most comprehensive study of its kind. Apart from its size, its main advantage is in the use of common measurement techniques in all of the participating countries. The methods for aerosol sampling were based on the recommendations of a European workshop12 while those for gases followed current best practice. This standardization allows, for the first time, comparisons to be made between countries, types of housing and animal species. The project findings may be useful in two ways. First, our scientific understanding of pollutant emissions has been advanced since there is now a solid database against which changes to animal husbandry systems can be evaluated, as well as enabling identification of the best and worst examples of air pollution in livestock buildings. This is important both for animal and human health as well as the environmental impact of livestock buildings, e.g. by the emission of ammonia. Second, EU and national legislation to control emissions of air pollution from livestock buildings will be better informed, since there are now uniform standards for assessment and a baseline against which the success of pollution control policies can be evaluated. An important example is the recent European Union directive on ‘‘Integrated Pollution Prevention and Control’’, which will require member states to set limits on certain pollutant emissions from large, new production sites for poultry (over 40 000 birds) and pigs (over 2000 fattening pigs over 30 kg liveweight or 750 sows). The relevance of the specific findings of the field survey is discussed in detail in the relevant papers. However, some general points can be made. Overall, the air quality in poultry houses was the worst amongst the three species studied, as shown by high concentrations of ammonia, dust and endotoxin. It was slightly better in most pig houses and substantially better in cattle houses. There are some important implications for animal and human health in poultry and pig houses, since the critical levels for an acceptable quality of air were approached and exceeded in some cases. The current recommendations1 for continuous exposure of animals (not humans) are 20 p.p.m. for ammonia, 3000 p.p.m for carbon dioxide, 3)4 mg/m3 for inhalable and 1)7 mg/m3 for respirable non-specific dust. The UK
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occupational human exposure limits13 are higher than these but are based upon an 8 h time-weighted average concentration whereas animals are forced to endure their atmospheres continuously. Furthermore, these limits do not take into account any synergism between different pollutants. There are a number of techniques that can be used to improve air quality.14,15 No single technique will provide a universal panacea because of the complexity of the sources. A technique which may lower the concentration of ammonia in a broiler house may raise the concentration of dust and vice versa. Instead, a combination of engineering devices and husbandry practices is needed to provide an integrated solution. Further research is desirable on practical systems to prevent the emission of pollutants at source or to cleanse the air in situ, though the latter strategy is less likely to be technically efficient, as shown by previous attempts using wet scrubbers or dry filters, for example. An additional complication arises from the multiple demands made on the ventilation system. At present, most systems are used to control the air temperature but currently there is interest in controlling relative humidity, e.g. in broiler houses, either to reduce the risk of heat stress in hot weather or to suppress the generation of dust from bedding. The concentration of aerial pollutants can also be controlled via adjustment of ventilation rate, though discharge of unwanted pollutants to the atmosphere is no longer an acceptable tactic because of concerns for environmental pollution or malodours. Control of these three variables, viz air temperature, relative humidity and pollutant concentration, is impracticable with present knowledge because of (1) ignorance of the financial and other penalties of not meeting the separate set-points, and (2) suitable control algorithms. Both of these deficiencies are high priorities for future research. Expressed per unit of liveweight (500 kg) cattle had an emission rate of ammonia that was lower than from pigs and poultry. However, the large number of cattle in Northern Europe ensures that this species is the largest source of ammonia emissions, despite the low concentration of ammonia in cattle sheds which is more than compensated for by the fast ventilation flow rates. The emission factors derived from this survey will be most useful to compilers of emission inventories: livestock are the largest source of ammonia emissions and livestock buildings represent about one-third of the total emissions.10 Most emission factors have been based upon the results from one or two experimental livestock buildings, albeit from measurements over a longer period than that used here. In general, our results provided support for the factors which have been employed but with some exceptions, e.g. layers kept on litter or weaners kept on slats.10 The greatest uncertainty in the estimates of emission
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C. M . W AT H ES E¹ A¸ .
rates was the calculation of ventilation rate, especially in naturally ventilated buildings. This arises from a number of causes, including identification of the functions of an opening as an inlet or outlet and the estimated rate of production of metabolic carbon dioxide.16 Overall, the accuracy of the estimates of emission rates is about $20%, which is sufficient for compilation of an inventory but this can be improved in experimental studies. Temperature control in mechanically ventilated buildings for pigs and poultry was generally good at cold temperatures but became less efficient at ambient temperatures of '17°C. There was some evidence for insufficient installed capacity of ventilation which would lead to a risk of heat stress in a warm summer, and this was confirmed by the high air temperatures which were recorded on occasions. The most likely explanation for this is the gradual increase over the years in stocking density, especially in broiler houses, whilst building designs have not altered substantially. There may be some benefit in re-examining the recommendations for ventilation capacity in pig and poultry houses, perhaps even considering the need for some form of cooling.
We thank the many technicians in all the partner countries, without whose help the project could not have been completed, and also Professor Th. Blaha, Head of the Unit of Epidemiology of the Hannover School of Veterinary Medicine, at Bakum, Germany, for his organizational and logistic support. We thank Chris Michael and his staff at Meaco Sales and Marketing for their enthusiasm and dedication in developing with us the novel wire-less data logging system. Finally, we thank the many farmers in the UK, Netherlands, Germany and Denmark who not only allowed access to their buildings for the measurements to be made, but also helped in so many other ways.
References 1
2
5. Conclusions This project has provided, for the first time, reliable information on the emission of the main aerial pollutants within and from a large number of livestock buildings in Northern Europe. The strength of the findings lies in the common methods of measurement which allows comparison to be made between countries, species and housing types. Evidence has been collected for a significant concentration of aerial pollutants within poultry and some pig houses which may be harmful to human and animal health. The emissions of ammonia from cattle buildings were substantial and add weight to current concern over this major source of environmental pollutant.
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Acknowledgements The work was funded mainly by the Commission of the European Union as Project No. PL900703. Supplementary funding was also received in the UK from the Ministry of Agriculture, Fisheries and Food, via Commission CC 0204; in Germany from the Hannover School of Veterinary Medicine and the Institut fu¨r Biosystemtechnik of the Bundesforschungsanstalt fu¨r Landwirtschaft; in the Netherlands from the Ministry of Agriculture, Nature Management and Fisheries; and in Denmark from the Ministry of Agriculture and Fisheries.
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Wathes C M Air and surface hygiene. In. ¸ivestock Housing (Wathes, C M; Charles, D R eds), 123—148 Wallingford: CAB International, 1994 Phillips V R; Holden M R; Sneath R W; Short J L; White R P; Hartung J; Seedorf J ; Schro¨ der M; Linkert K-H; Pedersen S; Takai H; Johnsen J O; Metz J H M; Groot Koerkamp P W G; Uenk G H; Scholtens R; Wathes C M The development of robust methods for measuring concentrations and emission rates of gaseous and particulate air pollutants in livestock buildings. Journal of Agricultural Engineering Research, 1998, 70, 11—24 Hinz T; Linke S A comprehensive experimental study of aerial pollutants in and emissions from livestock buildings. Part 1. Methods. Journal of Agricultural Engineering Research, 1998, 70, 111—118 Hinz T; Linke S A comprehensive experimental study of aerial pollutants in and emissions from livestock buildings. Part 2 Results. Journal of Agricultural Engineering Research, 1998, 70, 119—129 Pahl O; Phillips V R; Lacey J; Hartung J; Wathes C M Comparison of commonly used samplers with a novel bioaerosol sampler with automatic plate exchange. Journal of Aerosol Science, 1997, 28(3), 927—935 Pedersen S; Takai H; Johnsen J O; Metz J H M; Groot Koerkamp P W G; Uenk G H; Phillips V R; Holden M R; Sneath R W; Short J L; White R P; Hartung J; Seedorf J; Schro¨ der M; Linkert K-H; Wathes C M A comparison of three balance methods for calculating ventilation flow rates in livestock buildings. Journal of Agricultural Engineering Research, 1998, 70, 25—37 Seedorf J; Hartung J; Schro¨ der M; Linkert K-H; Pedersen S; Takai H; Johnsen J O; Metz J H M; Groot Koerkamp P W G; Uenk G H; Phillips V R; Holden M R; Sneath R W; Short J L; White R P; Wathes C M Ventilation control in livestock buildings in Northern Europe. Journal of Agricultural Engineering Research, 1998, 70, 39—47 Seedorf J; Hartung J; Schro¨ der M; Linkert K-H; Pedersen S; Takai H; Johnsen J O; Metz J H M; Groot Koerkamp P W G; Uenk G H; Phillips V R; Holden M R; Sneath R W; Short J L; White R P; Wathes C M Thermal and moisture conditions in livestock buildings in Northern Europe. Journal of Agricultural Engineering Research, 1998, 70, 49—57
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Takai H; Pedersen S; Johnsen J O; Metz J H M; Groot Koerkamp P W G; Uenk G H; Phillips V R; Holden M R; Sneath R W; Short J L; White R P; Hartung J; Seedorf J; Schro¨ der M; Linkert K-H; Wathes C M Concentrations and emissions of airborne dust in livestock buildings in Northern Europe. Journal of Agricultural Engineering Research, 1998, 70, 59—77 Groot Koerkamp P W G; Metz J H M; Uenk G H; Phillips V R; Holden M R; Sneath R W; Short J L; White R P; Hartung J; Seedorf J; Schro¨ der M; Linkert K-H; Pedersen S; Takai H; Johnsen J O; Wathes C M Concentrations and emissions of ammonia in livestock buildings in Northern Europe. Journal of Agricultural Engineering Research, 1998, 70, 79—95 Seedorf J; Hartung J; Schro¨ der M; Linkert K-H; Phillips V R; Holden M R; Sneath R W; Short J L; White R P; Pedersen S; Takai H; Johnsen J O; Metz J H M; Groot Koerkamp P W G; Uenk G H; Wathes C M Concentrations and emissions of airborne endotoxins and microorganisms in livestock buildings in Northern Europe. Journal of Agricultural Engineering Research, 1998, 70, 97—109
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Wathes C M; Randall J M (eds) Aerosol sampling in animal houses. Commission of the European Communities, Luxembourg, Eur. 11877, 1989 Health and Safety Executive Occupational exposure limits 1992. EH 40/92, Her Majesty’s Stationary Office, London, 1992 Dawson J R Minimizing dust in livestock buildings: possible alternatives to mechanical separation. Journal of Agricultural Engineering Research, 1990, 47, 235—248 Groot Koerkamp P W G Review on emissions of ammonia from housing systems for laying hens in relation to sources, processes, building design and manure handling. Journal of Agricultural Engineering Research, 1994, 59, 73—87 van Ouwerkerk E N J; Pedersen S J Application of carbon dioxide mass balance method to evaluate ventilation rates in livestock buildings. In: Proceedings of XIIth World Congress on Agricultural Engineering, Milan, Italy 29 August—1 September 1994: 516—529
Emissions of Aerial Pollutants in Livestock Buildings in Northern Europe: Overview of a Multinational Project C. M. Wathes1; V. R. Phillips1; M. R. Holden1; R. W. Sneath1; J. L. Short1; R. P. White1; J. Hartung2; J. Seedorf2; M. Schro¨der2; K. H. Linkert2; S. Pedersen3; H. Takai3; J. O. Johnsen3; P. W. G. Groot Koerkamp4; G. H. Uenk4; J. H. M. Metz4; T. Hinz5; V. Caspary5; S. Linke5 1 Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK; 2 Tiera¨rztliche Hochschule Hannover, Institut fu¨r Tierhygiene und Tierschutz, Bu¨nteweg 17 p, 30559 Hannover, Germany; 3 Danish Institute of Animal Science, Research Centre Bygholm, Dept. of Agricultural Engineering, PO Box 536, DK-8700 Horsens, Denmark; 4 Instituut voor Milieu- en Agritechniek, Postbus 43, 6700 AA Wageningen, The Netherlands; 5 Institut fu¨r Biosystemtechnik, Bundesallee 50, 38116 Braunschweig, Germany (Received 1 April 1997; accepted in revised form 12 January 1998)
multi-factorial environmental diseases, such as atrophic rhinitis in pigs. Third, livestock production systems are major sources of gaseous pollutants, such as ammonia and methane, which are involved in soil acidification and global warming. There are, however, few international field surveys of aerial pollutants in livestock buildings that have used common methods and thereby allow objective comparisons between production systems and animal species across national boundaries. Numerical models and supporting experimental studies of the dynamics of aerial pollutants in livestock buildings are also needed to complement the broad brush approach of field surveys. The overall aim of the work reported in the papers in this issue was to advance our scientific understanding of the emissions of aerial pollutants within and from livestock buildings in Northern Europe. This, in turn, will lead to improvements in the health and welfare of housed farm animals as well as of stockmen, through a reduction in the burden of aerial pollutants in livestock buildings, and to a greater protection of the countryside through reduced emissions of important environmental pollutants, such as ammonia. The main object of the project was to undertake a field survey of the emissions of aerial pollutants within and from a large number of livestock buildings in the partner countries, comprising Denmark, Germany, the Netherlands and England. This paper describes the overall aims and rationale behind the project, outlines the common methods used in the field survey, considers the likely implications for human and animal health and environmental impact, and makes some recommendations for measures to control and abate pollutant emissions within livestock buildings.
The principal object of this project was to undertake a large survey of the emissions of aerial pollutants within and from livestock buildings in Northern Europe. Measurements of aerial pollutants were made using common methods which were developed in the first year of the project. This allowed reliable comparisons to be made between results from different countries, species and types of building. The survey covered pigs, cattle and poultry and the major housing types. A total of 329 livestock buildings were surveyed. All the results are described in the accompanying papers in this issue. High concentrations of ammonia, airborne dust, endotoxin and microorganisms were measured in poultry and some pig houses: air quality was acceptable in cattle houses in terms of animal health. These burdens of aerial pollutants cause some concern because of their potentially harmful effects on human and animal health. Atmospheric ammonia is an important pollutant of the atmosphere and its rates of emission per animal housed were highest from cattle buildings. Measures based on husbandry and engineering techniques should be taken to reduce these emissions. ( 1998 Silsoe Research Institute
1. Introduction The air within many livestock buildings seethes with a dense miasma of microorganisms, dust particles and gases.1 These aerial pollutants give cause for concern for several reasons. First, primary and opportunist microbial pathogens may cause directly infectious and allergic diseases in humans and farm animals. Second, chronic exposure to some types of aerial pollutants may exacerbate 0021-8634/98/050003#07 $25.00/0/ag980278
3
( 1998 Silsoe Research Institute
4
C. M . W AT H ES E¹ A¸ .
2. The tasks 2.1. ¼ork plan The project started in March 1992 and was completed in February 1996. The research was carried out by Silsoe Research Institute, UK; Tiera¨rztliche Hochschule Hannover, Germany; Instituut voor Milieuen Agritechniek, the Netherlands; and the Danish Institute of Animal Science, Denmark. It comprised two main tasks: (A) the development of a common suite of sampling methods for use by the four partners involved in the field survey of aerial pollutant emissions; and (B) a field survey of aerial pollutant emissions within and from 329 livestock buildings in the four countries. Research towards a second object was undertaken by the Institut fu¨r Biosystemtechnik and concerned a more comprehensive study of aerial pollutant emissions (task C).
2.2 ¹ask A: Development of sampling methods for aerial pollutants in livestock buildings The development of a set of sampling methods for aerial pollutants in livestock buildings was carried out jointly by the four partners involved in the field survey. This task was essential if comparisons between countries were to be made. Each partner was given the responsibility of developing a particular technique on behalf of the partnership. Techniques were devised to measure the main environmental variables (Table 1). Each measurement technique involved technical development, supply and calibration of instruments to all partners, staff training, quality control of the data and overall technical advice. A full description of the techniques is given by Phillips et al.2
Table 1 Responsibilities for technique development Temperature, humidity and weather Aerial ammonia Airborne microorganisms and endotoxin Airborne dust Ventilation rate by CO 2 mass balance Ventilation rate by fan-wheel anemometry Data logging and statistical analysis
Silsoe Research Institute Instituut voor Milieu- en Agritechniek Tiera¨rztliche Hochschule Hannover Danish Institute of Animal Science Danish Institute of Animal Science and Instituut voor Milieu- en Agritechniek Institut fu¨r Biosystemtechnik Silsoe Research Institute
The primary aerial pollutants of interest were ammonia, dust, bacteria, fungal propagules and endotoxin. Each of these pollutants may affect adversely animal and human health,1 while ammonia is also an environmental pollutant that contributes to acid rain and other effects. The concentration of carbon dioxide was also measured not only because of concern over its potential hazard but also for calculations of ventilation rate.2 Temperature and humidity were measured to indicate the effectiveness of the environmental control system, as well as to ascertain any dependent association with emission of a pollutant.
2.3. ¹ask B: A field survey of aerial pollutant emissions in livestock buildings The field survey of the emissions of aerial pollutants within and from livestock buildings started at the beginning of the second year of the project, after the common sampling techniques had been developed. The survey covered the major types of livestock housing for cattle, pigs and poultry that were in common use in the four countries (Table 2). Usually, four replicates of each type of building were surveyed in each country. Where possible, according to husbandry practice, measurements were made in the same building once in the winter and once in the summer, defined arbitrarily as from November to April and from
Table 2 Numbers and types of livestock buildings across all four countries included in the field survey Species
Manure system
Dairy cows
Tie stalls or litter Cubicles Litter Slats with slurry Littter Slats with slurry Subtotal 118 Litter Slats or mesh floor with slurry Slats or mesh floor with slurry Litter or flushing systems Slats with slurry Subtotal 130 Aviary or perchery Cages with belts or deep pit Litter Subtotal 81 Grand total
Beef cattle Calves
Sows Weaner pigs Fattening pigs
Laying hens Broiler chickens
Number in survey 24 24 10 20 24 16 16 32 32 16 34 22 26 33 329
E MI S SI O NS O F AE RI AL P O LLU T AN TS IN LIVE S TO CK B UIL DI N G S
May to October, respectively. Each set of measurements was made over 24 h, starting at 06 : 00 h. The measurement programme in any one building took about a week and included erection of the equipment, measurement, removal, cleaning and disinfection of the equipment, preliminary data analysis and transfer to the next farm. A high standard of hygiene was adopted to avoid the spread of disease between farms. In each building, measurements were made in a vertical cross-section at seven locations which were representative of the location of the animals, the stockman and the exhaust of the ventilation system. An eighth location was sited outside the building so that the ambient conditions could be recorded. The ventilation rate of the building was estimated from a mass balance of the CO 2 emissions. The emission rate of other pollutants was calculated from the product of the ventilation rate and the outlet concentration or, for ammonia only, the difference in concentration between the ambient and outlet. The data were analysed usually by analysis of variance, after transformation, where appropriate. The effects of species, building type, season and time of sampling (day or night) were determined.
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balance of the production and clearance of the metabolic CO emitted in the expired air of the animals. Second, the 2 concentration of airborne dust was measured with modified personal samplers which separated the dust into the inhalable and respirable size fractions. Third, the concentration of airborne microorganisms was determined using a new design5 of bioaerosol sampler which permitted independent, remote sampling. Fourth, the ammonia analyser was a combination of a chemiluminescent nitric oxide (NO) analyser and an upstream thermal NH con3 verter. Samples of gases were presented to this analyser and an infra-red analyser for CO using a multi-point gas 2 handling system. Fifth, signals from the sensors for air temperature, air humidity, wind speed and direction were transmitted by radiotelemetry to avoid the encumbrance and inconvenience of cables which would have required a total length of at least 1)5 km. Finally, an automated data logging system was developed to control the gas analysis, gas handling system, etc., and to record all the data in a pre-processed form. The four sampling systems were in full time operation in 329 livestock buildings for over 2)5 yr in each country and worked satisfactorily apart from some minor breakdowns due to failure of various components such as gas pumps.
2.4. ¹ask C: A comprehensive study of aerial pollutant emissions from livestock buildings Full-scale measurements were made in two livestock buildings, a fattening piggery and a Louisiana style broiler house.3,4 The measurements of aerial pollutant emissions were made with similar methods to those used in Task B but at more locations and over a longer period than 24 h. An additional method of measuring ventilation rate with fan-wheel anemometers was used in the piggery, to supplement the findings of the CO mass balance. The 2 main focus of the measurements was the physical environment within the livestock building, though some limited observations of animal activity were also made.
3. Results 3.1. ¹ask A: Development of sampling methods for aerial pollutants Table 3 lists the measurement techniques which were developed in the first year of the project. The techniques were chosen according to their reliability, reproducibility and capital and running costs. A complete set of apparatus was provided to each of the four partners and was installed in a mobile laboratory. Several features of the sampling methods were novel. First, the ventilation rate was estimated from a mass
3.2. ¹ask B: A field survey of aerial pollutant emissions in livestock buildings Three balance methods6 for calculating ventilation flow rates were compared. These were based on the output of heat, moisture or carbon dioxide from the livestock within the buildings. A correction had to be made in the first two balances for the evaporation of water from sources other than the animals themselves. Partitioning of the metabolic heat production into sensible and latent components was also necessary. Over 24 h, the three methods gave comparable results except when there were small differences between indoor and outdoor air temperature, absolute humidity or concentration of CO . Estimates of the ventilation rate over 2 periods shorter than 24 h were also unreliable, unless they were corrected for diurnal changes in the production of metabolic heat, moisture and carbon dioxide. The measurements7 of ventilation rates in the 329 livestock buildings surveyed showed that the mean ventilation rates over 24 h were usually in agreement with recommended values. There was some evidence for insufficient maximum capacity in certain types of housing, e.g. sows and weaners on slats, caged laying hens and broiler chickens. This would have an important bearing on the risk of heat stress in summer, since removal of metabolic heat is primarily by the ventilation system.
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C. M . W AT H ES E¹ A¸ .
Table 3 Measurement techniques for aerial pollutants in livestock buildings Variable Ammonia concentration
Carbon dioxide concentration Ventilation rate Airborne dust concentration Airborne endotoxin concentration Air temperature Relative humidity Airborne microorganisms concentration Wind speed Wind direction
Technique Chemiluminescence NO x analyzer Photo-accoustic analyzer* Wet chemistry* Infra-red analyzer Photo-accoustic analyzer* Indirect mass balance of CO 2 Flow anemometer* Gravimetric filtration Mass oscillator* Gravimetric filtration
Location
Frequency
3]animal height 3]human height 1]outlet 1]ambient As aboves
Hourly
Hourly
As above
Hourly 12 h
Platinum resistance Capacitance sensor Impaction
As above but not ambient As above but samples pooled As above As above Single
6 min 6 min 12 h
Cup anemometer Wind vane
1]ambient 1]ambient
6 min 6 min
12 h
* Used in task C at various frequencies and locations s The same locations as used for measurements of ammonia concentration with exceptions listed for airborne dust and endotoxin concentration.
Mean air temperatures and humidities8 were also in broad agreement with recommended values. However, high air temperatures were recorded on occasions in the summer and were due to insufficient ventilation (vide supra). Furthermore, the temperature control system in some pig and poultry houses failed to maintain the target temperature once the ambient air temperature had reached approximately 17°C. The mean concentration9 of both inhalable and respirable dust was highest in poultry houses (3)60 and 0)45 mg/m3) and was lowest in cattle buildings (0)38 and 0)07 mg/m3). There was a diurnal rhythm of concentration in most building types due to changes in animal activity. Inhalable dust emission rates from pig and poultry buildings were about 40% higher in summer than winter. Dust emissions from cattle buildings and respirable dust emissions from pig and poultry buildings were not affected greatly by the season. The highest mean concentration10 of ammonia was recorded in poultry houses, with a range from 5 to 30 p.p.m. In pig houses, the ammonia concentration ranged between 5 and 18 p.p.m while in cattle buildings the concentration was less than 8 p.p.m. The emission rates of ammonia were highest from poultry and least in cattle, when expressed per liveweight unit, equivalent to 500 kg. A significant burden of airborne endotoxins11 was recorded in both pig and poultry houses (52—187 and
339—860 ng/m3, respectively for the inhalable fraction) but the concentration in cattle buildings was low. There was no correlation between the concentration of airborne endotoxin and Enterobacteriaceae. Concentrations of total bacteria ranged from 4)3 to 6)4 log (colony forming 10 units/m3) in cattle and poultry houses, respectively, and were between 10 and 100 times those of fungal propagules.
3.3. ¹ask C: A comprehensive study of aerial pollutant emissions from livestock buildings The results of the full-scale measurements in the Louisiana-type broiler house and the fattening piggery are given in Hinz and Linke.3,4 Comparison of the two main methods, e.g. fan-wheel anemometers and a CO 2 mass balance, for measuring ventilation rate in a mechanically ventilated building showed an acceptable agreement to within 6% (SD$11)7%) for measurements over 24 h. The correlation was poorer for a shorter measurement period. The measurements of the concentrations of airborne dust, endotoxin and ammonia gave similar overall results to those found in the field survey. There was little effect of sampling height, and measurements made in the central cross-section, as in the field survey, were representative of the building as a whole, which justified the choice of sampling positions in the field
E MI S SI O NS O F AE RI AL P O LLU T AN TS IN LIVE S TO CK B UIL DI N G S
survey. There was a clear influence of animal activity upon dust concentration in the piggery but not the broiler house, where dust concentration was dominated by the operation of the sidewall ventilation curtains.
4. Discussion This project achieved its object. The field survey of the concentrations and emission of aerial pollutants in livestock buildings is by far the most comprehensive study of its kind. Apart from its size, its main advantage is in the use of common measurement techniques in all of the participating countries. The methods for aerosol sampling were based on the recommendations of a European workshop12 while those for gases followed current best practice. This standardization allows, for the first time, comparisons to be made between countries, types of housing and animal species. The project findings may be useful in two ways. First, our scientific understanding of pollutant emissions has been advanced since there is now a solid database against which changes to animal husbandry systems can be evaluated, as well as enabling identification of the best and worst examples of air pollution in livestock buildings. This is important both for animal and human health as well as the environmental impact of livestock buildings, e.g. by the emission of ammonia. Second, EU and national legislation to control emissions of air pollution from livestock buildings will be better informed, since there are now uniform standards for assessment and a baseline against which the success of pollution control policies can be evaluated. An important example is the recent European Union directive on ‘‘Integrated Pollution Prevention and Control’’, which will require member states to set limits on certain pollutant emissions from large, new production sites for poultry (over 40 000 birds) and pigs (over 2000 fattening pigs over 30 kg liveweight or 750 sows). The relevance of the specific findings of the field survey is discussed in detail in the relevant papers. However, some general points can be made. Overall, the air quality in poultry houses was the worst amongst the three species studied, as shown by high concentrations of ammonia, dust and endotoxin. It was slightly better in most pig houses and substantially better in cattle houses. There are some important implications for animal and human health in poultry and pig houses, since the critical levels for an acceptable quality of air were approached and exceeded in some cases. The current recommendations1 for continuous exposure of animals (not humans) are 20 p.p.m. for ammonia, 3000 p.p.m for carbon dioxide, 3)4 mg/m3 for inhalable and 1)7 mg/m3 for respirable non-specific dust. The UK
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occupational human exposure limits13 are higher than these but are based upon an 8 h time-weighted average concentration whereas animals are forced to endure their atmospheres continuously. Furthermore, these limits do not take into account any synergism between different pollutants. There are a number of techniques that can be used to improve air quality.14,15 No single technique will provide a universal panacea because of the complexity of the sources. A technique which may lower the concentration of ammonia in a broiler house may raise the concentration of dust and vice versa. Instead, a combination of engineering devices and husbandry practices is needed to provide an integrated solution. Further research is desirable on practical systems to prevent the emission of pollutants at source or to cleanse the air in situ, though the latter strategy is less likely to be technically efficient, as shown by previous attempts using wet scrubbers or dry filters, for example. An additional complication arises from the multiple demands made on the ventilation system. At present, most systems are used to control the air temperature but currently there is interest in controlling relative humidity, e.g. in broiler houses, either to reduce the risk of heat stress in hot weather or to suppress the generation of dust from bedding. The concentration of aerial pollutants can also be controlled via adjustment of ventilation rate, though discharge of unwanted pollutants to the atmosphere is no longer an acceptable tactic because of concerns for environmental pollution or malodours. Control of these three variables, viz air temperature, relative humidity and pollutant concentration, is impracticable with present knowledge because of (1) ignorance of the financial and other penalties of not meeting the separate set-points, and (2) suitable control algorithms. Both of these deficiencies are high priorities for future research. Expressed per unit of liveweight (500 kg) cattle had an emission rate of ammonia that was lower than from pigs and poultry. However, the large number of cattle in Northern Europe ensures that this species is the largest source of ammonia emissions, despite the low concentration of ammonia in cattle sheds which is more than compensated for by the fast ventilation flow rates. The emission factors derived from this survey will be most useful to compilers of emission inventories: livestock are the largest source of ammonia emissions and livestock buildings represent about one-third of the total emissions.10 Most emission factors have been based upon the results from one or two experimental livestock buildings, albeit from measurements over a longer period than that used here. In general, our results provided support for the factors which have been employed but with some exceptions, e.g. layers kept on litter or weaners kept on slats.10 The greatest uncertainty in the estimates of emission
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C. M . W AT H ES E¹ A¸ .
rates was the calculation of ventilation rate, especially in naturally ventilated buildings. This arises from a number of causes, including identification of the functions of an opening as an inlet or outlet and the estimated rate of production of metabolic carbon dioxide.16 Overall, the accuracy of the estimates of emission rates is about $20%, which is sufficient for compilation of an inventory but this can be improved in experimental studies. Temperature control in mechanically ventilated buildings for pigs and poultry was generally good at cold temperatures but became less efficient at ambient temperatures of '17°C. There was some evidence for insufficient installed capacity of ventilation which would lead to a risk of heat stress in a warm summer, and this was confirmed by the high air temperatures which were recorded on occasions. The most likely explanation for this is the gradual increase over the years in stocking density, especially in broiler houses, whilst building designs have not altered substantially. There may be some benefit in re-examining the recommendations for ventilation capacity in pig and poultry houses, perhaps even considering the need for some form of cooling.
We thank the many technicians in all the partner countries, without whose help the project could not have been completed, and also Professor Th. Blaha, Head of the Unit of Epidemiology of the Hannover School of Veterinary Medicine, at Bakum, Germany, for his organizational and logistic support. We thank Chris Michael and his staff at Meaco Sales and Marketing for their enthusiasm and dedication in developing with us the novel wire-less data logging system. Finally, we thank the many farmers in the UK, Netherlands, Germany and Denmark who not only allowed access to their buildings for the measurements to be made, but also helped in so many other ways.
References 1
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5. Conclusions This project has provided, for the first time, reliable information on the emission of the main aerial pollutants within and from a large number of livestock buildings in Northern Europe. The strength of the findings lies in the common methods of measurement which allows comparison to be made between countries, species and housing types. Evidence has been collected for a significant concentration of aerial pollutants within poultry and some pig houses which may be harmful to human and animal health. The emissions of ammonia from cattle buildings were substantial and add weight to current concern over this major source of environmental pollutant.
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Acknowledgements The work was funded mainly by the Commission of the European Union as Project No. PL900703. Supplementary funding was also received in the UK from the Ministry of Agriculture, Fisheries and Food, via Commission CC 0204; in Germany from the Hannover School of Veterinary Medicine and the Institut fu¨r Biosystemtechnik of the Bundesforschungsanstalt fu¨r Landwirtschaft; in the Netherlands from the Ministry of Agriculture, Nature Management and Fisheries; and in Denmark from the Ministry of Agriculture and Fisheries.
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Wathes C M Air and surface hygiene. In. ¸ivestock Housing (Wathes, C M; Charles, D R eds), 123—148 Wallingford: CAB International, 1994 Phillips V R; Holden M R; Sneath R W; Short J L; White R P; Hartung J; Seedorf J ; Schro¨ der M; Linkert K-H; Pedersen S; Takai H; Johnsen J O; Metz J H M; Groot Koerkamp P W G; Uenk G H; Scholtens R; Wathes C M The development of robust methods for measuring concentrations and emission rates of gaseous and particulate air pollutants in livestock buildings. Journal of Agricultural Engineering Research, 1998, 70, 11—24 Hinz T; Linke S A comprehensive experimental study of aerial pollutants in and emissions from livestock buildings. Part 1. Methods. Journal of Agricultural Engineering Research, 1998, 70, 111—118 Hinz T; Linke S A comprehensive experimental study of aerial pollutants in and emissions from livestock buildings. Part 2 Results. Journal of Agricultural Engineering Research, 1998, 70, 119—129 Pahl O; Phillips V R; Lacey J; Hartung J; Wathes C M Comparison of commonly used samplers with a novel bioaerosol sampler with automatic plate exchange. Journal of Aerosol Science, 1997, 28(3), 927—935 Pedersen S; Takai H; Johnsen J O; Metz J H M; Groot Koerkamp P W G; Uenk G H; Phillips V R; Holden M R; Sneath R W; Short J L; White R P; Hartung J; Seedorf J; Schro¨ der M; Linkert K-H; Wathes C M A comparison of three balance methods for calculating ventilation flow rates in livestock buildings. Journal of Agricultural Engineering Research, 1998, 70, 25—37 Seedorf J; Hartung J; Schro¨ der M; Linkert K-H; Pedersen S; Takai H; Johnsen J O; Metz J H M; Groot Koerkamp P W G; Uenk G H; Phillips V R; Holden M R; Sneath R W; Short J L; White R P; Wathes C M Ventilation control in livestock buildings in Northern Europe. Journal of Agricultural Engineering Research, 1998, 70, 39—47 Seedorf J; Hartung J; Schro¨ der M; Linkert K-H; Pedersen S; Takai H; Johnsen J O; Metz J H M; Groot Koerkamp P W G; Uenk G H; Phillips V R; Holden M R; Sneath R W; Short J L; White R P; Wathes C M Thermal and moisture conditions in livestock buildings in Northern Europe. Journal of Agricultural Engineering Research, 1998, 70, 49—57
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Takai H; Pedersen S; Johnsen J O; Metz J H M; Groot Koerkamp P W G; Uenk G H; Phillips V R; Holden M R; Sneath R W; Short J L; White R P; Hartung J; Seedorf J; Schro¨ der M; Linkert K-H; Wathes C M Concentrations and emissions of airborne dust in livestock buildings in Northern Europe. Journal of Agricultural Engineering Research, 1998, 70, 59—77 Groot Koerkamp P W G; Metz J H M; Uenk G H; Phillips V R; Holden M R; Sneath R W; Short J L; White R P; Hartung J; Seedorf J; Schro¨ der M; Linkert K-H; Pedersen S; Takai H; Johnsen J O; Wathes C M Concentrations and emissions of ammonia in livestock buildings in Northern Europe. Journal of Agricultural Engineering Research, 1998, 70, 79—95 Seedorf J; Hartung J; Schro¨ der M; Linkert K-H; Phillips V R; Holden M R; Sneath R W; Short J L; White R P; Pedersen S; Takai H; Johnsen J O; Metz J H M; Groot Koerkamp P W G; Uenk G H; Wathes C M Concentrations and emissions of airborne endotoxins and microorganisms in livestock buildings in Northern Europe. Journal of Agricultural Engineering Research, 1998, 70, 97—109
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Wathes C M; Randall J M (eds) Aerosol sampling in animal houses. Commission of the European Communities, Luxembourg, Eur. 11877, 1989 Health and Safety Executive Occupational exposure limits 1992. EH 40/92, Her Majesty’s Stationary Office, London, 1992 Dawson J R Minimizing dust in livestock buildings: possible alternatives to mechanical separation. Journal of Agricultural Engineering Research, 1990, 47, 235—248 Groot Koerkamp P W G Review on emissions of ammonia from housing systems for laying hens in relation to sources, processes, building design and manure handling. Journal of Agricultural Engineering Research, 1994, 59, 73—87 van Ouwerkerk E N J; Pedersen S J Application of carbon dioxide mass balance method to evaluate ventilation rates in livestock buildings. In: Proceedings of XIIth World Congress on Agricultural Engineering, Milan, Italy 29 August—1 September 1994: 516—529