Introduction
Human health effects due to dust exposure has been an area of research in the last twenty years and today it is established beyound doubt that high dust exposure in animal confinement buildings is a respiratory health hazard.
Much has been learned about exposure under different working conditions and their relation to respiratory symptoms and disease and the interest has focused on work in swine confinement buildings because the highest exposure and the highest frequency of symptoms is found here. Poultry farming also carries a substantial, if not higher than swine farming, exposure to dust but the number of working hours spent inside and the number of persons employed is much lower than in swine farming. From a human health perspective dust exposure in pig farming is the most important because of the large number of people involved and the increasing number of working hours inside confinement buildings. Socio-economic changes with disappearence of smaller family-based farms and the development of agricultural industry in pig farming with all working hours spent inside the confinement building has increased exposure and will continue to do so in the future and will have a major impact on the respiratory health of the agricultural workers.
Exposure
The respiratory problems are mainly concerned with the high levels of dust exposure in poultry and pig farming. Much has been learned about dust concentrations, generation and transport of dust, and biological properties of dust in the last decade and the present state of knowlegde is summarized in (Pedersen et al. 2000, Ellen et al. 2000).
Generally exposure is very high compared to other work environments with a high content of microorganisms and endotoxins.
Evidence of harmful effects upon man
The inhalation of irritant substances causes inflammation in the airways which in some ways is analogous to smoking.
Several studies have demonstrated that working in pig confinement buildings is associated with symptoms of chronic bronchitis (cough and plegm), asthma-like symptoms like wheezing and shortnesss of breath during work, and with evidence of mild arways obstruction in cross-sectional studies. Some studies alo shows increased bronchial rectivity to irritants. Most important, dust exposure does not seem to be associated with the development of emphysema as in smokers. It must be emphasized, however, that the present evidence demonstrates that the effect of dust exposure is limited to the airways and not associated with cancer and cardio-vascular disease like smoking.Most studies have shown that work inside pig confinement buildings double or triple respiratory symptoms and that there is a clear dose-response relationship with the number of working hours inside buildings. It has also been learned from several studies that the many respiratory symptoms are not caused by sensitization to pig proteins in spite of high concentrations in the air. This is fundamentally different from the problems encountered by workers in laboratories with rodents like mice and rats where many persons become sensitized and develop allergic asthma. The reason for the lack of sensitization to pig proteins is not known. The problem is mostly irritation of the airways with other possible disease mechanisms (allergic alveolitis, allergic asthma, Organic Dust Toxic Syndrome, emphysema, lung fibrosis) being either rare or of no significance, for a general overview (Iversen 2000).
What is the major concern from the human health perspective in the future.
The present concern is about development of asthma in young people entering farming and about the development of chronic airways obstruction after many years of working in pig confinement buildings. The close association between asthma-like symptoms and dust exposure was once more confirmed in the largest study of farmers until now, the European study "Prevalence and Risk Factors of Airway Obstruction in Farmers" which includes 7988 farmers in Denmark, United Kingdom, Germany, Switzerland, and Spain (Radon et al., 2000. From the large prospective Danish study on young people trained in farming (Sigsgaard et al., 1999) the results from the first two years of follow-up demonstrated that the development of asthma was related to the total dust exposure with a relative risk of 1.0 in the lowest quartile rising to 5.5 in the quartile with the highest dust exposure).
Because of this dose-response relationship much effort has been put into studies which try to lower exposure under a certain threshold so that symptoms will not appear.
A study with dust reduction with an oil sprying method in a pig confinement facility (Senthilselvan et al., 1997) demonstrated substantial protection against acute effects on lung function measured by FEV1/FVC, bronchial reacticity measured by metacholine test, and inflammatory reactions measured by blood neutrophile counts in peripheral blood.
It is now beyond reasonable doubt documented that the major respiratory health problems in animal confinement buildings with heavy dust exposure, especially pig and poultry production, is airway inflammation caused by a non-allergic mechanism. This inflammation is associated with asthma-like work-related respiratory symptoms. The endotoxin content of the dust is probably the most important part of the dust for the inflammatory proces.
It has also been demonstrated that dust exposure is associated with an accelerated decline in FEV1 in pig farmers (Iversen et al. 1994, Senthilselvan et al. 1997, Vogelzang et al. 1998, Iversen et al. 2000). The extra loss in FEV1 approximately doubles in some farmers and will cause clinically significant disease in some farmers.
The acute reaction with its immediate development of symptoms and signs of inflammation seems to be related to the long term outcome (Schwarz et al. 1995, Kirychuk et al. 1998), a concept that was developed several years ago (Becklake 1995) and which has also been found in cotton industry (Glindtmeyer et al. 1994, Christiani et al. 1994). The acute reaction is now used for studying the effects of intervention and the most elegant and comprehensive study right now is the study by (Senthilselvan, Zhang et al. 1997) which clearly demonstrates the efficacy of this approach. This study also demonstrates that dust and endotoxin concentrations must be vey substantially lowered to abolish or significantly diminish acute inflammatory reactions in the airways.
The future establishment of threshold values for dust in confinement buildings
The exposure in present animal confinement buildings is still substantial (Takai 1999) and in many cases above the levels of endotoxin where long-term deleterious effects have been demonstrated in longitudinal studies in swine farmers (Vogelzang et al. 1998). We know from longitudinal studies of dairy and swine farmers that swine farmers do have an accelerated decline in FEV1, whereas dairy farmers do not (Iversen et al. 2000) and we also know that exposure in present European dairy farmers is 10-15% of the exposure in swine farmers (Takai et al.1999). These low values are below the values previously put forward as potential threshold values for working in swine confinement units (total dust 3.7 mg/m3, respirable dust 0.23 mg/m3, endotoxin 154 ng/m3) and recently proposed as threshold values in poultry houses (total dust 2.4 mg/m3, respirable dust 0.16 mg/m3, endotoxin 61 ng/m3) (Donham et al. 1999). There is thus an unresolved discrepancy between results from acute exposure studies and results from long term studies. Probably some of the explanation of this is the the use of different populations of persons where some are selected after years of exposure in farming and others, mainly the persons used in acute experiments, are socalled naive subjects to this environment. Important to consider is that the studies of Donham were done on farmers with years of experience.
The establishment of tresholds for exposure in animal confinement buildings is a high priority topic to avoid respiratory disease in farmers in the next generation.
References
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