In 1998 pesticide exposure was listed as one of the top respiratory health hazards in agricultural by
the American Thoracic Society [183]. Nevertheless, Kirkhorn and Garry in their review of agricultural
lung diseases suggested that, although there were notable exceptions, pesticides may not be either a
single or direct cause of chronic pulmonary diseases, such as asthma [184]. Other than pesticide
exposures, many factors such as variations in genetic makeup, physiological states, socioeconomic and
psychosocial factors, and other environmental factors may also contribute to the development of
respiratory symptoms and diseases. These factors must be considered along with pesticide exposures
when evaluating the effects of pesticide on respiratory health.
In occupational settings, it is often difficult to identify causes of disease because responses may be
delayed and so occur at home, or may even occur many years later for diseases with considerable
latency [75]. It is also critical when performing a study to use appropriate ‘normal controls’ to ensure
results are compared between comparable groups. In studies of occupational disease, a ‘healthy worker
effect’ is often observed, i.e., workers usually have lower rate of disease than the general
population [185]. This phenomenon is due to the exclusion of persons with disease from
employment [185]. Therefore, the effect of pesticides on respiratory health in occupational settings
may be underestimated when comparing with general populations. Le Moual et al. showed that a
“healthy worker effect” can also occur after employment commences when sick workers leave their
jobs perceived as “risky” and find new jobs with less exposures, or become more careful to avoid
being exposed within the same job [186]. The author suggested that many cross-sectional studies of
occupational asthma lacked sufficient information of health and job status both before and after
employment [186]. Spurious results can be obtained without taking account of the ‘healthy worker
effect’ into study design, data collection and statistical analyses.
McCauley et al suggested that valid diagnosis or confirmation of symptoms, diseases, or biological
markers of a health effect are critical for effectively studying the association between pesticide
exposures and health outcomes [187]. Data sources on health outcomes of occupational pesticide
exposures include workers’ compensation (WC) systems, hospital and occupational medicine specialist