Asthma is characterized by airway inflammation, increased airway responsiveness to a variety of stimuli, and generally reversible airway obstruction either spontaneously or with treatment (99-104). In occupational asthma, there is variable airway obstruction and/or airway hyperresponsiveness due to workplace exposure(s). There are two major types of occupational asthma:

1. Sensitizer-induced asthma, characterized by a variable time during which

"sensitization" to a high or low molecular weight agent present at the work site occurs

2. Irritant-induced asthma

Reactive airways dysfunction syndrome is a type of irritant-induced asthma secondary to acute, high-level exposure to an irritant such as anhydrous ammonia on the farm. Work-aggravated asthma is defined as preexisting or concurrent asthma worsened by irritants in the workplace. Exposure to organic dust can aggravate preexisting asthma, through allergic or irritant mechanisms (13,96).

Epidemiological studies of adult farmers have shown prevalence rates for asthma equal to or lower than that of the general population. Recent prevalence rates for asthma in farmers are in the range of 3.7% to 11.8%. However, the prevalence of wheezing in farmers has been found to increase with cumulative years of farm work performed from a level of 1.5% among farm work ers with less than 1 year to 4.7% among those with more than 10 years of this work. Specific agricultural jobs, such as production related to livestock and work with farm machinery and equipment, are among occupations with significantly elevated mortality for asthma with a proportionate mortality ratio (PMR) of 1.51. In addition, agricultural production related to crops had a PMR of 1.10. Although agricultural work was associated with an elevated PMR for asthma mortality, agriculture was not among the primary three industries with the greatest number of work-related asthma cases in the areas that were surveyed (98-105).

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