New-onset asthma and the effect of environment and occupation among farming and nonfarming rural subjects.

Background: Many factors, including environmental exposures, have been related to the increase in the prevalence of asthma, but only few have been tested for in longitudinal studies.

Objective: We studied farming students to determine whether their environment during childhood and as adults was a factor determining subsequent onset of asthma.

Methods: From 1994 to 1998, new cases of asthma were identified by means of an annual posted questionnaire followed by a telephone interview in a prospective cohort consisting of 1964 farming-school students and 407 nonfarming subjects aged 16 to 26 years. For each case, we selected a control subject from the cohort with no asthma in a case-based design, and all underwent an interview and a clinical examination.

Results: We found 122 new cases of asthma. In a multiple regression model the odds ratio for new asthma was 3.3 (95% CI, 1.7-6.3) for smoking; 3.4 (95% CI, 1.6-7.0), 2.5 (95% CI, 1.1-5.3), and 7.0 (95% CI, 1.2-41.6) for exposure to swine, dairy production, and welding, respectively; and 11.7 (95% CI, 2.4-56.4) for bronchial hyperresponsiveness at baseline. Being born and raised on a farm significantly reduced the risk odds ratio (0.5 [95% CI, 0.3-0.98]), whereas atopy had no influence.

Conclusion: Exposure to swine and dairy confinements, welding, smoking, and bronchial hyperresponsiveness are risk factors for nonallergic asthma, and being born and raised on a farm reduces the subsequent risk. These findings support the theory that immune and inflammatory responses can be influenced by environmental exposure to early childhood, reducing the risk of asthma later in life.