Asthma and lower airway disease
New-onset asthma and the effect of environment and occupation among farming and nonfarming rural subjects

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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.

Section snippets

Methods

Between 1992 and 1994, all of the 1964 farming-school students in Denmark who were in their second of 3 terms (mean age, 19.2 years; range, 16-26 years) together with 407 nonfarming rural subjects recruited from conscripts into the army were invited into a prospective cohort designed to study the risk factors for asthma development. The nonfarming rural subjects were living in a parish of no greater than 10,000 inhabitants, and they had no intention of making a career in farming. During the

Results

There were 122 new cases of asthma identified, with 107 of these being from the farming students and 15 from the nonfarming rural subjects. Of the subjects with new asthma, 87% were male compared with 91% of the nonasthmatic subjects being male. Table II describes the demographic characteristics of the 112 subjects with new asthma (cases) and the 116 subjects who had not had asthma (control subjects). The age distribution was the same in cases and control subjects. Among farming students, the

Discussion

We have identified that baseline BHR, smoking, swine and dairy production, and welding all increase the risk of onset of asthma in young adults. More importantly, we also found that being raised on a farm significantly reduced the subsequent risk of asthma. This is the first evidence from a longitudinal study of environmental exposure protecting against the subsequent development of asthma.

Several cross-sectional studies in children have shown less atopy, hay fever, and asthma present in

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    Supported by the Danish Medical Research Council, the Danish Agricultural Research Council, Helsefonden, and the P.C. Petersens foundation.

    Disclosure of potential conflict of interest: O. F. Pedersen has received royalties from GM Electronics Kilwinning, United Kingdom. T. Sigsgaard has received research support from the Danish Work Environment Research Fund and the Danish Medical Research Fund. The rest of the authors have declared that they have no conflict of interest.

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