Ecological associations between asthma prevalence and potential exposure to farming

Farming and exposure to livestock have been proposed as protective against the development of asthma in children. An ecological study was conducted to examine the support for these relationships in the USA. County-specific aggregate measures were used to examine the relationship between the prevalence of asthma and wheeze in 7th and 8th grade school children (aged 12-14 yrs) and selected measures of potential exposure to farming in the state of North Carolina. Binomial regression models were fitted to quantify these relationships, with adjustments for parental smoking, socioeconomic status, sex and race. Regression coefficients were reported for an increment of one interquartile range (IQR) in each indicator of exposure. The prevalence of asthma showed decreasing trends for most indicators of farm exposure. Regression results suggested that the largest decreases in asthma prevalence were associated with the number of farms (-5.0% per IQR increment), acreage of hay (-7.2% per IQR increment) and beef cattle (-7.8% per IQR increment). The pattern of results was similar for wheezing. In conclusion, the findings of this study are consistent with the hypothesis that certain farm exposures are protective against childhood asthma. Further research with individual-level data is needed to identify the specific protective exposures.