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22 So, microbial exposure during infancy may promote development of non-allergic phenotype and decreased risk of allergic diseases and asthma,1 but it may also be a risk factor for occupational asthma, and other occupational respiratory diseases such as organic dust syndrome and chronic bronchitis.4 In the present study, farming environment during infancy was associated with higher FEV1 and FVC but not with FEV1/FVC. These spirometric findings suggest that a non-farming environment is associated with lower lung volumes, not airflow limitation. This is an unexpected finding, since farming environment  is mainly associated with obstructive disorders. However, as our lung function measurements [https://en.wikipedia.org/wiki/PDE4B PDE4B] consisted of FEV1 and FVC, the type of lung function impairment cannot be definitely identified. One potential mechanism for the present findings suggesting lower lung volumes in a non-farming population may be obstruction in small airways caused by patchy collapse in early exhalation. In  such a case, FEV1 and FVC are decreased and FEV1/FVC is normal.23 However, the possible contribution of airway obstruction on the suspected restrictive-type impairment would require a bronchodilator test. Confirmation of a true restrictive abnormality would require total lung capacity measurement either with a body plethysmograph or using gas dilution methods.24 We have previously shown, in this cohort, that an early farming environment is associated with reduced risk of atopic sensitisation, asthma and atopic diseases at the age of 31?years.2 These associations are, therefore, potential explanations for the present observations. However, in additional analyses, when we further adjusted for atopy and doctor-diagnosed asthma, the size of the estimates for the association between early childhood farming environment and current lung function were reduced by 50% at the most. This suggests that although atopy and asthma probably mediate the effects  on improved lung function, other pathways are also likely to be operational. These results imply that an early farming environment may improve lung function, however, not simply by preventing development of atopy and asthma. Differences in physical activity could also explain the observed differences in lung function. Physical activity is shown to be associated with higher lung function, measured with FEV1 in adulthood, in men and women of all ages.25 In the present study, analyses were adjusted for current, but not for childhood, physical activity. It is possible that children living on farms have a higher level of physical activity, as they may participate in physically demanding tasks on farms, as compared with their peers living in non-farming environments.