Biological plausibility
Suggested mechanisms for the beneficial health effects of residential green spaces include protection against inflammatory responses through more diverse microbial exposure, and improvement of air quality due to removal of air pollutants. In the present study, there was little evidence of any protective effect of greenness on the risk of asthma. In contrast, the results indicated adverse effects of maternal exposure to greenness during pregnancy on the risk of developing asthma in childhood. This effect was most prominent with exposure to high density of greenness in the spring. The mechanisms underlying such adverse effects are not yet fully understood, but are likely to be related to allergens, such as pollen, released into the air from high density of vegetation biomass during the spring season.
None of the previous studies had evaluated the role of the timing of exposure during pregnancy (e.g. trimesters) for the effect of exposure to greenness during pregnancy on the risk of asthma. It has been suggested that during the third trimester of gestation and the first year of life the immune system maturates towards a balanced Th1/Th2 response, which is critical for healthy immunity. As environmental exposures acting during these critical time periods of pregnancy are likely to influence the development of the immune-related diseases, such as asthma and allergy, it is important to fill in this gap in the knowledge.