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.