Introduction
Increasing evidence has shown that exposure to green vegetation in the vicinity of home is related to health. While several studies have reported beneficial associations between residential greenness and health outcomes, literature on respiratory health has been contradictory: some studies have reported beneficial but other studies harmful effects in relation to greenness.
According to some recent studies, residential greenness is associated with decreased risk of childhood asthma. However, other studies have reported harmful effects or inconclusive results associated with an increase in greenness. These observational studies have highlighted the complexity of such multidimensional relations between green spaces and asthma. Inconsistencies in previous findings could be explained by differences in the study design, geographic region of the study, outcome definitions, and exposure assessment in these studies. For example, exposure assessment has differed in the definitions used for green spaces and in the timing of studied exposures. The most commonly used indicator of greenness is the Normalized Difference Vegetation Index (NDVI), the values of which change throughout the year depending on the season and vegetation growth. However, in many studies the greenness exposure was measured at one point in time, which overlapped with the study period and was most frequently assessed during the summer (i.e. during maximum vegetation).
Although several studies have reported that the prenatal period represents the time window of environmental exposures related to special susceptibility to asthma, it is still unknown what kind of role the timing of exposure to green spaces during pregnancy plays for the risk of asthma. Previous observational studies have reported an association between greenness during pregnancy and the risk of asthma, but only Sbihi et al. and Lin et al. actually investigated the residential greenness during pregnancy and the risk of asthma and allergic diseases, and none of the previous studies evaluated trimester-specific exposure in the relation between exposure to greenness during pregnancy and childhood asthma. Even though greenness can be rather stable around homes, families often move during pregnancy or after the birth of a child due to the increasing need for space. Both changes in the residential address and season of pregnancy may influence cumulative exposure to greenness.
The objective of the present study was to investigate the potential association between residential greenness during pregnancy and early life and the development of asthma up to the age of 27 years, with special focus on trimester-specific and seasonal effects. We addressed this research question using the prospective population-based Espoo Cohort Study with a full residential history of the families during pregnancy and early life. We studied the timing of exposure during pregnancy by assessing trimester-specific exposure to greenness in relation to the development of asthma. Furthermore, we considered exposure to greenness during spring and summer separately to assess potential effect of the season on the studied relation.