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.