Introduction
The
world has experienced considerable growth of urban areas in recent
decades, with approximately 54% of the world population living in
cities.1 While this development provides some
benefits, such as better health care, education and social services,
unplanned, uncontrolled and rapid urbanization has also been associated
with environmental degradation, land changes, as well as loss of green
areas.2 There is increasing evidence that exposure to
greenness may improve human health. A recent review suggested that
exposure to green spaces can substantially improve physical and mental
health and wellbeing, and reduce the risk of adverse birth
outcomes.3 Moreover, green spaces may reduce the
adverse effects of environmental exposures to air pollution, noise,
extreme temperatures, and loss of biodiversity, while providing benefits
for citizens and producing
economic
value by increasing the quality of landscapes.4 At
the same time, there has been an increase in the incidence of chronic
inflammatory diseases, including allergy,5 which
cannot be explained by genetic reasons.
Epidemiological studies on the association between urban green spaces
and development of allergic diseases have reported inconsistent
results.6,7 This heterogeneity of results may be
related to different definitions of green spaces (i.e., exposure),
seasonal qualitative and quantitative variation in greenness, features,
proximity and accessibility of green spaces, timing and duration of
exposure, exposure to other environmental factors, such as air
pollution, as well as differences in the study design and outcome
definitions used.6 Several studies have reported
pregnancy and early-life being important critical time periods of
exposure.8,9 A recent review suggested that a variety
of environmental factors acting on the mother during pregnancy may have
long-term effects on the immune system of their children, which will
affect their susceptibility to develop inflammatory diseases, including
allergic diseases.8 Garcia-Serna et al.9 also emphasized the first two years of life as the
critical time period for the development of immune system, reporting
that both prenatal and postnatal environmental factors can induce an
unbalanced Th1/Th2 response, thus increasing the risk of inflammatory
diseases later in life.
Furthermore, exposure to ambient
air pollutants during pregnancy and early-life have been reported to
increase the risk for development of asthma and allergic
diseases10,11, but green spaces may mitigate such
effects.12,13 However, potential association between
exposure to green spaces during pregnancy and early-life and occurrence
of allergic diseases has been explored less.14Furthermore, greenness has spatio-temporal variation, which depends on
the climatic conditions. The season of exposure is particularly
important due to increased pollen exposure in green spaces in certain
seasons, as increased pollen has been associated with an increased
specific IgE sensitisation and allergic responses, which have been shown
to play a role in the development of allergic
diseases.15
We hypothesised that prenatal and early-life exposure to residential
green spaces reduce the risk of allergic rhinitis in childhood and young
adulthood. We tested this hypothesis in the longitudinal Espoo Cohort
Study by estimating the associations between exposure to green spaces
during pregnancy and early-life, in both spring and summer seasons, and
development of allergic rhinitis up to 27 years of age. We also
elaborated the role of exposure to air pollution as a modifier for the
studied associations.