Synthesis with previous knowledge
There are some previous studies which suggested that exposure to green
spaces may be protective against allergic rhinitis, although the
evidence has been inconsistent.15,34 A study including
1251 school children from Italian and Austrian alpine valleys showed an
inverse association between NDVI within 100 m around schools and the
prevalence of allergic rhinitis.39 The results from a
study that included seven birth cohorts suggested that the effect of
NDVI differed by geographic location; green spaces within 500 m were
inversely associated with allergic rhinitis in GINI/LISA North and PIAMA
cohorts, but positively associated in BAMSE and GINI/LISA
South.34 In addition, the modifying effect of outdoor
levels of NO2 was also heterogenous across those
cohorts. A protective effect of green spaces was observed in PIAMA
cohort among those who were exposed to middle levels of
NO2, but an increased risk was observed in BAMSE cohort
among those exposed to higher levels of
NO2.34 Ruokolainen et al.40 and Paciência et al. 41 also
found previously a protective effect of exposure to green spaces on
allergic sensitization in children, although this has previously been
reported as a risk factor for allergic rhinitis.42Also other studies have provided evidence that exposure to green spaces
may increase the risk of allergic rhinitis.34,36 Our
results showed how the season of exposure in early-life may be an
important factor that influences the development of allergic rhinitis:
our results suggested that NDVI exposure during spring, when pollen and
air pollution exposure is at its highest (in the study area), is
associated with a higher risk of allergic rhinitis. Seasonality of green
space exposure is particularly important when considering the
concentration of pollens. Green spaces may lead to a greater spread and
higher concentrations of allergenic pollen, which has been associated
with an increased prevalence of allergic diseases.43In Finland, pollen concentrations from birch trees, the most common
cause of pollen sensitization rates [34.0% (26.8-41.2%)], are
highest in April and May,44-46 which could explain the
observed results and the higher risk of allergic rhinitis during spring.
Although there is increasing evidence showing the influence of prenatal
and early-life exposures on the development of allergic
diseases,47 it remains unclear whether there is a
specific time period (i.e., developmental window) within which
individuals may be particularly sensitive to environmental exposures.
Our study provides novel insights on time-specific critical periods in
the life course – prenatal and early-life – during which exposures to
green spaces may affect human health. Our results showed that early-life
may be a critical period for exposure to environmental factors that
affect development of allergic rhinitis during late childhood and early
adulthood. Some previous studies have also suggested that early-life
exposure to green spaces may enhance immune function so that it protects
against inappropriate inflammatory responses and development of allergic
diseases.48-50