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