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.