Synthesis with previous knowledge
Our findings are consistent with findings from three previous observational studies focusing on similar associations between prenatal exposure to greenness and childhood asthma. A population-based birth cohort study including children from New York City showed a positive association between high residential urban tree canopy cover at the time of recruitment in the third trimester of pregnancy and asthma at 7 years of age. Tischer et al. conducted a study in Spain and found that higher residential NDVI exposure at the time of birth was associated with an increased risk of asthma in the Euro-Siberian region (North Spain), but not in the Mediterranean region (South Spain). The authors suggested that non-native species in the green urban areas in the north might be potential sources of exposure to harmful allergens. Moreover, a study from an urban area of China reported a positive relation between residential NDVI during the entire pregnancy and any allergic disease by 2 years of age.
On the other hand, five previous observational studies reported contradictory findings suggesting negative or no clear association between maternal exposure to greenness during pregnancy and the risk of asthma. The results from a Portugues population-based birth cohort study provided evidence that high residential NDVI exposure at birth was associated with a lower risk of asthma at 7 years of age. In contrast, birth cohort studies from both Germany and USA reported inconsistent associations between NDVI at the birth address and asthma in childhood. Furthermore, a Canadian study found that annual increased average NDVI exposure during pregnancy was protective against asthma before school age, but they did not find any significant association with increased risk of different phenotypes of asthma, including transient and chronic asthma.
There are also two epidemiologic studies reporting an association between residential greenness during childhood and increased risk of asthma or allergy. Parmes et al. investigated 8 063 children from nine European population-based studies and reported that increased greenness in land cover was associated with an increased risk of lifetime and current asthma at the age from 3 to 14 years, but there was substantial heterogeneity between the results of these cohorts. In a recent study from Denmark combined information on land cover, biodiversity measured as the bioscore in the first two years of life, and diagnosis of childhood asthma from the registers and found out that green space was associated with a higher risk of asthma, but with a reduced risk of developing severe asthma.