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.