Summary
Objectives: The aim of this study was to investigate the associations between surrounding greenness levels and asthma among children, and to explore a possible change of this association by the distance of the residence to a city park.
Design: A nested case–control study.
Setting: Children aged 4−6 years residing at their
current address since birth in Kaunas, Lithuania, whose mothers were recruited in 2007–2009 to the KANC newborns cohort study.
Participants: The participants were 1489 children whose parents in 2012–2013 filled in the questionnaires and agreed to participate in the study. Primary and secondary outcome measures: We estimated clinically diagnosed asthma risk factors. The surrounding greenness was measured as the average of the satellite-based Normalised Difference Vegetation Index (NDVI) within the buffers of 100, 300 and 500 m from each child’s home address, and the distance to a city park was defined as the distance to the nearest city park. Multivariate logistic regression was performed to study the relationship between the greenness exposures and asthma adjusted for relevant covariates. Results: An increase in the NDVI (>median) in buffers of 100, 300 and 500 m was associated with a slightly increased risk of asthma, while an IQR increase in NDVI-100 m statistically significantly increased the risk of asthma (OR 1.43, 95% CI 1.10 to 1.85). The stratified analysis by surrounding greenness revealed indications of stronger associations for children with higher surrounding greenness (NDVI-100>median) and those living farther away from parks (>1000 m), compared to NDVI-100≤median and the distance to a city park >1000 m (OR 1.47, 95% CI 0.56 to 3.87). Conclusions: A higher level of the surrounding greenness was associated with a slightly increased relative risk of asthma in children. Further investigation is needed to elucidate the influence of city parks and neighbourhood greenness levels on asthma.

Relevance
Inconsistent associations between green space expos- ure and childhood asthma have stimulated studies on the relations between the surrounding greenness levels, the distance from the place of residence to city parks and asthma. This is the first study in Eastern Europe to evaluate the long-term impact of quantified greenness levels in the urban environment on the risk of asthma in 4−6-year-old children. In the present study, using individual-level data of a population-based sample and adjusting for important risk factors for asthma, we inves- tigated the associations between objectively estimated residential greenness levels as the NDVI, residential dis- tance to the nearest city park (>1 hectare (ha)), and the number of children clinically diagnosed with asthma controlling for various covariates. Furthermore, in the stratified analysis, we investigated the joint effect inter- action between surrounding greenness, distance to a city park and the risk of asthma in children controlling for PM2.5 and NO2 influence.