Complex interplay between greenness and air pollution in respiratory health.

For many, it seems self-evident that a greener city improves the quality of life and health for its citizens. Epidemiological studies are providing increasing evidence that the presence and amount of vegetation around locations where one spends a lot of time (home, work and school) have numerous beneficial effects on physical and mental health, including increased longevity.1 Interestingly, the evidence supporting a positive role of vegetation on allergic and respiratory health is much weaker, possibly because of the complex role played by air pollution. Beneficial effects attributed to vegetation may be due to less air pollution, as levels tend to be lower in vegetated areas. Some vegetation types may also actively reduce air pollution levels, although the scientific consensus is that this occurs only to a limited extent.2 Air pollutants may also interact with vegetation, such as pollen,3 to influence associations with health. These mechanisms are relevant in urban settings where air pollution levels can be high. Various metrics are used to estimate ‘vegetation exposure’ in epidemiological studies. The Normalised Difference Vegetation Index (NDVI) is common and assesses surrounding levels of green vegetation, often termed ‘greenness’. The NDVI is calculated from satellite data giving the ratio of visible and near-infrared light reflection and ranges from −1 (water), 0 (barren rock) to +1 (dense green vegetation). It provides a measure …