April 24, 2019

Interdisciplinary research on the health benefits of contact with nature has been growing in recognition considering contemporary interest in urban greening. Air quality improvement via air pollution reduction by urban trees is routinely cited in popular media and promoted as a strategy to reduce asthma. If you are in a densely forested area, there are fewer cars and other pollution sources, which can lead to the “displacement” of emission sources that are causing poor air quality. However, this effect has limits in an urban environment.

A recent interdisciplinary review indicates that the relationship between urban trees, air quality and asthma may be more complicated than expected. Dr. Gina Lovasi, Professor and Co-Director of the Urban Health Collaborative, Dornsife School of Public Health, was one of the authors of the review. The authors assessed the scientific literature with an emphasis on empirical studies on the biophysical process through which urban trees affect air quality, as well as studies of asthma-related outcomes across space and time. Monitoring air quality in urban environments has not consistently found lower levels of harmful particles in the urban areas with more trees.

Urban trees can degrade air quality and may even increase asthma. Trees produce pollen, a major contributor to seasonal allergies that can trigger asthma exacerbations. In prior work analyzing hospitalizations over time, spikes in rates that correspond with the highest tree pollen concentration can be identified. The relationship between urban trees, air quality, and asthma are very complex, and there are substantial differences in how natural science and epidemiology approach these issues.

Other health outcomes, such as physical activity and mental health, have seen trends pointing toward more positive health effects of trees. If the health benefits from trees depend on how people perceive them and subsequently feel, more planning needs to be put into where and how trees are planted and select species with lower allergenic pollen production to limit harms.

This review was published in Landscape and Urban Planning.