Living near greenery protects against asthma

A recent study suggests that there is a nontherapeutic way to lower the risk of asthma, and that is living in greener areas.

In the UK Biobank cohort, participants at the highest quartile of exposure to residential green space, assessed using the normalized difference vegetation index (NDVI) at a 500-metre buffer, had a roughly 9-percent lower risk of incident asthma compared with those at the lowest exposure quartile (hazard ratio [HR], 0.9113, 95 percent confidence interval [CI], 0.8634–0.9619; p=0.000756), reported principal investigator Dr Mingkai Huang from the State Key Laboratory of Respiratory Disease at Guangzhou Medical University in Guangzhou, China. [ERS 2024, abstract OA969]

For each interquartile range increase in NDVI, the risk of incident asthma dropped by around 3 percent (HR, 0.9717, 95 percent CI, 0.9553–0.9883; p=0.000899).

The estimates were slightly attenuated but remained significant when green space was measured at 1,000 metre surrounding the residence (quartile 4 vs 1: HR, 0.9473, 95 percent CI, 0.8973–0.9996; p=0.048; per interquartile range increment: HR, 0.9793, 95 percent CI, 0.9628–0.9961; p=0.0159).

In mediation analysis, 32.5 percent of the effect of residential green space on asthma risk was mediated by PM_2.5 (p=0.026).

Huang noted that the association between NDVI and asthma incidence was stronger among participants with increased predisposition to asthma (high polygenic risk score), among nonsmokers, and among those in poor socioeconomic status.

Greenery and severe asthma

Exposure to residential greenness also proved to be beneficial to participants with severe asthma in the Chinese C-BIOPRED cohort. However, its protective effect was observed only in the nonsmoking population (quartile 4 vs 1: HR, 0.8314, 95 percent CI, 0.7409–0.9331; p=0.00181; per interquartile range increment: HR, 0.9570, 95 percent CI, 0.9225–0.9928; p=0.0192).

More importantly, a higher level of residential green space was associated with improvements in asthma-related indicators such as eosinophil count, Asthma Control Questionnaire – 5 score, Asthma Quality of Life Questionnaire score, and exacerbation times.

Taken together, the findings from the UK Biobank and the Chinese C-BIOPRED cohorts provide evidence of the beneficial role of sustainable natural environments in people’s respiratory health, Huang said.

Greenery and cleaner air

In a separate presentation, Prof Alessandro Marcon from the University of Verona in Verona, Italy, highlighted the role of trees in urban areas in removing air pollutants and improving the health of cities, further supporting the idea that green spaces are essential for respiratory wellbeing.

“Trees remove particulate matter (PM) by dry deposition, and gases such as sulfur dioxide (SO₂), ozone, and nitric oxide (NO) are removed by stomatal uptake in the leaves,” Marcon explained.

Indeed, a US study showed that vegetation significantly reduced air pollutants, removing up to 30 percent of SO₂, 10 percent of PM₁₀, 11 percent of PM_2.5, and 14 percent of nitric dioxide (NO₂). [Environ Sci Technol 2019;53:13228-13237]

“If we look at each county, we could restore tree coverage by covering grasslands and shrublands with trees. At the average level of tree coverage for each county in the US, we could have an additional 20 percent to 37 percent reduction in air pollution by vegetation,” Marcon noted.

However, he stressed that urban forests can only complement but not replace air pollution emission control.