Greening cities saves lives and transforms urban health

By Priyanjana Pramanik, MSc.Reviewed by Susha Cheriyedath, M.Sc.Dec 2 2024

Italian study reveals how urban greenery reduces deaths and promotes well-being, urging decisive action to make green spaces accessible to all.

Study: The mortality impacts of greening Italy. Image Credit: Fahroni / Shutterstock

In a recent study published in the journal Nature Communications, researchers estimated the health benefits of increasing green spaces in residential areas across Italy. They concluded that more green spaces lead to fewer deaths, highlighting the need for more decisive action to increase green space accessibility in urban areas.

Background

Green spaces, including greenery, playgrounds, and parks, are known to promote physical and mental health and reduce the burden of mortality and morbidity. This relationship may arise because green spaces encourage activities like walking and cycling, reduce heat, noise, and air pollution, lower stress, and improve immunity through exposure to diverse natural microbes. The World Health Organization (WHO) guidelines recommend that all homes should have at least half a hectare of green space within 300 meters.

Researchers use a metric from satellite data called the Normalized Difference Vegetation Index (NDVI) to measure the amount of vegetation in urban areas. They have shown that higher NDVI values have a negative association with mortality, with a pooled relative risk of 0.96 per 0.1 NDVI increase within 500 meters of homes, according to a meta-analysis. One study found that increasing NDVI in metropolitan areas in the United States could prevent up to 38,187 deaths among older adults, while a similar study in Europe found that more green spaces could save up to 42,968 lives. However, despite the significant associations, these studies did not conclusively prove that greening and no other factors decreased mortality.

About the Study

In this study, researchers estimated how much mortality could be reduced in Italy by increasing green space in residential areas. They conducted a nationwide study that used high-resolution satellite data at 10 meters.

The NDVI in the greenest period for the year 2022 and land cover data for 2021 were analyzed at a 10-meter resolution for areas within 300 meters of homes. Population-weighted exposures (PWE) to NDVI were calculated for all municipalities, representing varying levels of greenness. The total population of adults in Italy in 2022 was 48,628,328 individuals. The 95th, 75th, 50th, and 25th percentile NDVI proportions were 0.54, 0.43, 0.36, and 0.30, respectively. Counterfactual exposure for the Health Impact Assessment (HIA) was set at the 75th percentile (NDVI = 0.46), corresponding to a tree cover of approximately 30% within 300 meters of residences.

For the HIA, the analysis focused on 39.8 million adults in 3,720 municipalities with a PWE below the 0.46 NDVI target. An additional analysis of land cover types included the proportion of built-up areas, green areas, and tree cover using Spearman correlations and generalized additive mixed effects models (GAMM).

Findings

In the HIA, researchers approximated preventable deaths in Italy in 2022 to be approximately 28,433 (95% CI 21,400–42,350). Deaths were higher in provincial capitals, particularly those with populations of 120,000 or more.

The preventable years of life lost were 279,324 (95% CI 210,247–415,980), predominantly in provincial capitals and among people over 80. Specifically, greening could prevent 2.1% of deaths in less populated areas and up to 6.3% in more densely populated areas.

The analysis of land cover found a negative correlation between the greenest NDVI and built-up areas. However, positive relationships were seen with green areas and tree cover, highlighting the critical role of tree density in improving NDVI and reducing mortality. Sensitivity checks adjusting the terms of the model and excluding sparsely populated areas did not significantly change the findings.

Conclusions

Larger cities (provincial capitals) show higher mortality reductions from greening (up to 6.3%). In contrast, smaller, less populated municipalities showed lower reductions (as low as 2.1%). The greenness target is set at 0.46 NDVI based on the greenest 25% of the population and aligns with WHO recommendations for 300-meter residential greenness.

Tree cover significantly influences green space benefits, with built environments decreasing greenness. Urban trees reduce pollution, heat, and stress while improving social cohesion, physical activity, and mental health. Trees also mitigate urban heat islands, reducing risks during heat waves. Greening interventions could include planting more trees, creating green corridors, adding green roofs, and restoring industrial areas.

The study's strengths include its nationwide scope, high-resolution satellite imagery, and detailed mortality and exposure data, making its findings replicable globally. However, limitations include the reliance of results on the assumption that residential exposure equals the overall exposure of an individual, which may not be true. Researchers assumed that all population groups equally benefit from greenness and that there is a linear relationship between greenness and mortality. This study also did not consider whether green spaces are accessible or their quality.

Overall, the findings indicate that increasing green spaces in residential areas could significantly reduce mortality. The researchers advocate for strong policy actions to enhance the quantity and accessibility of urban green spaces to improve public health.