The mortality impacts of greening Italy

Rome: Green spaces have been shown to be beneficial to humans, but quantifying these benefits is a challenge for epidemiology. In this health impact assessment study, we exploit satellites to estimate for the whole of Italy the number of deaths that could be prevented in the 49 million adult population by greening residential areas.

The exposure was assessed by calculating the normalized difference vegetation index at 10-m resolution within a 300-m distance from homes in 7904 municipalities. In this study we estimate, by achieving nationwide the level of residential greenness currently reached by the 25% of the population, a total of 28,433 (95% confidence interval: 21,400–42,350) preventable deaths and 279,324 (210,247–415,980) preventable years of life lost in Italy in 2022, representing the 5% of the total mortality burden. More green means fewer deaths, thus strong action is needed to increase the amount and accessibility of green spaces in all human settlements.

Green spaces, such as parks, playgrounds, and residential greenery, have been shown to have beneficial effects on people. They can promote mental and physical health and reduce morbidity and mortality among citizens1,2,3,4,5,6,7,8,9,10,11,12. The normalized difference vegetation index (NDVI) is a widely used satellite-derived metric that can quantify the vegetation in human settlements (e.g., street trees or general vegetation in public and private spaces)11,13. Several studies have highlighted the inverse relationship between NDVI and mortality2,3,4,5,6,7,8,9,10. Based on this evidence, a meta-analysis has estimated that the pooled relative risk (RR) for all-cause mortality per 0.1 increase in NDVI within a buffer of 500 m or less from a participant’s home was 0.96 (95% CI 0.94–0.97)11.

Several hypotheses have been considered to explain these findings. It has been proposed that physical activity is an important health determinant associated with green spaces. Green spaces can be places for recreational physical activity. In addition, green areas may encourage walking and cycling as forms of active transport1,7,11,12,14,15. However, a mediation analysis of one of the studies4 included in the meta-analysis showed that physical activity accounted for 2.1% (95% CI 0.2;19.3%) of the association between green space and mortality11. Greenness can also have a number of beneficial health effects through ecosystem services, and has been shown to have a protective effect by reducing air pollution, noise, and the heat island effect11,12,16,17,18. Air pollution was included as a covariate in some of the studies2,3,6 included in the meta-analysis, but the resulting RRs are not significantly different from those of studies that did not consider air pollution. After including air pollution in a mediation analysis, James and colleagues4 found that PM2.5 (fine particulate matter) could explain 4.4% (95% CI 2.4;7.7%) of the association between greenness and mortality, while Vienneu et colleagues6 estimated a mediation of 2.4% (−0.2;5.5%) by PM1011.

Other proposed mechanisms to explain the health benefits of green spaces include stress reduction, enhanced relaxation, and restoration. One theory that explains the benefits of being in green spaces is the psychosomatic stress reduction theory. According to this idea, exposure to natural environments, such as views of them, could help people who are stressed out by putting them in a more positive emotional state1,19,20. In another study conducted in Dutch cities, the relationship between urban greenery and perceived general health was shown to be most strongly mediated by stress and social cohesion21. Finally, there is evidence linking green spaces to immunological function22. Li and colleagues23,24 found an association between forest visits and enhanced immune responses, including the expression of anti-cancer proteins (such as granzymes A and B, granulysin, and perforin). It has also been suggested that exposure to a variety of microbes in their natural habitats may regulate immunity25.

In terms of policy, the World Health Organization (WHO) suggests that a minimum of 0.5 hectares of green space should be available within a 300-m linear distance from homes1. The 3–30–300 rule for urban greening was proposed by urban forester Cecil Konijnendijk. It states that every home, school, and workplace should have at least three well-established trees in sight; every neighbourhood should have at least 30% tree cover; and every residence should be no more than 300 m from the nearest large public green space26. One way to link epidemiological evidence to public health policy is through health impact assessment (HIA), which combines meta-analytic effect measures with exposure and outcome data to estimate the health impacts under a counterfactual scenario11,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43.

Using the exposure-response function from Rojas-Rueda et colleagues11, two large-scale health impact assessment studies were conducted in urban areas in the United States and in Europe to estimate the impacts of greenness on mortality27,28. According to the American HIA study27, an increase of 0.1 NDVI units at the census tract level in the 35 most populous metropolitan areas could have prevented an estimated 38,187 deaths of people aged 65 and older in 2019. According to the European HIA study28, which analysed 978 selected cities and 49 greater cities using 250-m resolution NDVI data, achieving a greenness level estimated to be in line with WHO recommendations1 for access to green space could have reduced deaths by 42,968 in 2015.

However, the health benefits of exposure to greenness are likely to occur in all human settlements, not just selected cities or the more urbanised areas4,7,11. In addition, the use of different counterfactuals for each city (0.1 NDVI increase for all cities in the American study27 and city-specific modelled targets in the European study28) may limit the comparability of the areas evaluated. Finally, there is uncertainty in the choice of target exposures due to the lack of specific recommendations for NDVI values1,27,28.

Therefore, it may be worth exploring different methodological strategies and benefiting from updated and high-resolution data to assess the health impacts of greening interventions in all human settlements. To ensure comparability, it may be desirable to conduct these assessments using a single counterfactual exposure for all areas. Ideally, this NDVI target exposure should be as realistically achievable as possible, population-based, and possibly related to the existing green space recommendations. Finally, it would be desirable for this assessment to be replicable as far as possible around the world, using available software and data.

In the present study we want to meet all these needs by capitalising on satellite-derived residential exposure data at 10-m resolution for the whole of Italy in 2022. The aim of this health impact assessment is to estimate the total adult mortality burden that could be prevented in all Italian municipalities by greening residential areas up to the level of greenness currently achieved by the 25% of the population.

The total adult population (≥20 years) in all 7,904 Italian municipalities in 2022 is 48,628,328. All exposures to greenest period NDVI (2022) and to specific land cover classes (2021) at 10 m resolution were calculated within 300 m of homes (population-buffers). Maps of greenest period NDVI, population-weighted exposure (PWE) to greenest period NDVI, and population counts are shown in Figs. 1 and 2 and Supplementary Fig. 1, respectively. Population-weighted percentiles of exposures to greenest period NDVI, exposures to land cover classes, and adult population counts are reported in Supplementary Table 1. The 25th, 50th, 75th and 95th population-weighted percentiles of PWEs to greenest period NDVI in all the Italian municipalities are 0.30, 0.36, 0.43 and 0.54, respectively. The 25th, 50th, 75th and 95th population-weighted percentiles of greenest period NDVI in all the Italian 300 m population buffers are 0.27, 0.36, 0.46 and 0.60, respectively. The unweighted percentiles of these measures and the weighted percentiles of these measures when excluding sparsely populated areas in the exposure assessment are reported in Supplementary Tables 2 and 3, respectively.

The results of the HIA are reported in Tables 1–3 and in Supplementary Figs. 2 and 3 for all the 39,803,860 adult inhabitants of the 3720 Italian municipalities that have a PWE to greenest period NDVI below the counterfactual exposure of 0.46 in 2022. Considering the whole of Italy, the PWEs to tree cover proportion, green area proportion and greenest period NDVI are 21.2%, 32.3% and 0.33, respectively.