By Seema Gupta, MD, MSPH

Clinical Assistant Professor, Department of Family and Community Health, Joan C. Edwards School of Medicine, Marshall University, Huntington, WV

Dr. Gupta reports no financial relationships relevant to this field of study.

SYNOPSIS: A national prospective cohort study of nurses found that higher levels of green vegetation were associated with decreased mortality.

SOURCE: James P, Hart JE, Banay RF, Laden F. Exposure to greenness and mortality in a nationwide prospective cohort study of women. Environ Health Perspect 2016 Apr 14 [Epub ahead of print].

For the first time in history, a majority of humanity is urbanized, with more than 50% of the planet’s 7.1 billion people living in urban areas. As population and urban development increases at an unprecedented rate, it creates a tremendous stress on local, regional, and global air and water quality. Urbanization leads to land use and land cover change, which is a major driver of global environmental change. The growing disease burden in urban areas attributable to nutrition and lifestyle choices remains a major public health challenge. For example, urbanization is associated with profound changes in diet and exercise that in turn increase the prevalence of obesity with attendant increases in risk of type 2 diabetes and cardiovascular disease.1 Growing evidence suggests close contact with nature benefits health. There is emerging research and policy interest in the potential for using the natural environment to enhance health and well being. Positive health effects of green space have been observed on cardiovascular diseases, mental health, self-reported general health, sleep patterns, and longevity, just to name a few.2 Similarly, the use of green space has been associated with higher levels of physical activity and lower levels of obesity within communities.3 Evidence also supports the hypothesis that living in areas with higher amounts of green spaces reduces mortality, mainly related to cardiovascular disease.4 However, this evidence is limited in scope, and many studies have relied on aggregated data limiting inferences regarding the effect of greenness on individual health.

James et al used data from the Nurses’ Health Study prospective cohort to examine greenness around the homes of 108,630 adult women. Researchers mapped home locations and used high-resolution satellite imagery to determine the level of vegetation within 250 meters and 1,250 meters of the homes. Researchers followed study participants between 2000 and 2008, tracking changes in vegetation and participant deaths. During the study, 8,604 participants died.

The authors analyzed models after adjusting for mortality risk factors such as age, race/ethnicity, smoking, and individual- and area-level socioeconomic status. Researchers found that women living in the highest quintile of cumulative average greenness (accounting for changes in residence during follow-up) in the 250 meters area around their homes experienced a 12% lower rate of all-cause, non-accidental mortality (95% confidence interval [CI], 0.82-0.94) compared to those in the lowest quintile. Results remained consistent for the 1,250 meters area, although the relationship was slightly attenuated.

Cause-specific mortality analyses demonstrated that the associations were strongest for respiratory, cancer, and kidney disease mortality. When researchers compared women in the areas with highest greenness to those in the lowest, they found a 34% lower rate of respiratory disease-related mortality (95% CI, 0.52-0.84), a 41% lower rate of kidney disease mortality (95% CI, 0.33-1.05), and a 13% lower rate of cancer mortality (95% CI, 0.78-0.97). Researchers did not note any statistically significant association between greenness and mortality from coronary heart disease, diabetes, or infections.

James et al suggested that a large proportion of the association between greenness and mortality may be explained through mental health pathways of depression risk and social engagement, which subsequently affects mortality.

COMMENTARY

Going green could be good for health and society in many ways. This study was the first prospective examination of the relationship between exposure to greenness and mortality across the entire United States, and it demonstrated that nurses who lived in areas with more vegetation experienced lower death rates regardless of age, race/ethnicity, smoking, or socio-economic status. The findings were consistent across all regions of the country (including urban and rural areas) and were most robust for cancer, respiratory, and kidney disease mortality. These findings substantiate previous research in the field.

Biophilia is a theory that suggests an instinctive bond between humans and other living systems; therefore, the theory suggests people have evolved to prefer certain natural environments that are essential to their prospering.5 The findings of this study with regard to respiratory and cancer mortality benefits may be attributed to a reduction in air pollution exposure and increased physical activity in the areas of greater greenery. Renal disease mortality benefits also may be related to greater physical activity. Higher exposure to greenness consistently has been linked to lower levels of depression, anxiety, and stress. So it seems that greenness may lower all-cause as well as cause-specific mortality in women, which perhaps can be explained by benefits in mental health, social engagement, physical activity, and lowering of air pollution. If these findings are to be believed, and there is no reason they should not be, we may find ourselves giving recommendations for planting trees and other vegetation to our patients while saving the environment — something we never learned in medical school.

REFERENCES

  1. Patil RR. Urbanization as a determinant of health: A socioepidemiological perspective. Soc Work Public Health 2014;29:335-341.
  2. Nieuwenhuijsen MJ, Kruize H, Gidlow C, et al. Positive health effects of the natural outdoor environment in typical populations in different regions in Europe (PHENOTYPE): A study programme protocol. BMJ Open 2014;4:e004951.
  3. Lachowycz K, Jones AP. Greenspace and obesity: A systematic review of the evidence. Obes Rev 2011;12:e183-189.
  4. Gascon M, Triguero-Mas M, Martínez D, et al. Residential green spaces and mortality: A systematic review. Environ Int 2016;86:60-67.
  5. Wilson EO. Biophilia. Cambridge:Harvard University Press;1984.