Proportion of the association between greenness and mortality explained by mediators (source: James et al)

CNN reported last month on a US study that finds living near nature is linked to longer lives, at least for women:

The trees, shrubs and plants outside your home might offer something more than just pretty scenery. A new study found that living in, or near, green areas can help women live longer and improve their mental health.

Researchers from Harvard T.H. Chan School of Public Health and Brigham and Women’s Hospital examined more than 108,000 women enrolled in the Nurse’s Health Study — a nationwide investigation into risk factors for major chronic diseases in women — from 2000 to 2008.

They compared risk of death with the amount of plant life and vegetation near the women’s homes and found that women living in the greenest areas had a 12% lower death rate than women living in the least green areas. The levels of vegetation were determined using satellite imagery from different seasons and years.

When the researchers looked at specific causes of death (the study only looked at natural causes), they found that women living in areas with the most vegetation had a 34% lower rate of respiratory disease-related mortality and a 13% lower rate of cancer mortality compared with those with the least vegetation around their homes.

The result held regardless of adjustment for age, race/ethnicity, smoking status, individual level socioeconomic status, and area-level socioeconomic status. The authors propose how the mechanism might work:

The association between greenness and mortality was explained primarily by improving mental health and increasing social engagement, as well as by lowering air pollution exposure and increasing physical activity.

You can read the study here, Exposure to Greenness and Mortality in a Nationwide Prospective Cohort Study of Women, Peter James, Jaime E. Hart, Rachel F. Banay, and Francine Laden.

That sounds great and on the face of it is strong evidence for planting more trees, although it might also be taken as evidence against redeveloping suburbs at higher density. Of course, as with any study, caution is appropriate. As the authors acknowledge, there’s conflicting evidence on this topic:

Some studies have observed contradictory findings. One ecological study conducted on the city level found that all-cause mortality was higher in greener cities…An ecological analysis across the entire United Kingdom found that higher greenness was associated with lower cardiovascular and respiratory mortality among males; however, no significant associations were found among women.

Also, while the association was strongest for respiratory, cancer, and kidney disease mortality, there was no link between greenness and mortality due to coronary heart disease and diabetes. The authors don’t offer a theory for this curious result but it seems odd to this layperson; if greenness promotes activity, shouldn’t it reduce heart disease and diabetes to some degree?

It’s not clear what the mechanism underlying the correlation between greenness and mortality is. The four explanatory factors hypothesised by the authors only explain 27% of the association when taken together (see exhibit). What accounts for the rest? What’s also interesting is physical activity and air pollution, although statistically significant, make a very small contribution to the proffered explanation.

The spatial measure used for the study is quite blunt. Most of the respondents are urban (85%) but we don’t know if they live in the suburbs or the inner city. It’s plausible that higher greenness reflects suburban locations; if so, it might be that there’s something else about the suburbs that’s not measured by the study – something different from greenness – that has a net benefit for women’s health e.g. perhaps they’re more socially homogeneous? Or maybe elderly nurses with the best health prospects simply tend to prefer suburban locations for some reason?

I’m not sure this study by itself  is enough to conclude, as CNN implies, that “planting trees, shrubs and plants outside your home” might extend the lifespan of the average woman. Fortunately, there are plenty of other reasons to promote greening of private and public land. For example, with good design, street trees can enhance the visual amenity of streets; provide shade, shelter and points of interest for pedestrians; reduce home cooling costs; attenuate noise; support bird life; and remove pollutants and carbon dioxide from the air.

I’d like to see a major national program to “green the streets of Australia”. It’s hard to think of a more cost-effective way of improving the quality of urban streetscapes and the amenity of residents than planting street trees in all suburbs. If it turns out there’s a health benefit too, that’s terrific.