Women in the Greenest US Neighborhoods Died at a 12% Lower Rate. Nobody Has Shown the Park Did It

What does living near a park do to your health?

Women in the greenest US neighborhoods died at a 12% lower rate than women in the least green ones. An association rather than a proven effect.

James and colleagues published that analysis in Environmental Health Perspectives in 2016, and you can read it in full at PMC5010419. They followed 108,630 women in the Nurses’ Health Study and recorded 8,604 deaths between 2000 and 2008.

Women in a top fifth of greenness around their homes had a 12% lower rate of all-cause nonaccidental mortality than women in a bottom fifth, on a confidence interval running from 0.82 to 0.94.

Split it by cause and it gets stranger. Respiratory deaths ran 34% lower and cancer 13%, while for heart disease the study found nothing it could tell apart from chance at all, which is the opposite of what almost anybody would guess about fresh air and exercise.

They adjusted for age, race and ethnicity, smoking, and income and education at both a personal and a neighborhood level. We would call that a serious set of controls for work of this kind.

It’s still not a trial. Nobody assigned anybody a green address. And what a headline calls green space turns out to be stranger than it sounds.

What does greenness mean in this research?

A satellite index, averaged over circles drawn around each home address. Nobody counted a park, a trail or a street tree. Nobody went and looked at a single tree.

It’s NDVI, the Normalized Difference Vegetation Index, off NASA’s Terra satellite, which returns an image every 16 days at a resolution of that resolution. Think of a square roughly 820 feet on a side.

James and colleagues averaged those readings inside two circles, one with a radius of 820 feet, and one at 4101 feet, about three quarters of a mile.

Now the part changing how you read every green space headline. NDVI reports how much living plant matter bounced light back at a satellite. It can’t tell you what those plants are, who owns them, or whether you may walk on them. Its authors say so outright.

So a public park, a golf course, a cemetery, a highway median and a watered lawn can all return one value. A gated country club scores greener than a small city park with a bald outfield, and no analysis in this literature can tell those two apart.

That’s what living near green space means in this literature. Not a park. A figure for how much green vegetation a satellite saw within about 820 feet of a mailbox.

Is any of it cause rather than association?

Yes, in two Philadelphia trials. What they randomized wasn’t a park. It was cleaning up trashed vacant lots, which is about as unglamorous as public health research gets.

Branas and colleagues randomly assigned 541 vacant lots to treatment or control, then tracked police records and 445 sampled residents for 38 months. Published in PNAS in 2018, open in full at PMC5866574.

Citywide the effects were modest. Gun assaults fell 2.7%, burglary 6.3%, nuisance complaints 12.8%. In neighborhoods below the poverty line that same work produced much larger figures, with gun assaults down 29.1%, burglary 21.9%, nuisances 30.3%.

Residents near a treated lot said they felt unsafe going outside 57.8% less. Their sense of crime nearby fell 36.8%. Vandalism, 39.3%.

South and colleagues took the mental health side in JAMA Network Open, also in 2018, at PMC6324526. That trial covered 110 lot clusters and 342 residents from October 2011 to November 2014. Feeling depressed fell 41.5%, and among 139 residents below the poverty line it fell 68.7%.

Note what was in the box. Trash hauled out. Ground graded. Grass seeded. A few trees, a low wooden fence, then mowing. That paper prices all of it near 46 cents a square foot to build and about 5 cents a square foot a year to keep.

We would call that the strongest causal evidence in this field, and it describes a dumping ground rather than a park.

How would green space affect health?

Physical activity accounted for 2.1%. Mental health explained 30.6%. So the park-makes-you-jog story, which is the one nearly every article about green space leads with, turns out to be the thinnest strand in the whole dataset.

Those shares don’t add up, and a combined estimate falls below mental health alone. Mediation behaves that way when pathways run through one another. Mood, company and movement aren’t separate things in a life.

Federal reviewers reached a matching conclusion from another direction. The Community Preventive Services Task Force examined park, trail and greenway construction on its own in July 2021 and found the evidence insufficient, because its 17 studies disagreed with each other about physical activity.

Those same reviewers do recommend that construction when it arrives bundled with something else. Community input on a plan. Walking groups or classes. Better crossings, transit connections, longer opening hours. Bundled that way, use rose 18.3% and moderate to vigorous activity rose 17%.

Nine studies of construction alone did find a median 32.1% rise in how much a space got used. People turn up. Whether they move more across a whole week, rather than simply moving the same walk into a nicer setting, is what nobody has pinned down.

Do trees near your house clean your air?

Barely. Nowak and colleagues at the Forest Service added up every tree in the contiguous United States and got an average air quality gain of under one percent.

They published it in Environmental Pollution in 2014. City air improved 0.13% a year for PM2.5, country air 0.51% for ozone, and those two figures bracket everything in between. A few spots reached 2 to 4.5 percent under kind weather, and a peak hour in heavy forest could touch 16 percent.

National totals still matter. Modeled health benefit came to 6.8 billion dollars for 2010, with more than 850 deaths avoided and 670,000 episodes of acute respiratory symptoms prevented. But 96.3 percent of that removal happened on rural land, while 68.1 percent of the value settled in cities, where people are.

That same paper lists effects running the other way. Trees slow wind and lower the mixing height, which can push concentrations up instead of down. Trees also give off volatile organic compounds, which are precursors for ozone and fine particles.

Planted one way, trees do earn their keep. A roadside belt that’s both tall and dense can cut downwind pollutants by roughly 30%. Read those two words twice. A row of young street trees with daylight through it’s neither tall nor dense.

If cleaner air is your reason for wanting a park nearby, we would say this is the weakest case available.

How much cooler is a block with trees?

About 3 degrees F across a whole urban forest, and far more in the shade of one tree. Somebody sat down with 308 studies and came out with urban forests 3.0 degrees F cooler on average than city land without them.

A citywide average is honest and small. Local ones are neither.

EPA’s compendium chapter on trees and vegetation, published in 2008, collects those measurements. Shaded walls and roofs ran 20 to 45 degrees F below the peak temperature of the same surfaces in sun. Peak air temperature inside a grove came in 9 degrees F below open ground.

Two of its figures describe ordinary streets. Suburbs with mature trees measured 4 to 6 degrees F cooler than new suburbs without them. Grass sports fields ran 2 to 4 degrees F cooler than ground bordering them.

Two more effects come with figures attached, and both of them land where you live. Parks and street trees can cut energy demand in nearby buildings by 10 percent. Trees in a yard or along a curb soak up 15 to 27% of annual rainfall that would otherwise run off down a storm drain.

One more figure comes off a computer rather than a thermometer, and a model isn’t a body count. A 10% rise in tree cover would mean roughly 50 fewer deaths a year in Salt Lake City and 3,800 fewer in New York City.

Heat has the shortest chain of reasoning between a tree and you, which is probably why it’s the one effect nobody argues about. You can stand under it.

Is there a target amount of green space?

No US agency publishes a figure for how much green space a home should be near. Nothing above is something you can pass or fail.

That absence is worth sitting with. You can look up whether your water went over a lead limit last spring, or whether your county blew an air standard yesterday afternoon. Ask how many trees a street needs and nobody in Washington has an answer.

What you’ve instead is one large cohort association, two randomized trials on vacant lots, a federal review that came back insufficient, and a stack of shade measured with thermometers.

James and colleagues name their own weak point. People in better health may choose greener neighborhoods, and that preference alone could produce their whole result. They also note that participants were nurses, over 90% of them White non-Hispanic, and all were women, which limits how far it travels.

Those Philadelphia trials carry their own caveats. South and colleagues lost 22.6% of participants to follow-up, used a single distress scale with no clinical diagnosis, and never checked whether anybody set foot on a greened lot.

So, what survives. Shade you can measure in your own yard with a thermometer. A mortality association you can read but can’t bank. And one cheap piece of work, tested twice in Philadelphia, that was never a park at all: somebody hauled trash out of a vacant lot and kept mowing it.