If a person lives in an area rich in foliage, their grass may indeed be greener, at least when it comes to risk of developing Alzheimer’s or Parkinson’s diseases. That is according to a large study led by Jochem Klompmaker, Jaime Hart, and Peter James at the Harvard T.H. Chan School of Public Health, Boston. In the December 20 JAMA Network Open, they reported that, across the contiguous U.S., people living in towns with lots of green space were least likely to have either neurodegenerative disease. This correlation was strongest in those over 85 and in blacks. The authors think protection may partly stem from less pollution in greener areas.
First author Klompmaker and colleagues correlated residential ZIP code “greenness” with hospital admissions with an AD or PD diagnosis among 61.7 million Medicare beneficiaries over age 65. Eighty-four percent were Caucasian, and 88 percent had an income high enough to exclude them from Medicaid. From 2000 to 2016, there were 7.7 million cases of AD and 1.2 million of PD diagnosed among this cohort.
“This analysis is the largest dataset examined for how the natural environment influences risk of hospitalization for AD and PD,” wrote Caleb (Tuck) Finch, University of Southern California, Los Angeles.
ZIP code greenness refers to the “normalized difference vegetation index,” or the density of vegetation captured on satellite images by infrared sensing. Any bright pixel was deemed to contain chlorophyll—be it grass, shrubs, trees, crops, etc.—and was reported as a ratio of green to non-green pixels.
For each ZIP code, the researchers calculated the average density during the summer months annually, the percentage of parkland according to the U.S. Geological Survey Protected Areas database, and the percentage of surface water, aka blue space, based on the Joint Research Centre Global Surface Water dataset. On average, half of each ZIP code was greenery, and 7 percent was parkland. Forty percent of ZIP codes contained more than 1 percent blue space.
Klompmaker adjusted correlations with all three factors for median income per ZIP code, county-level tobacco smoking, year, U.S. region (Northeast, South, Midwest, and West), and individual characteristics, including sex, age, race and ethnicity, and Medicaid eligibility as a proxy for socioeconomic status.
After all these adjustments, people in the greener ZIP codes by quartile were less likely to have AD or PD than those in less-green environments. The reductions in risk were 5 and 6 percent per interquartile range (IQR) for AD and PD, respectively. The percentage of parkland or blue space did not influence AD risk, but each lowered PD risk by about 3 percent per IQR.
Greenness correlated with even more protection in black people, dropping their risk of AD and PD by 8 and 12 percent, respectively, per IQR. Ditto for people over 85 and people eligible for Medicaid, whose risk of both diseases fell 7 or 8 percent.
People living in the South seemed to buck the trends. There, more green and blue space nudged up AD and PD risk by 2 to 3 percent per IQR (see image at right). Greenness less benefitted people with a low income, or people living in areas with high rates of poverty.
Why might greener be healthier? The authors think people who live in such areas might exercise more or be more sociable. Greener environments also have less pollution. Both exercise and pollution are known to decrease or increase AD risk, respectively (Dec 2020 news; Sep 2022 news; Aug 2021 news). Indeed, when the scientists adjusted AD and PD hazard ratios for air pollutant levels in each ZIP code, the relationships between greenness and disease risk weakened, and the influence of parkland or blue space fell to almost zero.—Chelsea Weidman Burke
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- Klompmaker JO, Laden F, Browning MH, Dominici F, Jimenez MP, Ogletree SS, Rigolon A, Zanobetti A, Hart JE, James P. Associations of Greenness, Parks, and Blue Space With Neurodegenerative Disease Hospitalizations Among Older US Adults. JAMA Netw Open. 2022 Dec 1;5(12):e2247664. PubMed.