More greenery right around the home was linked to less dementia in Stockholm

Does it matter for the brain whether you can see trees from your window? A long-running study of older people in Stockholm suggests it might. People with more greenery within about 100 metres of their home were less likely to develop dementia over the following years. The study only shows a link: people who live among trees may differ from others in ways the researchers could not fully measure.

Earlier studies on green surroundings and dementia have given mixed results. One reason, the team argues, is that most of them measured greenery crudely, for a whole postcode or neighbourhood, rather than right around each person's home. Many also relied on health records to find dementia cases, which can miss people. This study tried to do better on both counts.

What they did

The data come from SNAC-K, a study that since 2001–2004 has followed people aged 60 and over living in Kungsholmen, a central district of Stockholm. Participants are examined in person by physicians, nurses and psychologists, every six years for younger participants and every three years for those aged 78 and older. Dementia was diagnosed by a physician, checked by a second one, and settled by an outside neurologist if they disagreed; for people who died between visits, death records and medical files were reviewed.

The analysis included 2,993 people who did not have dementia at the start. For each of their home addresses over the years, the researchers measured greenery from satellite images, using a standard index that picks up green leaves by the way they reflect light. They did this within circles of 100, 250 and 500 metres around the home, and averaged the five years before a person's diagnosis or the end of follow-up.

The analysis took into account age, sex, education, occupation, income in the neighbourhood, marital status, smoking, drinking, physical activity, body weight, medications and early retirement, as well as air pollution, road traffic noise, and nearby water such as lakes or the sea.

What they found

Over up to 16 years (9.7 on average), 477 people (16%) developed dementia.

• Close greenery mattered most. A step up in greenery within 100 metres of the home, the size of the gap between a home at the lower quarter mark and one at the upper quarter mark of greenery in the sample, was linked to about a 20% lower rate of dementia (hazard ratio 0.79).
• Further away, weaker links. For greenery within 250 or 500 metres, the link was weaker. Greenery measured 5 to 10 years before diagnosis showed a similar but smaller and statistically uncertain link.
• Clearer in women. In women, the same step in greenery went with about a 31% lower rate (hazard ratio 0.69). In men, the link was small and could have been chance (0.93).
• Genetic risk. The link appeared in people without APOE4, a common gene variant that raises the risk of Alzheimer's disease, but not clearly in carriers. The authors stress that this difference was not statistically significant and should be read cautiously.
• Noise, pollution and water did not change the greenery link.

The results held when people who moved house during the study were left out, and when the analysis was corrected for people who dropped out.

What to keep in mind

• An association, not proof. People in less green places were on average older, poorer, less educated, less active and exposed to more noise and pollution. The team adjusted for these, but some differences may remain. The authors' summary speaks of greenery that "may reduce" dementia risk; the study itself cannot show cause and effect.
• One wealthy city district. Kungsholmen is a generally affluent urban area. The authors say the results may apply mainly to similar city settings.
• Drop-outs. 499 people left the study early. They were older and less educated, though their surroundings were similar to those who stayed, and correcting for them did not change the results.
• Satellite greenness is not the same as use. The index measures leaves, not whether people actually went into parks or gardens.

Why it matters

The study measured both greenery and dementia more carefully than many earlier ones, and found a link for the greenery closest to home. The authors suggest several possible routes: green surroundings may lower stress, encourage walking and social contact, and shield people from noise and pollution. None of these was tested here. If the link holds up in other cities and in studies that can track such pathways, it would add a brain-health argument to the many reasons cities already have to keep trees and green space near where people live.