Extreme heat is linked to rapid cognitive decline

With 2023 expected to be the hottest year on record, a new study points to another potential consequence of heat waves: a faster decline in memory and thinking skills in older adults.

The study, of nearly 9,500 older adults in the US, found that those with the greatest exposure to heat waves over a 12-year period also showed a sharp decline in cognitive function – critical mental skills such as memory, reasoning and judgment.

This association was observed specifically among older black Americans and those living in favelas, groups that typically have fewer resources to protect themselves from the scorching summer heat.

Experts stressed that the results only show a relationship between heat and cognitive decline, and extreme temperatures cannot be blamed.

Cognitive decline is complex and affected by many factors, said lead researcher Eunyoung Choi, a postdoctoral fellow at New York University’s School of Global Public Health in New York City.

“Isolating the specific effect of extreme heat from this complex network is a challenging task,” she said.

Meanwhile, there are reasons why frequent exposure to heat waves can affect mental ability in the elderly, according to Choi.

For example, there can be direct effects: extreme heat can impair mental performance in the short term, and sustained exposure over time can promote inflammation and damage to brain cells.

Elevated temperatures can also act in indirect ways, Choi said. She pointed to the well-known connection between heart health and brain health.

Certain cardiovascular and metabolic conditions, including high blood pressure, diabetes and heart disease, are associated with an increased risk of cognitive impairment, possibly because they impair blood flow to the brain. Extreme heat may make these conditions worse.

What’s more, Choi said, extreme heat makes it difficult to sleep or leave the house for exercise or social activities, all of which can negatively affect cognition in older adults.

Why are low-income people and black Americans affected more? There may be a role for “cognitive reserve,” Choi said — the brain’s ability to adapt and maintain function even as age or disease begins to alter brain tissue itself.

Disadvantaged people — who face chronic stress and fewer educational and job opportunities, for example — may start out with lower cognitive reserves and see it erode faster over time, Choi said.

She added that this could make them more vulnerable to any additional stressors, such as extreme heat.

Kristina Dahl, principal climate scientist at the nonprofit Union of Concerned Scientists, agreed that all of these scenarios are plausible.

“I think this study is very interesting,” said Dahl, who was not involved in the research.

She said scientists know something about the acute effects of extreme heat on cognition. She noted that a lot of people have experienced “that sluggish feeling in the brain” that can come with summer heat waves.

The new study, Dahl said, is beginning to link cumulative heat exposure to cognitive decline in the long term.

However, like Choi, she cautioned that it is difficult to separate the effects of extreme heat from other things going on in people’s lives.

“I’d like to see more studies like this,” Dahl said. “One study is thought-provoking, but we need more than one to better understand what’s going on.”

The findings – recently published in the Journal of Epidemiology and Community Health – are based on 9,448 American adults ages 50 and older who took cognitive tests repeatedly over a 12-year period. The researchers used temperature data from a government database to estimate participants’ long-term exposure to heat waves.

Overall, 17% of participants were considered to be highly exposed – experiencing extreme heat for at least two weeks a year, on average.

And Choi’s team found that exposure to heat was not related to cognitive outcomes for older adults at first.

But the picture was different when they looked at people’s trajectories over time: higher exposure to extreme heat was associated with faster declines in cognitive outcomes — especially among older blacks and those living in disadvantaged neighborhoods (regardless of race).

Among the black participants, those exposed to a higher degree of extreme heat were expected to experience a 42% decrease in cognitive scores between the ages of 65 and 85. That compares to a 32% decrease among black adults who were exposed to less heat.

Similarly, the number of people living in slums with high exposure to extreme heat was expected to decrease by 37%, compared to 29% among low-income people with less heat exposure.

And while there are a lot of questions for future studies, Dahl said it’s time to protect vulnerable Americans from extreme heat.

She said this could include ensuring easy access for people in low-income areas to cooling centres; Prevent utilities from cutting people’s electricity over unpaid bills, and ensure that outdoor workers have enough shade, water, and rest.

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