Extreme heat linked with uneven cognitive decline

Extreme heat is a well-known threat to our health, with older people being especially vulnerable to developing hyperthermia and heatstroke, and subsequently dying. Now, new research from the US suggests that heat poses a longer-term health risk for many older people, too — though that risk is not spread equally. 

Eun Young Choi at New York University and colleagues found evidence that cumulative exposure to unusually hot days over many years hastens cognitive decline. However, this was the case only for Black people — not White or Hispanic people — and for residents of poorer, but not wealthier, neighbourhoods. "High exposure to extreme heat can affect cognitive decline, but does so unequally across the population," the researchers write in the Journal of Epidemiology and Community Health. 

The team looked at data collected on 9,448 older people from across the US from 2006 to 2018. This data included the results of cognitive function tests at the ages 65 and 85, as well as information on their race/ethnicity, gender, level of educational achievement, and neighbourhood socio-economic status.

For each participant, the researchers calculated their long-term exposure to 'extreme heat days', defined as a day on which the temperature is in the top 5% of high temperatures recorded historically for that particular location. Those who scored in the top 20% for the average number of extreme heat days from 2006 up until the year before their most recent cognitive assessment were classed as having a 'high' cumulative exposure to extreme heat. Overall, 17.3% of the participants fell into this high exposure category (which corresponded to exposure to at least 13.1 extreme heat days on average per year). 

The team's analysis revealed that, for Black (but not White or Hispanic — the two other race categories in this study) participants, high levels of extreme heat exposure was linked to a faster deterioration in cognitive function over time. Black participants in this 'high' exposure category had an average cognitive score of 13.7 at age 65, which declined by 42% to an average of 8.0 by 85. Those with 'low' exposure to extreme heat started off with statistically the same average score at 65 (13.9). But, by age 85, their scores had decreased by less — 32% — to 9.4. 

A separate analysis, considering neighbourhood wealth, revealed that for residents of neighbourhoods of above average or average wealth, a high level of exposure to extreme heat was not associated with levels of cognitive decline. However, for people living in poorer neighbourhoods, it was. For these people, the average cognitive score of 14.5 at age 65 declined by 37%, averaging at 9.1 by age 85. People in similarly poor neighbourhoods in areas that had not suffered high levels of extreme heat saw a smaller decline, by an average of 29%, to 10.2 at age 85.

Compared with younger people, older adults are known to be more vulnerable to the impacts of extreme heat. There are various reasons for this. Body temperature regulation deteriorates in older age, and older people are more likely to be taking medications that interfere with effective sweating. Over time, cumulative exposure to extreme heat in older people could trigger a "cascade of detrimental effects" in the brain, the researchers write. These harmful effects include damage to cells, inflammation and oxidative stress, which can deplete a person's cognitive reserve and so contribute to cognitive deterioration.

There are also a number of reasons why older people in poorer neighbourhoods may be at risk of faster cognitive decline. They are less likely to have air conditioning at home. They may also find it harder to access green spaces in which to cool off in, or be more socially isolated. Limited access to quality healthcare facilities may also drive their risk higher, with delays in treatment for cognitive issues exacerbating declines linked to high heat exposure. 

When it comes to race, Black older adults may, in contrast to the White people in the study, be more likely to suffer the effects of extreme heat as a result of decades of systemic racism, the researchers write: "Black older adults may have disproportionately experienced systemic disadvantages throughout life due to structural racism, racial segregation, and discriminatory policies that may diminish cognitive reserve." The cumulative effect of these disadvantages could have led to chronic stress, which is also linked to vulnerability to cognitive decline. 

The study does have a few limitations. For example, the team didn't have data on who had access to air conditioning, so couldn't control for it in their analysis (though they did control for many other factors that might have influenced the results). But these new findings highlight yet another heightened vulnerability of disadvantaged groups to the effects of climate change. 

Of course, these effects are set to get worse. By 2050, it's predicted that there will be twice as many extreme heat days in the US each year, the team notes. In the UK, heatwaves are already becoming more intense, more prolonged, and more frequent. "Concerted efforts are required at every level to empower vulnerable communities, map their specific needs, bolster green infrastructure, and advance our knowledge of key factors leading to disproportionate harm," the researchers conclude. 

Read the paper in full: https://doi.org/10.1136/jech-2023-220675