The strongest associations between severe vision impairment and temperature were found in males, those between the ages of 65 and 79 and Caucasians. Increased pollution from forest fires (pictured) is one theory to explain it. Photo: Patrick Perkins/Unsplash.

As global warming becomes more a fact of life than a theoretical possibility, the potential link between average area temperature and vision deterioration becomes a cause of concern and may drive a higher-than-expected increase in consequences over the coming years. So says a study from Ophthalmic Epidemiology that offers data establishing the connection as well as speculation on possible mechanisms.

Researchers at the University of Toronto recently investigated the relationship between vision and temperature, noting that the odds of severe visual impairment among older Americans in the continental United States has increased with a rise in average area temperature. A potential causal association would mean that the predicted rise in global temperatures could impact the number of older Americans affected by severe vision impairment, as well as any associated health and economic burden.

This study was a secondary analysis of the US Census Bureau’s annual American Community Survey (ACS), which was conducted through mail, telephone and in-person interviews. Data from six consecutive years of the cross-sectional survey was assessed. The subsample analysis included community-dwelling and institutionalized adults aged 65 and older in the United States who lived in the same state in which they were born (n=1,707,333). Severe vision impairment was determined by each participant’s response to the following question: “Is this person blind or do they have serious difficulty seeing even when wearing glasses?” Average annual temperature data from the National Oceanic and Atmospheric Administration was combined into a 100-year average and mapped to corresponding US Census Bureau's public use microdata areas from the ACS.

Compared with those who lived in regions with average area temperature <50°F, the odds of severe vision impairment consistently increased with elevation in average area temperature irrespective of age, sex, income and education attainment. Higher average temperature was consistently associated with increased odds of severe vision impairment across all cohorts (i.e., age, sex, race, income and educational attainment cohorts) with the exception of older Hispanic adults. The researchers proposed that the observed decreased association may instead reflect the impact of other unmeasured factors, such as regional differences in nutrition and access to healthcare providers.

In the sample of all adults 65 and older, compared with participants who lived in counties with average temperature <50°F, the odds of severe vision impairment were 14% higher for those who lived in counties with average temperature of 50° to 54.99°F (OR=1.14), 24% higher for those whose counties averaged 55° to 59.99°F (OR=1.24) and 44% higher for those residing in temperatures 60°F or above (OR=1.44).

“The factors that may contribute to the observed relationship between vision and temperature in this present study are unclear and warrant future research,” the team wrote in their paper. Theories they considered included higher susceptibility to eye diseases in warmer environments due to increased ultraviolet light exposure, as well as the impact of infections and air pollution on disease patterns.

“Those living in warmer environments may be at greater risk of vision impairment because high temperatures, particularly heatwaves, are associated with increased concentrations of air pollutants such as particulate matter and ozone,” the researchers suggested. “As average temperature tends to correlate with ultraviolet exposure, individuals living in hotter environments may experience greater folate degradation and thus be at increased risk of AMD-related vision impairment.”

They concluded, “Combined with the projected 5.4°F increase in average temperature by the year 2100 due to global warming, the number of Americans impacted by vision impairment and the associated economic burden may increase substantially over the coming decades if, in future research, area temperature is found to be causally linked to vision loss.”