The effects of pollution on human health are often subtle yet wide-reaching. Case in point, a new study out Tuesday seems to show that heavy exposure to certain automobile fumes can raise the risk of developing a degenerative disease that steadily erodes eyesight.
The disease is known as age-related macular degeneration, or AMD. AMD is characterized by the progressive destruction of the macula, a part of the retina that lets us see straight ahead with clear, sharp focus. The progression of AMD can vary, with some people experiencing little problems with their vision for many years, while others quickly worsen. Though the disease doesn’t cause complete blindness, it’s overall one of the leading causes of irreversible vision loss in people over the age of 50.
Like many degenerative diseases, AMD can be caused by several risk factors. Age is obviously a major contributor, but genetics and environment play big roles, too. White Americans over the age of 50, for instance, are twice as likely to have AMD (2.1 percent of the over 50 population) than people of any other race (0.9 percent).
According to the authors of the new study, published in the Journal of Investigative Medicine, there’s been little to no research on how air pollution could affect our chances of getting AMD. Some studies have shown that air pollution exposure can increase the risk of eye problems such as conjunctivitis and dry eye. Smoking is also thought to double the risk of developing AMD.
The researchers, based in China, looked at national health data from the country and cross-referenced it with air quality data. The health records of people over 50, in addition to determining if they were ever diagnosed with AMD during a 11-year time period leading up to 2010, were used to roughly estimate where they lived, based on where they often got medical treatment. And for air quality, they focused on two major sources of air pollution often emitted by motor vehicles: nitrogen dioxide (NO2 ) and carbon monoxide (CO).
People were divided evenly into four groups, based on how high their average daily exposure levels of NO2 and CO were. Those who lived in areas with the highest NO2 and CO, the authors found, were nearly twice as likely to be diagnosed with AMD than people living in the lowest quartile of NO2 and CO exposure.
“This study indicates the air pollution exposure as a risk factor for AMD,” the authors wrote.
There are a few caveats to this study, as always. One is that population studies like this can only show a correlation between two things, not show that one causes the other. And though the authors accounted for other risk factors for AMD, such as age or high blood pressure, they weren’t able to track people’s smoking history.
Another important consideration is that China has much higher levels of air pollution on average than many other countries, including the U.S. And since the researchers didn’t find any increased AMD risk for more moderate levels of exposure to either NO2 or CO, there might be areas of the world where these findings aren’t really relevant. The relative risk of getting AMD was also small in the study, with only 0.036 percent of people in the sample size of nearly 40,000 developing the condition during the study period.
That said, China is obviously one of the largest countries in the world, with more than 1.3 billion residents. It’s also not the only country with high levels of air pollution to worry about—much of Asia, including the more populous India, is similarly struggling with poor air quality. And even in countries with relatively good air quality, such as the UK, car exhaust is still thought to contribute to thousands of preventable deaths every year.
Because the retina is connected to the central nervous system, the effects of air pollution on the brain may extend beyond AMD—a study by the authors published in 2018, using much of the same data, found an increased risk for dementia in older people most exposed to these pollutants.
Correction: This article originally misstated where the study was published. Gizmodo regrets the error.