Air pollution could cloud your vision in later life, according to a large study that found a link between fine particle air pollution and macular degeneration, an age-related eye disease that can lead to irreversible blindness.
The findings serve as a clear reminder of the many ways that air pollution can be harmful to our health, even though it's still early days for this research.
"Our findings add to the growing evidence of the damaging effects of ambient air pollution, even in the setting of relative low exposure of ambient air pollution," the study authors write in their paper.
Air pollution is a global problem many can't escape, with the World Health Organisation (WHO) estimating over 90 per cent of the world's population lives in places where air quality levels exceed the limits set for pollutants that pose health risks.
The biggest public health concerns about poor air quality circle around pollutants such as particulate matter (dust, soot, and more), ground-level ozone, nitrogen dioxide, sulfur dioxide and other gases, which are emitted from motor vehicles, heavy industry, and wood fires.
Fine particles, called PM2.5 for short, are especially concerning. These microscopic particles less than 2.5 micrometres in size can penetrate deep into the lungs and enter the bloodstream, causing inflammation around the body.
Repeated exposure to pollutants like these can irritate people's eyes and throat, cause breathing difficulties. Furthermore, ambient air pollution accounts for 43 percent of deaths from chronic obstructive pulmonary disease, and for over a quarter of all deaths from lung cancer, heart disease, or stroke.
In this study, the focus was on age-related macular degeneration (AMD), a condition where a person's vision worsens with age, leading to increased vision loss and potentially even blindness.
The disease is linked to leaky blood vessels in the back of the eye and small blobs of fat and protein that build up on the macula, the part of the eye at the centre of the retina. Genetics and being a smoker are amongst the main risk factors for this condition.
For their analysis, the researchers pulled data on thousands of people enrolled in the UK Biobank and estimated the annual air pollution levels around their homes using other publicly available datasets.
From 2006 onwards, almost 116,000 people were asked to report if their doctor diagnosed them with macular degeneration.
Of that larger group, 52,062 people also had their eyesight examined and retinal thickness measured, as an indicator of any changes to their eye health.
What the study found is that people who were exposed to higher levels of fine particle air pollution had higher rates of self-reported AMD.
Exposure to other pollutants, including nitrogen dioxide but not coarse particulate matter, was also associated with changes in retina thickness, detected on imaging.
But don't be swayed by the big numbers alone. Only a tiny fraction of people were actually diagnosed with AMD during the study – and remember, while this observational study can bring our attention to trends and patterns observed across a population, it can't establish a cause.
In other words, researchers do what they can in population-wide studies like these to account for other factors, such as lifestyle, that influence disease risk but suffice to say, trying to untangle the precise health impacts of exposure to air pollution in a world where everything is interconnected is not always clear cut.
The researchers suggest that air pollution may affect the eye in a roundabout way through inflammation and oxidative stress, two defence mechanisms where the body is fighting against foreign material and trying to detoxify chemical species, respectively. But more research will be required to examine that plausible link.
It's not the first time though that air pollution has been linked to eye disease. A 2019 study examining the global burden of glaucoma found higher average levels of fine particulates were associated with more cases of glaucoma, which affects the optic nerve.
"The good news is that ambient air pollution can be controlled and the diseases it causes prevented," writes Philip Landrigan, a public health physician and epidemiologist from Icahn School of Medicine at Mount Sinai, New York, who was not involved in the study.
Enforcing air quality standards and reducing emissions from coal-fired power plants – by transitioning to clean fuels and ultimately to renewable energy sources – would both be effective strategies for reducing air pollution.
We saw how quickly the skies cleared in the first few months of the coronavirus pandemic, which grounded air traffic and pulled cars off the road as people stayed at home. Although such drastic changes weren't ultimately sustainable, the momentary relief from the air pollution that usually blankets cities has shown us what's possible.
"Cities and countries will need to switch to non-polluting energy sources, encourage active commuting, enhance their transportation networks, [and] redesign industrial processes to eliminate waste," writes Landrigan.
"These changes will not be easy. They will need to overcome strong opposition by powerful vested interests. But, fortunately, the technical, institutional, and policy tools needed to control air pollution are already at hand."
In the meantime, more research will be needed to build the evidence around the long-term risks that air pollution poses to eye health.
The research was published in the British Journal of Ophthalmology.