Unknown Unknowns

The images in June 2023 of the New York cityscape shrouded in a sepia-coloured blanket of smoke – a by-product of over 400 drifting Canadian wildfires – marked one of the worst days of air pollution in recent US history. Reports carried headlines straight from dystopian fiction – e.g., “smoke apocalypse” (1) – and centered on incidents caused by the sudden drop in air quality, such as the actress Jodie Comer abruptly halting a Broadway performance because she was struggling to breathe (2).

Amid stricter global regulations on carbon emissions, air pollution still accounts for around 11.65 percent of global deaths, and remains one of the leading factors of disease burden (3). Government regulations – such as the UK’s Clean Air (Human Rights) Bill (4) – tend to focus on breathing issues, such as asthma and chronic obstructive pulmonary disease, but many diseases are linked to air quality. Stroke, cardiovascular diseases, trachea, bronchus and lung cancers, autoimmune and intestinal diseases, and even Alzheimer’s disease have all been associated with air pollution by the World Health Organization (WHO) (5).

Now, a study in the Journal of Clinical Medicine (6) has reviewed recent medical literature to determine ambient air pollution’s links to vision-threatening posterior segment ocular diseases. Andrzej Grzybowski, Professor of Ophthalmology at the University of Warmia and Mazury, Poland, and one of the study’s main authors, explains that air pollution’s impact on ocular health is poorly perceived. “As ophthalmologists, when we think about air pollution, we usually refer to dry eye syndrome, as the ocular surface disorder.” Of course, the anterior segment of the eye comes into direct contact with the external environment, and its susceptibility to airborne pollutants is well documented. However, emerging evidence from research conducted by Gryzybowski and others suggests that the posterior segment of the eye is similarly vulnerable to the health effects of polluted air.

The new study contends that better technologies and more standardized methods to track air pollutant levels could become cost-effective preventatives for ocular diseases. We need to have “more and better-evidenced studies,” says Grzybowski, noting that, in cases such as central retinal artery occlusion (CRAO), there are only a few available studies so far. 

To quantify air pollution, the study measured particulate matter, with PM2.5 referring to particles with a ≤2.5 μm diameter and PM10 referring to particles with a ≤10 μm diameter. Although both these particle sizes can be inhaled and enter the lungs, PM2.5 can also enter the bloodstream. Grzybowski says, “We showed significant associations between PM2.5 and glaucoma, including primary open-angle, primary angle-closure, and normal-tension glaucoma. An increased risk of AMD was linked to increased exposure to PM2.5, NO2, and CO.” He continues, “Single studies suggested that increased exposure to PM2.5 and PM10 is associated with diabetic retinopathy; THC and NMHC increased the risk of retinal vein occlusion; and CO, NO2, and PM10 are linked to an increased risk of CRAO.”

It is becoming clear that toxic air pollutants can be identified as a potential modifiable risk factor for visual impairment, particularly posterior segment ocular diseases, the paper concludes. But environmental cleaning can take decades, which will affect any efforts to minimize the adverse effects of toxic components contained in the air for the human organism. “We need to learn more – first, about the pathophysiology of these influences,” cautions Grzybowski. “Finally, the list of different disorders that might be aggravated by air pollution is not closed. This is the area of unknown unknowns.” 

There is a glimmer of hope, however. As Grzybowski explains, the topic is of increasing importance. “We will certainly learn more about it in the future – hopefully not only in the context of the increasing number of disorders related to air pollution, but also in terms of minimizing its harmful effects to ocular health.” He adds, “Air pollution is a complex issue that requires collaborative efforts on a global scale.” 

_{^{Hero and Teaser Image Credit: Smoke Image sourced from Pixabay.com}}