Poor air quality is one of the leading environmental health risks, and its effects on the eye are not insignificant. Air pollution has been associated with increased morbidity and mortality, as well as pulmonary and cardiovascular diseases and eye diseases such as glaucoma and age-related macular degeneration (AMD). Studies suggest air pollution causes oxidative stress and inflammation, which is particularly concerning for retinal health, since the retina is one of the highest oxygen-consuming tissues in the human body and is in closer proximity to the outside environment than many other structures of the body.

A recent study for the UK Biobank examined the effects of air pollution on self-reported AMD patients and on in vivo measures of subretinal layer thickness and confirmed that exposure carries retinal risks.

The cross-sectional study included 115,954 UK Biobank participants aged 40 to 69. The researchers used self-reported cases of AMD to identify overt disease but excluded participants with other self-reported ocular conditions, high refractive error and poor SD-OCT imaging. The researchers measured retinal pigment layer thickness as a structural biomarker for AMD in 52,602 participants. Self-reported AMD patients underwent photoreceptor sublayer and retinal pigment epithelium (RPE) layer thickness examinations.

The researchers found that individuals exposed to higher amounts of fine ambient particulate matter with an aerodynamic diameter less than 2.5μm had higher odds of self-reported AMD, thinner photoreceptor synaptic region, thicker photoreceptor inner segment layer and thinner RPE. Unlike coarse particulate matter (from grinding, agriculture, windblown dust), fine ambient particulate matter is primarily from combustion processes, and these particles are able to reach smaller airways and alveoli and be transmitted through the blood, resulting in cascades of physiological events.

The study also found that higher levels of particulate matter absorbance and nitrogen dioxide were associated with thicker photoreceptor inner and outer segment layers and a thinner RPE layer and that higher particular matter (larger than 10μm) levels were associated with thicker photoreceptor outer segment and thinner RPE. Nitrogen dioxide is also a product of combustion, most often from traffic and industrial sources. “It’s one of the most notable ambient air pollutants associated with health effects,” the researchers wrote in their paper.

Higher exposure to nitrogen oxides was associated with a thinner photoreceptor synaptic region. “Nitrogen oxide is produced from the reaction of nitrogen and oxygen gases in their air during combustion,” the researchers noted. “It contributes to the formation of fine particles and ground-level ozone.”

The researchers concluded that greater exposure to these air pollutants was associated with self-reported AMD and differences in retinal layer thickness. “Ambient air pollution could plausibly be associated with AMD through oxidative stress or inflammation. Oxidative damage induces many adverse biological effects including lipid, protein, DNA oxidation, initiation of proinflammatory processes and RPE apoptosis.”

The investigators added that cigarette smoking may also contribute to particulate matter air pollution and noted the strong link between AMD and smoking; however, they found no significant association between smoking status and self-reported AMD.

Chua SYL, Warwick A, Peto T, et al. Association of ambient air pollution with age-related macular degeneration and retinal thickness in UK Biobank. Br J Ophthalmol. January 25, 2021. [Epub ahead of print].