A recent study published in Ophthalmic Epidemiology highlights the connection between air pollution and keratoconus, revealing that fine particulate matter (PM2.5 and PM10) and nitrogen dioxide (NO2) may aggravate the condition. Researchers found that pollution could affect corneal structure directly and stimulate known risk factors such as eye rubbing and atopy. With keratoconus prevalence varying globally, the findings underscore the importance of understanding environmental influences on ocular health and disease progression. Photo: Getty Images. Click image to enlarge.

The impact of air pollution on ocular health is becoming more demonstrable as time passes. It has been shown to affect cataract risk, chronic retinal eye disease and dry eye, and research is examining a potential link with corneal pathologies such as keratoconus. In a new study published in Ophthalmic Epidemiology, researchers looked to build upon a previous study of theirs from 2021 to see if air pollution could play a role in the physiopathology of keratoconus and found that some particles could indirectly exacerbate other known risk factors of keratoconus and disturb the structure of the epithelium.

The three most implicated factors in the pathogenesis of keratoconus are UV radiation, eye rubbing and atopy. Others have speculated on the involvement of genetic factors, syndromic associations, obesity and environmental factors, the latter of which was the focus of this study. Researchers specifically investigated the effects of particles matter (PM) of 2.5μm and 10μm, but also nitrogen dioxide (NO₂) and the correlation with the prevalence in the scientific literature. They performed a literature review across four databases—PubMed, Research Gate, Google Scholar and the International Journal of Keratoconus and Ectatic Corneal Diseases—amounting to 44 studies that met the inclusion criteria, compared to the previous 17 in their original research sample.

The prevalence of keratoconus ranged from 4.29 per 100,000 inhabitants (Taiwan) to 6,690 per 100,000 (Saudi Arabia). The mean pollution rate in the selected studies was 26.88μg/m³ for PM_2.5 , 58.23μg/m³ for PM₁₀ and 24.79μg/m³ for NO₂. “These current analyses confirm the positive correlation between the fine particles levels and the prevalence of keratoconus,” The authors stated in their paper. “PM₁₀ levels appear to be more strongly correlated with prevalence than PM_2.5 levels, consistent with our previous findings.” Also, NO₂ levels appear to be correlated with keratoconus prevalence despite a more limited number of studies.

This research suggests pollution can affect the cornea both directly and indirectly, they said. “The indirect pathway involves the stimulation of recognized risk factors for keratoconus, such as eye rubbing and atopy,” they wrote. “Air pollution is known to reduce the stability and quality of the tear film, but also increase eye irritation and inflammation.” The direct pathway involves modifying corneal structure by several means. “Exposure to PM_2.5 has been shown to lead to modifications in the epithelial structure and their microvilli, which become shorter and fewer,” they explained, offering one example.

Patients with keratoconus may be more sensitive to the accumulation of reactive oxygen species, associated with air pollution, in the cornea, which could elevate the rate of cell death and contribute to corneal thinning, according to the study authors. “This process may induce the development of keratoconus in genetically predisposed individuals and exacerbate its evolution where it’s already present,” they wrote.

In this study, patient origins weren’t generally accounted for, and as epidemiological studies do, it considered an entire population of a given area, but not the proportion of patients from other cities or regions. The authors mentioned these two limitations and added that a third limitation concerns the particle levels used in the analyses. “These are annual averages, making it impossible to assess whether variations throughout the year or significant peaks have an impact on keratoconus prevalence,” they wrote. “A more comprehensive study of a specific area, incorporating meters such as air pollution, air temperature, and pollen concentration, could provide valuable insights into these questions.”

Although further research is needed, this study appears to show that air pollution influences keratoconus development indirectly by stimulating eye rubbing and atopy, and that fine particles may directly affect corneal structures and increase cell apoptosis, the authors concluded.