Findings from a literature review published in Ophthalmic Epidemiology reaffirmed the deleterious effects of air pollution on keratoconus (KC) prevalence.
Give me some background.
KC is a progressive asymmetrical corneal ectasia with three known risk factors, including:
- Exposure to ultraviolet (UV) rays
- Eye rubbing
- Atopy
A 2021 study by the same research team demonstrated that fine particulate matter (PM) and nitrogen oxide (NO2) might be an emerging risk factor for KC—as they may indirectly exacerbate risk factors such as atopy and eye rubbing.
Tell me more about air pollution.
PM refers to the mixture of solid particles and liquid droplets found in the air and can be subdivided into:
- PM10: Inhalable particles, with diameters that are typically 10 μm and smaller
- PM2.5: Fine inhalable particles, with diameters that are generally 2.5 μm and smaller
Per the Environmental Protection Agency (EPA), fine particles (i.e., PM2.5) pose the greatest risk to health as they:
- Can embed deep into the lungs and enter the bloodstream after inhalation
- Are the main cause of reduced visibility (haze) in parts of the United States
Note: Most particles form in the atmosphere as a result of complex chemical reactions with pollutants from power plants, industries, and automobiles, such as sulfur dioxide (SO2) and NO2.
Now, talk about the study.
Investigators included 44 relevant studies according to strict selection criteria from four databases:
- PubMed
- Research Gate
- Google Scholar
- International Journal of Keratoconus and Ectatic Corneal Diseases
The levels of PM and NO2 were extracted from available World Health Organization (WHO) databases and correlated with prevalences from epidemiological studies.
Findings?
The prevalence of KC ranged from 4.29 per 100,000 inhabitants in Taiwan to 6,690 per 100,000 in Saudi Arabia.
The mean pollution rates in the selected studies were as follows:
- PM2.5: 26.88 ± 25.26 μg/m3
- PM10: 58.23 ± 60.98 μg/m3
- NO2: 24.79 ± 12.58 μg/m3
Further, Pearson correlation tests demonstrated a significant positive correlation between the prevalence of KC and all analyzed particle rates:
- PM2.5: R = 0.58, p < 0.001
- PM10: R = 0.67, p < 0.001
- NO2: R = 0.58, p < 0.001
Expert opinion?
Of note, PM10 levels appeared to more strongly correlate with KC prevalence than PM2.5 levels, which was consistent with findings from the previous study.
The researchers concluded that KC patients may be more sensitive to air pollution as the resulting accumulation of reactive oxygen species may increase the risk of epithelial cell death and contribute to corneal thinning.
- Meaning: Air pollution “may induce the development of KC in genetically predisposed individuals and exacerbate its evolution where it’s already present,” the study authors noted.
Limitations?
These included:
- Patient origins were not accounted for in this study
- The epidemiological studies considered the entire population of a given area and not the proportion of patients from other cities or regions
- The PM levels used in the analyses were annual averages, making it impossible to assess whether variations throughout the year or significant peaks may impact KC prevalence
Take home.
These findings suggest that fine PM may be a risk factor for KC—likely caused by pollutants indirectly exacerbating risk factors such as atopy and eye rubbing.
Further, atmospheric pollution may also have a direct effect on the cornea, leading to disturbances in the structure of the epithelium and increasing cell apoptosis.
Next steps?
Additional research is warranted to validate these findings and further develop insights using additional factors, such as air temperature and pollen concentration.