Nitrogen Dioxide Exposure May Induce Conjunctivitis

Close up of an eye with conjunctivitis
Researchers explore the association between conjunctivitis ambient nitrogen dioxide exposure in Jinan, China.

Even short-term environmental exposure to nitrogen dioxide (NO2) can induce conjunctivitis, particularly infective conjunctivitis, according to research published in Atmospheric Environment. These findings underline the importance of stricter air control measures to promote ocular health, the study concludes.

Adverse effects resulting from air pollution can be particularly severe in developing regions with poor air quality. A team of researchers in Jinan, China, conducted a study to examine the association between short-term exposure to NO2 and outpatient conjunctivitis — allergic, infective, and unspecified conjunctivitis — visits between 2014 and 2019. 

Daily mean concentrations of NO2, fine particulate matter (PM2.5), particulate matter (PM10), and sulfur dioxide (SO2) were recorded (49.74 µg/m3, 72.13 µg/m3, 137.32 µg/m3, and 36.05 µg/m3, respectively); 8-hour maximum mean ozone (O3) concentration was 72.10 µg/m3, indicative of severe air pollution. The daily mean temperature was 15.50 ° Celsius. 

Overall, 42,123 outpatient visits for all 3 types of conjunctivitis were recorded during the study period (8504 infective, 6429 allergic, and 27,190 unspecified). Within this caseload, 62.1% of patients were between 18 and 65 years of age, and 22,933 were women.

Investigators found that overall, NO2 was significantly associated with the number of outpatient conjunctivitis visits across all subtypes at lag0, lag01, lag02, and lag03; estimates were larger at lag0, with an excess risk of 1.00% (95% CI, 0.13%-1.87%). Investigators also found significant associations between NO2 and infective conjunctivitis, with a similar lag pattern to conjunctivitis as a whole (excess risk, 2.60%; 95% CI, 0.69%-4.54%). No significant results were noted for allergic or unspecified conjunctivitis (excess risk, -0.43% and 0.16%, respectively). 

Stratified analyses of lag0 found significant associations between NO2, conjunctivitis, and infective conjunctivitis among women. Age-specific analyses found an association between NO2 and infective conjunctivitis in people younger than 18 years of age and people between 18 and 65 years. When stratified by season, data showed more significant estimates in the cold season for both conjunctivitis and infective conjunctivitis. 

After the addition of other pollutants to the models, excess risk for associations between NO2, conjunctivitis, and infective conjunctivitis remained significant; estimates increased slightly when PM2.5, PM10, SO2, and O3 were adjusted (excessive risk, 2.65%, 2.38%, 2.67%, and 1.83%, respectively). 

Study limitations include the ecological biases inherent to a time-series study, potential misclassification of exposure measurement due to a lack of individual exposure measurement data, the use of a single outpatient center, limiting generalizability, and the exclusion of pollen, which is an important risk factor for allergic conjunctivitis. 

“To the best of our knowledge, this is the first study to simultaneously explore the associations of NO2 with conjunctivitis and its 3 specific subtypes,” the research concludes. “Our study suggests that short-term exposure to NO2 might serve as a trigger to induce conjunctivitis, especially infective conjunctivitis.” 


Guo H, Zhang S, Zhang Z, et al. Short-term exposure to nitrogen dioxide and outpatient visits for cause-specific conjunctivitis: A time-series study in Jinan, China. Atmospheric Environment. Published online February 15, 2021. doi:10.1016/j.atmosenv.2021.118211