Dry eye is a multifactorial ocular surface disease. The tear film is unstable and/or ocular surface damage occurs due to an abnormal amount or quality of tear fluid or fluid dynamics, which leads to ocular discomfort and visual dysfunction. The clinical manifestations include dry eyes, discomfort, pain, and loss of tear film homeostasis on the ocular surface, which leads to reduced tear production and excessive evaporation[8]. Various names such as dry eye syndrome and dry eye disease that have appeared clinically in China are collectively called dry eye. One of the most significant features of this condition is that women are more susceptible[9]. Moss et al.[10] reported that the incidence of dry eye in women is about 1.5 times that in men. This is consistent with our results. In the past, the understanding of dry eye was limited to a severe lack of tears. Recent studies have shown that the condition also includes eye discomfort due to unstable tear film caused by normal tear volume and other reasons. The primary or external causes of dry eye are environmental factors, which mainly refer to excessive tear evaporation caused by smoke, sand, air conditioning, etc. One study reported that ocular surface system dysfunction can cause symptoms through two related factors affecting tear film: high osmotic pressure and tear film instability[11]. Previous studies have described a link between outdoor air pollution and dry eye in the population. Dry eye is related to increased ozone concentration and decreased humidity[12]. Yongsub et al.[13] reported that air pollution can increase ocular surface damage. However, an association between the atmospheric pollutants O3 and NO2 and the prevalence of dry eye has not yet been reported. Our results demonstrated the relationship between environmental pollution and dry eye prevalence through case analysis.
Subjects in the 41–61 age group have the most dry eye disease. Correlation analysis showed that O3 levels affected the outpatient visit rate of participants younger than 60 years old, but there was no obvious correlation between changes in air pollutant O3 and dry eye in older subjects. This may be due to the increase in other ocular surface disease due to age, which makes them more susceptible to dry eye. NO2 concentration was relevant to the number of dry eye patients in all age groups. The analysis of the correlation between climate and dry eye revealed that the number of outpatient visits for dry eye was significantly higher in the summer compared to the other seasons. This may be due most patients living in closed spaces during the hotter months. However, due to an insufficient number related cases, we could not obtain more accurate data to support this hypothesis.
Although the study used a large sample of clinical patient data, it does have some limitations. First, we did not have data on the concentrations of pollen and microorganisms in the environment, which are also important outdoor environmental factors related to dry eye. The overlap between air pollutant concentration and allergens (e.g., pollen) may affect our results. However, our data show that NO2 concentrations peak in winter, while pollen levels are higher in spring and summer. Secondly, the hazardous substances in the surveyed environment are not fully accurate because people spend more time indoors. The lack of indoor air pollutant measurement is one of the limitations of our research. Penetration of environmental pollutants into the indoor environment generally occurs under high-concentration pollution conditions, and we recommend conducting real-time indoor ventilation measurements to address this issue. In addition, since this was a registration study, we could not take into account the geographical mobility of the population in the subsequent period. There are also some individual differences such as body mass index, social demographics, education level, and overall health that could not be further analyzed. This is an inherent limitation of research using pre-existing databases. Finally, although we did have access to outpatient records, uncertainty was inevitable because it was impossible to confirm the exact diagnosis of each case without personal medical records.