Air pollution is a major global health concern, contributing to increased morbidity and mortality, especially for allergic diseases like allergic conjunctivitis (AC). AC, a common ocular surface disease, significantly impacts quality of life and incurs substantial economic costs. Evidence suggests air pollution and climate change exacerbate AC symptoms and increase incidence. Prevalence varies widely across regions due to factors including environmental risk factors (air pollution and climate), ethnic differences, disease heterogeneity, and allergen species. Seasonal variations in AC are also influenced by climate, with high temperatures, low humidity, and low latitude associated with higher prevalence. While some studies have explored the link between air pollution and AC in other regions, research in Northeast China, a region experiencing significant air pollution and climate change, is lacking. This study aims to fill this gap by analyzing the relationship between AC incidence, air pollutants (PM2.5, PM10, CO, SO2, NO2, O3), and meteorological factors (air temperature, wind speed, relative humidity) in Northeast China over a five-year period. This is crucial for informed resource allocation for AC prevention and control.

Existing literature demonstrates a global prevalence of AC ranging from 5.2% in Tibet to 40% in the USA, highlighting regional variations. These differences are attributed to diverse factors like environmental conditions, ethnicity, disease heterogeneity, and allergen types. Studies in Shanghai have shown a positive correlation between AC outpatient visits and increased levels of temperature, O3, and NO2. Air pollution encompasses both particulate matter (PM10, PM2.5, DEPs) and gaseous pollutants (CO, O3, NO2, SO2). PM10 includes mold, dust, and pollen, while PM2.5 comprises organic compounds, metallic particles, and combustion particles. Traffic-related air pollutants (TRAPs) are a significant source in urban areas. While studies exist in other regions of China, notably Hong's study in Southeast China, a comprehensive analysis of the impact of air pollutants and meteorological variables on AC in Northeast China was previously absent.

This study analyzed data on allergic conjunctivitis incidence (IAC), air pollutants, and meteorological factors across 20 prefecture-level cities in Northeast China from 2014 to 2018. The data included annual and monthly averages for various pollutants (PM2.5, PM10, SO2, CO, NO2, O3) and meteorological parameters (air pressure, air temperature, relative humidity, visibility, precipitation, wind speed, and wind direction). Spatial and temporal patterns of IAC, air pollution, and meteorological factors were examined. Correlation analysis was used to assess the association between IAC and environmental factors using both monthly and annual paired data. Threshold values of atmospheric pollutants and meteorological factors that induce a significant increase in AC cases were determined.

The study revealed a significant increasing trend in AC incidence (7.6% annually) from 2014-2018, with higher rates in provincial capitals. Spatially, AC prevalence was highest in Liaoning Province, particularly in Dalian and Anshan. Temporally, peak AC incidence occurred in late summer and early autumn (July-September), with the highest incidence in August. Provincial capital cities showed higher concentrations of PM10, PM2.5, SO2, NOx, and CO, with O3 concentrations highest in Jilin city and coastal cities of Liaoning province. Annual average O3 concentrations increased over the study period, while other pollutants generally decreased. Higher concentrations of most pollutants occurred from October to March. Initially, monthly correlation analysis showed no significant relationship between IAC and environmental factors, potentially due to the omission of pollen data. However, annual provincial data analysis revealed significant positive correlations between IAC and PM2.5, NO2, air pressure, wind speed, CO, SO2, PM10, O3, and air temperature. A significant negative correlation was found between IAC and relative humidity. Comprehensive analysis using PM2.5, O3, temperature, and relative humidity indicated that increasing temperature and decreasing humidity were associated with higher IAC, while higher levels of PM2.5 and O3 worsened AC, especially in dry and hot conditions. Threshold values for pollutants and meteorological factors were also established. The PM10 and PM2.5 thresholds were higher than current Chinese and WHO standards, suggesting a relatively lower environmental risk at those levels for AC. Conversely, the SO2 threshold was comparable to WHO standards but lower than Chinese standards, suggesting a greater environmental risk. The NO2 and O3 thresholds were below current standards, indicating these are major environmental risk factors for AC.

This study demonstrates a clear link between AC incidence and both air pollution and meteorological conditions in Northeast China. The positive correlations with various pollutants, particularly SO2, NO2, and O3 (even below current standards), and the negative correlation with relative humidity, highlight the complex interplay of these factors in influencing AC. The threshold values identified provide valuable insights for environmental policy and public health interventions. The lack of a significant correlation in monthly data, potentially due to the absence of pollen data, suggests that other factors beyond air pollution and the meteorological parameters studied in this research contribute to intra-annual AC variations. The higher PM10 and PM2.5 thresholds than current standards may warrant further investigation and adjustments to existing standards.

This study is the first to comprehensively investigate the relationship between AC, air pollution, and meteorological conditions in Northeast China. The findings emphasize the significant impact of air pollution and climate factors on AC incidence and severity. The identified thresholds provide crucial information for developing effective environmental policies and public health strategies to mitigate the impact of air pollution on AC. Further research should investigate the role of pollen and other allergens, alongside the potential synergistic effects of pollutants and meteorological factors.

The study's limitations include the potential exclusion of pollen data, which may affect the interpretation of monthly correlations. The reliance on prefecture-level city averages may mask variations at a more granular level. The study also focuses on Northeast China, and its findings may not generalize to other regions with different climatic or pollution characteristics. Further, more refined data on the specific sources of air pollution might improve the understanding of the interactions between pollutants and AC.