Air pollution and meteorological conditions significantly contribute to the worsening of allergic conjunctivitis: a regional 20-city, 5-year study in Northeast China

The study investigates whether and how air pollution and meteorological conditions are associated with the incidence of allergic conjunctivitis (AC) in Northeast China. Motivated by rapid industrialization and urbanization, and rising allergic diseases, the authors note that conjunctiva is directly exposed to ambient air, suggesting susceptibility to pollution and climate effects. Prior evidence links ambient pollutants and weather changes to AC onset and exacerbation, but Northeast China, characterized by heavy industry, winter heating, straw burning, and complex topography limiting pollution dispersion, has not been studied. The objective is to quantify spatiotemporal patterns in AC incidence across major cities from 2014–2018 and assess correlations with key air pollutants and meteorological variables, identifying thresholds beyond which AC incidence increases, to inform public health and environmental policy.

The paper summarizes global and regional AC prevalence variability: USA 40%, Japan 30%, Pakistan 19.2%, Ethiopia 11.1%. Within China, schoolchildren in Shanghai show 28% prevalence, whereas Tibet shows 5.2%, highlighting regional differences potentially driven by environmental factors, ethnicity, disease heterogeneity, and allergens. Seasonal patterns are noted (e.g., higher incidence in May and September in Korea), possibly due to diurnal temperature variations increasing aeroallergens like pollen. Air pollution components include particulate matter (PM10, PM2.5), diesel exhaust particles, and gaseous pollutants (CO, O3, NO2, SO2). Traffic-related air pollutants are major urban sources. A prior Shanghai study found outpatient AC visits correlated with temperature, O3, and NO2. Northeast China’s unique pollution profile includes increased haze days, winter heating emissions, straw burning causing extreme PM2.5 spikes (>1000 µg m−3), and mountainous terrain inhibiting dispersion. Despite this, the region lacked AC-focused environmental studies prior to this work.

- Study area and period: 20 prefecture-level cities across three provinces (Heilongjiang, Jilin, Liaoning) in Northeast China, 2014–2018. - Data: Incidence of allergic conjunctivitis (IAC) from health records; ambient air pollutants (PM2.5, PM10, SO2, CO, NO2, O3); meteorological variables (air pressure, air temperature, relative humidity, visibility, precipitation, wind speed at 2 m and 10 m, wind direction). - Analyses: - Spatiotemporal analysis of IAC, pollutants, and meteorological factors at city and provincial scales, including monthly and interannual patterns. - Correlation coefficient analysis using paired monthly data at city level (2014–2018) and annual provincial paired data (2014–2018) to assess associations between IAC and environmental variables. - Identification of threshold concentrations for pollutants and meteorological factors representing turning points above which AC incidence increases, and comparison of these thresholds to national (China) and WHO air quality standards. - Presentation: Summary statistics by year and province (Table 1); monthly variation plots for 2017; spatial distributions of IAC and pollutants.

- Spatial patterns: Higher IAC in Liaoning Province; provincial capitals (Shenyang, Changchun, Harbin) show significantly higher IAC than other cities. Within Liaoning, larger cities such as Dalian and Anshan have higher IAC. - Temporal patterns: IAC increased significantly from 2014 to 2018, with an average annual growth rate of 7.6%. By province, increases were Jilin 16.7%, Heilongjiang 5.9%, and Liaoning 5.7%. Seasonally, IAC peaks July–September with a maximum in August. - Air pollution and meteorology patterns: O3 increased interannually; PM2.5, PM10, SO2, NOx, and CO generally decreased but remained seasonally elevated during October–March due to heating and straw burning. O3 peaks in June–July; other pollutants peak in October and January. Visibility shows peaks in May and September/October and a minimum in July/August due to rainfall. Temperature and pressure are inversely related seasonally; relative humidity peaks in August and is lowest in April; wind speed peaks in April–May and November. - Correlations (annual provincial paired data, 2014–2018): - Positive correlations: IAC with PM2.5 (r=0.57, p<0.05), NO2 (r=0.54, p<0.05), air pressure (r=0.63, p<0.05), wind speed (r=0.54, p<0.05); and very significant positive correlations with CO (r=0.92, p<0.01), SO2 (r=0.85, p<0.01), PM10 (r=0.83, p<0.01), O3 and air temperature (very significant, coefficients reported up to 0.75). - Negative correlation: IAC with relative humidity (r=−0.61, p<0.05). - Monthly paired city-level data did not show significant links, likely due to unmeasured confounders such as pollen. - Relative importance: Although both meteorology and pollution are associated with AC, air pollution appears to have a more prominent effect. - Thresholds (turning points above which AC increases): PM10=70 µg m−3; PM2.5=45 µg m−3; SO2=23 µg m−3; NO2=27 µg m−3; O3=88 µg m−3. - Comparison to standards: PM10 and PM2.5 thresholds exceed China/WHO standards, implying lower environmental risk relative to current standards; SO2 threshold aligns with WHO but is much lower than China’s standard, indicating greater risk under China’s standard; NO2 and O3 thresholds are below current standards, marking them as major environmental risk factors for IAC.

The findings demonstrate that higher concentrations of ambient pollutants and certain meteorological conditions are associated with increased incidence of allergic conjunctivitis in Northeast China. Spatial clustering of higher IAC in provincial capitals aligns with higher emissions and pollutant concentrations in these urban centers. Seasonal peaks of IAC during hot months correspond to higher O3 and elevated temperatures, while the negative association with relative humidity suggests dry conditions exacerbate AC. Strong positive correlations with CO, SO2, PM10, O3, and temperature indicate that both primary and secondary pollutants, along with heat, contribute to AC burden, with air pollution exerting a stronger overall influence than meteorology. The identification of pollutant thresholds below or near current standards for NO2 and O3 underscores the need to re-evaluate health-based limits to protect against AC. These results support targeted pollution control (notably NO2 and O3 precursors) and public health advisories during hot, dry periods in the region.

This regional 20-city, 5-year analysis provides the first comprehensive assessment linking ambient air pollutants and meteorological factors to allergic conjunctivitis incidence in Northeast China. The study documents rising IAC, urban hotspots, strong pollutant–IAC correlations, and specific pollutant thresholds relevant to policy. It highlights that air pollution, more than meteorology, drives AC variation, with NO2 and O3 presenting particular risks at levels below current standards. These insights inform environmental protection strategies and potential adjustment of health-based air quality standards. Future research should incorporate aeroallergen (e.g., pollen) measurements, finer spatial and temporal resolution, and individual-level clinical data to better disentangle pollutant–allergen interactions and causal pathways.

Monthly paired analyses did not detect significant associations, likely because key confounders such as pollen were not measured; the study acknowledges that plant pollen, an important driver of intra-annual AC variation, was not fully accounted for. The analysis relies on aggregated city/provincial data, which may obscure local heterogeneity and individual-level exposure-response relationships. Detailed methodology for determining thresholds is not provided in the excerpt, potentially limiting interpretability.