Background <p>Evidence remains sparse and inconclusive regarding the acute effect of ambient sulfur dioxide (SO₂) exposure on allergic conjunctivitis (AC). The main objective of this study was to assess the temporal relationship between acute SO₂ exposure and AC risk.</p> Methods <p>This study employed a case-crossover design incorporating daily data on outpatient visits for AC from five hospitals across five Chinese cities, spanning from January 2014 to December 2022. Daily pollution and meteorological data were retrieved through national air quality surveillance system. To examine the link between SO₂ and AC, we utilized conditional logistic regression models combined with random-effects meta-analyses.</p> Results <p>Over the study period, there were 109,985 outpatient visits for AC, with 63,423 (57.7%) males and 46,562 (42.3%) females. We observed a significant positive association between SO₂ and AC outpatient visits, with an adjusted odds ratio (OR) of 1.045 (95% CI: 1.011, 1.079) for every standard deviation (SD) increase in SO₂ concentrations. This positive association remained consistent across sex, age, and season. Furthermore, the association remained significant and robust when adjusting for other pollutants, such as fine particulate matter, carbon monoxide, nitrogen dioxide and ozone, in both two-pollutant and multi-pollutant models.</p> Conclusions <p>This is the largest study in China to demonstrate that short-term SO₂ exposure increases the risk of AC. Our findings emphasize that reducing SO₂ pollution is important to protect ocular health and provide insights for future standards and policies.</p>