Fluctuations in emergency department (ED) utilization may be related to pollutant concentration variations that can heighten acute pathologies, exacerbate chronic conditions or indicate increased human activity. This study investigates the association between particulate matter (PM2.5-PM10) concentrations in patients' residential areas and ED visits. Retrospective, single-centre, time-series cohort study on a 100km urban area around a level 1 hospital in Florence, Italy, with 307,279 ED visits of patients of age > 14 years between 01/01/2019 and 31/12/2022. PM2.5 and PM10 daily concentrations, recorded by 14 air-quality stations, were interpolated at patients' residences, in a 250m grid, with inverse distance weighting and averaged on various time windows (lag) up to 30 days before ED visit. The association between ED visits and pollutant concentrations as percent excess of incidence rate (%Er) with 95% confidence intervals were investigated with generalized additive models. Significant association was found with short-term PM2.5 and PM10 fluctuations (PM2.5: 1.24%Er, 0.59-1.99 95%CI; PM10: 1.79%Er, 1.08-2.72). Such association was stronger for the subgroups of deceased patients (PM2.5 5.71%Er, 1.44-8.99; PM10 6.38%Er, 2.02-10.83), elderly patients presenting with dyspnoea (PM2.5 4.18%Er, 2.36-6.33; PM10 4.99%Er, 2.77-7.32) and younger patients suffering traumatic events (PM2.5 1.29%Er, 0.32-2.51; PM10 2.14%Er, 0.88-3.50). Exposure-response curves showed that particulate matter is associated with ED visits even at concentrations below the WHO guard levels. A short-term rise in PM2.5 and PM10, even below guard levels, was associated with increased ED utilization, particularly related to elderly patients with dyspnoea and the concomitant lurking occurrence of vehicle accidents in the younger population. Trial registration: NCT06491290; 09/07/2024; retrospectively registered.