Short-Term PM2.5 Exposure and Daily Cancer Mortality Across 47 Japanese Prefectures: A Nationwide Time-Stratified Case-Crossover Study, 2013-2022.

Short-term exposure to fine particulate matter (PM2.5) is an established risk factor for cardiopulmonary mortality, but its acute association with cancer mortality remains poorly characterized. We examined associations between short-term PM2.5 fluctuations and daily all-cancer and site-specific mortality across all 47 Japanese prefectures from January 1, 2013, to December 31, 2022. Using a nationwide time-stratified case-crossover design, we fitted prefecture-specific conditional quasi-Poisson regression models and pooled the estimates by random-effects meta-analysis, expressing associations as the excess relative risk (ERR) per 10 μg/m³ increase in PM2.5 (lag 0-2 days). Sensitivity analyses varied lag structures, meteorological spline specifications, and co-pollutant adjustment. A 10 μg/m³ increase in short-term PM2.5 was associated with a 0.88% increase in all-cancer mortality (95% CI: 0.57-1.18; p < 0.001). The association was consistent across sex, age, and primary site (all interaction p > 0.40), with significant estimates for breast (ERR 2.06%), colorectal (1.06%), stomach (0.81%), and lung (0.77%) cancers. Associations attenuated with longer moving-average windows and were no longer statistically significant from lag 0-5 onward. A measurable fraction of cancer deaths was attributable to PM2.5 above the WHO 24-h guideline (15 μg/m³), with the largest fractions tracking prefecture-specific coefficients rather than ambient concentration. These findings indicate that short-term PM2.5 exposure is associated with higher daily mortality across diverse cancer types in Japan, even at relatively low ambient levels. Because the individual-level effect size is small and attenuated to non-significance after adjustment for either NO₂ or SO₂, these findings are best interpreted as supporting population-level air-quality measures rather than individual clinical intervention.