Air pollution in Japan has decreased over time, but concerns remain about fine particulate matter (PM2.5) and its impact on health. Ground measurements in China and Japan show that PM2.5 pollution varies by season and region. Fukuoka is heavily affected by pollution from outside Japan, while Tokyo's pollution is mostly from local sources. Winter to spring PM2.5 peaks are due to outside pollution, while summer to autumn peaks are from local sources. Overall, PM2.5 pollution is decreasing in Japan due to lower emissions and concentrations in China.