In January 2013, extremely high concentrations of fine particles (PM_2.5) were observed around Beijing, China. In Japan, the health effects of transboundary air pollution have been a matter of concern. We examined the association between the levels of outdoor PM_2.5 and other air pollutants with primary care visits (PCVs) at night due to asthma attack in Himeji City, western Japan.

A case–crossover study was conducted in a primary care clinic in Himeji City, Japan, involving 112 subjects aged 0–80 years who visited the clinic due to an asthma attack between 9 p.m. and 6 a.m. during the period January–March, 2013. Daily concentrations of particulate matter, ozone, nitrogen dioxide, and some meteorological elements were measured, and a conditional logistic regression model was used to estimate the odds ratios (OR) of PCVs per unit increment in air pollutants or meteorological elements.

Of the 112 subjects, 76 (68 %) were aged <15 years. We did not note any association between daily PM_2.5 levels and PCVs due to asthma attack at night. A positive relation between ozone and PCVs due to asthma attack was detected. The OR per 10 ppb increment in daily mean ozone the day before the visit was 2.31 (95 % confidence interval 1.16–4.61).

These findings do not support an association between daily mean concentration of PM_2.5 and PCVs at night. However, we did find evidence suggesting that ozone is associated with PCVs.