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Studies have shown that short- and long-term exposure to particulate matter (PM) can increase the risk of asthma morbidity and mortality. However, the effect of medium-term exposure remains unknown. We aim to examine the effect of medium-term exposure to size-fractioned PM on asthma exacerbations among asthmatics with poor medication adherence. We conducted a longitudinal study in China based on the National Mobile Asthma Management System Project that specifically and routinely followed asthma exacerbations in asthmatics with poor medication adherence from April 2017 to May 2019. High-resolution satellite remote-sensing data were used to estimate each participant's medium-term exposure (on average 90 days) to size-fractioned PM (PM 1 , PM 2.5 , and PM 10) based on the residential address and the date of the follow-up when asthma exacerbations (e.g., hospitalizations and emergency room visits) occurred or the end of the follow-up. The Cox proportional hazards model was employed to examine the hazard ratio of asthma exacerbations associated with each PM after controlling for sex, age, BMI, education level, geographic region, and temperature. Modelling results revealed nonlinear exposure-response associations of asthma exacerbations with medium-term exposure to PM 1 , PM 2.5 , and PM 10. Specifically, for emergency room visits, we found an increased hazard ratio for PM 1 above 22.8 µg/m3 (1.060, 95 % CI: 1.025–1.096, per 1 µg/m3 increase), PM 2.5 above 38.2 µg/m3 (1.032, 95 % CI: 1.010–1.054), and PM 10 above 78.6 µg/m3 (1.019, 95 % CI: 1.006–1.032). For hospitalizations, we also found an increased hazard ratio for PM 1 above 20.3 µg/m3 (1.055, 95 % CI: 1.001–1.111) and PM 2.5 above 39.2 µg/m3 (1.038, 95 % CI: 1.003–1.074). Furthermore, the effects of PM were greater for a longer exposure window (90–180 days) and among participants with a high BMI. This study suggests that medium-term exposure to PM is associated with an increased risk of asthma exacerbations in asthmatics with poor medication adherence, with a higher risk from smaller PM. [Display omitted] • We examined the effects of PM on asthmatics with poor medication adherence. • We first reveal the effect of medium-term exposure to PM on asthma exacerbations. • Exposure to smaller particles has a greater effect on asthma exacerbations. [ABSTRACT FROM AUTHOR]