BACKGROUND AND AIM: Studies have shown an acute association between the occurrence of thunderstorm conditions in the presence of high pollen and severe asthma, but minimal work has studied these events over a multi-year time series, using detailed measurements of storms and pollen. METHODS: We estimate the change in risk between thunderstorm asthma events and asthma-related emergency room visits in the Minneapolis-St. Paul metropolitan area for the years 2007-2018. We defined thunderstorm asthma events as daily occurrence of 2 or more lightning strikes during periods of high pollen, interpolated using weather and pollen monitor data and modeled lightning counts. Daily counts of asthma-related emergency department visits were acquired from the Minnesota Hospital Association. We use a 2-stage approach combining zip code level quasi-Poisson time-series regressions in a meta-analysis to estimate an overall relative risk of hospitalizations during thunderstorm asthma events. RESULTS:We observed a 4.4% (RR = 1.044; 95% CI (1.013,1.076), increase in asthma emergency department visits on the day of thunderstorm asthma event and a 3.2% (RR 1.032; 95% CI (1.003,1.062) increase on the day after the storm event. Our findings were robust to adjustment for temperature, humidity, wind, precipitation, ozone, PM2.5, day of week, and natural splines for seasonal variation in asthma cases. Occurrence of lightning alone or pollen alone shows no association with risk of severe asthma. CONCLUSIONS:Our results support an association between co-occurrence of lightning and pollen and risk of asthma hospitalization. This model, while previously described with specific epidemic events, has not been well studied in a time series model to evaluate long-term risk, using measurement of both lightning and pollen, and never using unique small area exposure assignment and outcome counts. Our findings indicate that thunderstorm asthma may be an unidentified environmental risk factor for severe asthma. KEYWORDS: asthma, pollen, thunderstorms, climate