AbstractBackgroundOutdoor Alternaria and Cladosporium spores are ubiquitous. Few studies have assessed their impact on asthma hospitalizations providing conflicting results, mainly focused on vulnerable paediatric populations. We aimed to study the impact of outdoor Alternaria and Cladosporium concentrations on acute hospitalizations in the Capital Region of Denmark.MethodsThis is a bi‐directional case‐crossover study with 26 years of national registry data at individual level on acute asthma hospitalizations and daily average data on Alternaria and Cladosporium, pollen (Artemisia, Poaceae), maximal temperature, and air pollution. Conditional logistic regression models were applied to assess the associations. Concentration quartiles at lag 0 were used for categorizing the exposure.ResultsFor lags 0–2, the odds of hospitalization were significantly higher for both Alternaria and Cladosporium at concentration quartile 2–4 compared with quartile 1. When stratified for age and sex, odds of hospitalization at Alternaria quartiles 2–4 were significantly higher in males below 40 years at lag 0–2, and at lag 0 in females (18–30 years), while quartiles 2–4 of Cladosporium concentrations were associated with significantly higher odds in boys (0–17 years) at lag 1–3, males (18–39 years) at lag 0–1, females (18–39 years) at lag 1–2, males (40–64 years) at lag 0–2, females (40–64 years) at lag 0 and 2, in seniors (65+ years) male at lag 1–2 and female at lag 0–1. The effect of Alternaria varied significantly depending on the level of Cladosporium (p < .0001).ConclusionAmbient Alternaria and Cladosporium spores can induce asthma hospitalizations. Males are more susceptible to both genera. Males and females under age 40 years are more susceptible to Alternaria.