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The relationship between air pollution and pneumonia is poorly understood.To examine relationships between short-term air pollution exposure and number and severity of pneumonia cases along the Wasatch Front in Utah, a region with periodic high levels of outdoor air pollution.We applied time-stratified case-crossover analyses with distributed lag to patients presenting to seven emergency departments with pneumonia over a 2-year period. We compared levels of particulate matter less than or equal to 2.5 μm in aerodynamic diameter, nitrogen dioxide, and ozone at patient residences with emergency department cases, hospitalizations, objectively defined severe pneumonia, and mortality. We calculated direct cost impacts of particulate matter less than or equal to 2.5 μm in aerodynamic diameter reduction.We evaluated 4,336 pneumonia cases in seven hospitals. Among adults aged 65 years and older, we found consistently positive associations between particulate matter less than or equal to 2.5 μm in aerodynamic diameter within 6 days of presentation and instances of pneumonia (Lag Day 1 adjusted odds ratio, 1.35 per 10 μg/m3 over 12 μg/m3; 95% confidence interval, 1.16–1.57), severe pneumonia (Lag Day 1 adjusted odds ratio, 1.38; 95% confidence interval, 1.06–1.80), and inpatient mortality (Lag Day 5 adjusted odds ratio, 1.50; 95% confidence interval, 1.03–2.16). Smaller associations were found between nitrogen dioxide exposure and pneumonia occurrence, severity, and inpatient and 30-day mortality. Ozone exposure was modestly associated with increased instance and severity of pneumonia in younger adults. Particulate matter less than or equal to 2.5 μm in aerodynamic diameter and nitrogen dioxide effects were greatest in colder months, and ozone effects were greatest in warmer months. Reduction of particulate matter less than or equal to 2.5 μm in aerodynamic diameter levels to less than 12.0 mg/m3 could prevent 76–112 cases of pneumonia per year in these hospitals serving approximately half of the Wasatch Front's population, reducing direct medical facility costs by $807,000 annually.Among older adults, short-term ambient particulate matter less than or equal to 2.5 μm in aerodynamic diameter exposure is associated with more emergency department visits and hospitalizations for pneumonia, severe pneumonia, increased mortality, and increased healthcare costs. Nitrogen dioxide and ozone modestly increase pneumonia risk and illness severity.