Particulate matter (PM) and other air pollutants are reportedly associated with both lung and systemic inflammation; however, an association between air pollutants and pneumonia outcomes has not been well established. Therefore, we evaluated the effect of air pollutants on the short-term outcomes of emergency department patients with pneumonia. We collected data on PM2.5 (aerodynamic diameter < 2.5 µm), PM10 (aerodynamic diameter < 10 µm), sulfur dioxide (SO2), nitrogen dioxide (NO2), and ozone from 11 air-quality monitoring stations in Kaohsiung City between January 1, 2008, and December 31, 2013. Medical records were extracted for non-trauma patients aged > 17 years who had visited the emergency department with a principal diagnosis of pneumonia. In-hospital mortality and the association of air pollutant exposure with the need for invasive respiratory and/or vasopressor support (IRVS) within 72 h were evaluated. Interquartile range (IQR) increments of PM2.5 and PM10 were associated with an increased IRVS risk with odds ratios (ORs) of 1.211 (95% confidence interval [CI], 1.031–1.419) and 1.194 (95% CI, 1.020–1.394) on lag 1, respectively, and per-IQR increments of NO2 were associated with an increased IRVS risk with an OR of 1.146 (95% CI, 1.004–1.308) on lag 2. IQR increments of PM2.5 and NO2 were associated with an increased in-hospital mortality risk with ORs of 1.202 (95% CI, 1.100–1.429) and 1.175 (95% CI, 1.014–1.360), respectively. During the warm season, IQR increments of PM2.5, PM10, and NO2 corresponded with an increased IRVS risk, with ORs of 1.333 (95% CI, 1.078–1.644), 1.348 (95% CI, 1.090–1.665), and 1.321 (95% CI, 1.101–1.585), respectively. For patients with pneumonia, PM2.5, PM10, and NO2 exposures were risk factors for a poor prognosis. Exposure effects appeared to be greater during the warm season. Regulations focused on PM2.5, PM10, and NO2 levels should be considered to improve patient outcomes.