More than 40,000 individuals were exposed to particulate matter following the World Trade Center (WTC) attacks. This population has increased rates of asthma and worsening of preexisting asthma. WTC asthma is on the spectrum of disorders ranging from reactive airways dysfunction to irritant-induced asthma. It is a non-immunologic phenomenon, with direct airway epithelial damage causing release of pro-inflammatory mediators. Risk factors for development of WTC asthma relate predominantly to the magnitude of irritant exposure.