The attack on theWorldTradeCenter (WTC) site and the subsequent collapse of the towers on September 11 generated an aerosol containing a wide range of toxic and irritant agents. A partial list of these materials includes pulverized concrete, gypsum, pulverized glass, asbestos, silica, fibrous glass, heavy metals, soot, volatile organic compounds, acid mists and organic products of combustion, among them polycyclic aromatic hydrocarbons (PAHs). A recurring theme with regard to exposure conditions at the ‘‘Ground Zero’’ site was the uneven provision and use of adequate respiratory protection, the clinical consequences of which became apparent over the ensuing several months. The populations at risk for adverse health consequences, in decreasing intensity of exposure, included those whowere caught in the blackout (then ‘‘greyout’’) of the collapse cloud; the Ground Zero first responders and workers and volunteers involved in the rescue and recovery effort over the first few days; those involved in restoration of essential services and infrastructure (electric, gas, transportation, etc.), debris removal crews and their support services, building clean up teams, persons who eventually reoccupied offices, commercial and school buildings near the WTC site, and community residents. The extent of the clean-up effort, now 8months in duration, and the necessity of moving truckloads of debris throughpublic streets to the barge-loadingoperation at the Hudson River for transport to Staten Island increased the number of persons at risk for exposure. By October 2001, the Mount Sinai-Irving J. Selikoff Center for Occupational and Environmental Medicine (COEM) began evaluating individuals, who presented with respiratory complaints, related to their exposure to airborne irritants. Exposure-related factors (when they were at or near ‘‘Ground Zero,’’ performing what tasks, over what time period, with what level of respiratory protection) appeared to be significant determinants of the severity of respiratory reactions; but host biological factors appeared to play a role as well, with some exhibiting greater susceptibility to the irritant-induced effects. Health effects among the individuals seen in the Clinical Center included new-onset (i.e., post-9/11) sinusitis, laryngitis, tracheitis, reactive upper airways dysfunction (RUDS), bronchitis, and reactive airways dysfunction syndrome (RADS) and irritant-induced asthma. Those who had sinusitis or asthma prior to 9/11 experienced amarked worsening of their symptoms. Symptoms of upper and/or lower airway irritation were frequently reported to be worsened or provoked by re-exposure to airborne irritants (tobacco smoke, vehicle exhaust, cleaning solutions, etc.), by exercise, and by cold air. While initially respiratory complaints and illnesses were identified primarily among workers and volunteers at or near the WTC site, over the ensuing months, similar problems were found among office reoccupants and community residents, especially those situated downwind (South and East) of Ground Zero. While therewas initial concern about persistent sequelae of acute musculoskeletal injuries sustained by workers and volunteers at the site, relatively fewer such injuries occurred during rescue and recovery and debris removal thanwould be expected for a project of this magnitude. A particularly prominent clinical finding was the prevalence of persistent psychological distress among thosewho initially presented with respiratory conditions. Symptoms consistent with the classical picture of post-traumatic stress disorder and major depression were persistent among many