PM2.5 exposure assessment of the population in Lower Manhattan area of New York City after the World Trade Center disaster

On 11 September 2001, the explosion and the collapse of the World Trade Center (WTC) Twin Towers in New York City (NYC), USA, generated a massive release of dust and inhalable toxic substances to the atmosphere as a result of the pulverization of various building materials, furniture, and computers. Many concerns were raised as Particulate Matter (PM) levels in Lower Manhattan might not meet the current National Ambient Air Quality Standards (NAAQS) (65 μg m−3). The current study aims to provide a first estimate of the scale of population exposures during this episode. Data collected from existing monitoring stations in September showed the occurrence of a series of high peaks of PM2.5 registered in the Lower Manhattan area after the 11 September. An interpolation technique was used within a Geographical Information Systems (GIS) environment to estimate outdoor PM2.5 concentrations over NYC. Monthly average of 24 h outdoor PM2.5 concentration of Lower Manhattan was 20.2 μg m−3 and did not exceed the NAAQS value. PM2.5 concentrations in indoor micro-environments were simulated by a deterministic micro-environmental model (INTAIR) and linear regression equations. Three typical population groups were identified for the NYC area—home-makers, office/shop-workers, and students/children—and their 12 h nighttime and daytime exposures were estimated from 14 September until the end of September, either as mean exposure (daytime and nighttime) or as exposure weighted by residential population distribution (nighttime only). Average nighttime and daytime exposures of the Lower Manhattan population were calculated to be 37.3 and 23.6 μg m−3, respectively (daily average: 30.45 μg m−3), in which the various group movements and activities, smoking habits of individuals, and special population movements due to access restrictions and risk avoidance behaviors were considered. Within the study period, assuming the real nighttime population distribution followed the residential population pattern, approximate one quarter of the population was exposed to 20–30 μg m−3 PM2.5; one half of the Lower Manhattan population was exposed to 10–20 and 30–60 μg m−3; around one fifth of the population was exposed to over 60 μg m−3 during nighttime. The results indicated that although the outdoor PM2.5 concentration was lower than the NAAQS value, personal exposure levels, which were generally higher than the outdoor PM2.5 concentration, might still be a reason of concern.