NCJ Number
190517
Date Published
1998
Length
26 pages
Annotation
This document describes the “rules-of-thumb” for emergency responders for air toxic releases in urban environments.
Abstract
Releases of hazardous gases and aerosols are not an everyday occurrence, but do happen in populated urban environments occasionally. These releases may stem from on-site accidents, may result during transport of hazardous chemicals, or may be premeditated as in a chemical or biological agent terrorist attack. At accident sites, emergency responders are typically responsible for maintaining order, determining safe zones, and performing rescue and evacuation. Emergency responders need to be cognizant of the rather complicated transport and dispersion processes that occur around building structures in order to make sound decisions that may save lives. The “rules-of-thumb” depicts common situations that may occur in an urban environment and gives warnings about potentially fatal consequences if the situation is misinterpreted. The locally measured wind may not match large-scale wind due to building-induced circulation. This is called apparent wind anomalies. For winds nearly face-on to the building wall, concentrations of hazardous material can build-up in between buildings and take a relatively long time to flush out (agent trapping in vortices). Recessed entryways or architectural alcoves may trap and hold air contaminants for some time after the plume has passed by (agent entrapment). For winds parallel to the street, the plume can become contained within the street canyon and can travel up side streets (on-axis channeling effects). The plume can get channeled by streets near the source and end up traveling off the prevailing wind direction axis (off-axis channeling effects). The air contaminant can move short distances against the prevailing wind direction in recirculation zones along the sides and top of the building (eddy transport of agent). The prevailing wind switches direction occasionally, so that the upwind safe zone may now be downwind (large-scale wind variability). The local wind can switch direction very rapidly, so that the plume may switch from one side of the building to the other in a matter of seconds (small-scale wind variability). After the plume has left the area of release, the ground and building surfaces may still be contaminated due to deposition of the toxic agent (agent deposition). When the plume is passing over, it is probably safer to remain indoors. After the plume has passed by, it may be safer to move outdoors (indoor effects). 71 references