Positive-pressure filtration systems draw contaminated air into the building through special filters that remove the contaminants. This creates a slight positive pressure in the building, which in turn prevents the nonfiltered, contaminated air from leaking into the building. Experience suggests that such systems, however, are expensive and difficult to properly engineer. A less expensive but less protective measure is to engineer the building controls so that the HVAC system could be completely shut down. Little outside air would be drawn into the structure if all doors and windows were closed.
The ability of an action to adequately protect people in a healthcare facility depends on the characteristics of the toxic agent(s) involved, the size and nature of the release, meteorological conditions, the characteristics of the population affected, and the ability of the threatened structures to provide protection from outdoor agent concentrations. Deciding to shelter-in-place requires a prediction of the outdoor plume concentration of the toxic agent that will occur in the risk area, an estimation of the concentration that will occur inside the buildings at risk, and a calculation of the indoor estimated level of exposure. Deciding to evacuate requires an estimation of how long it will take to move patients and staff out of the building and when they will reach a safe distance compared to the outside concentration that people will experience while evacuating; that is, calculating exposures to those who evacuate in the plume will be considered against exposures to those who have not left (Sorensen, Shumpert, and Vogt 2002).
The indoor concentration of a contaminant is determined by infiltration rates into a building and the inside circulation of air. The inside environment will have a lower peak concentration of a contaminant than the outside air. The lower the air exchange, the lower the peak concentration. Infiltration is measured by air changes per hour (acph) between the outside and the inside or the number of times each hour that an enclosure’s total volume of air is exchanged with outside air. An average air exchange rate for office buildings is estimated to be 0.66 acph and an industrial building to be 0.31 acph with the HVAC system(s) off and doors and windows closed (Engelmann 1990). Little is known about the movement of contaminants inside buildings, especially large and complex structures such as hospitals.
Overall exposure to contaminants in a closed indoor environment will be similar to the overall outdoor exposure because contaminants remain in the building after the plume has passed. If reducing total exposure is the goal, as opposed to reducing peak concentration, then the facility needs to be evacuated or ventilated after the plume has passed or the outdoor concentration is less than the indoor concentration (Rogers et al. 1990).
Sheltering for a biological hazard is slightly different from that for a chemical vapor in that hospital HVAC systems are designed to filter out most aerosols in the size range of biological agents (0.5 to 10 microns) (Weik and Weik 2001). Thus, leaving the HVAC system on would be warranted if the hospital is not under positive pressure.
Sheltering for a radiological hazard is somewhat different from that for chemical hazards in that a building will provide some protection against radiation from inhalation exposure as well as exposure from atmospheric clouds and ground deposition. The amount of protection against radiation is determined by the type of structure and location in the structure. Interior rooms and basements in buildings such as hospitals would typically reduce exposure from a cloud source by 70 percent to 90 percent and from a ground source by 98 percent to 99 percent (Schleien 1983).
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