Indoor Release

Distinguishing between a biological release and a chemical release:
A biological agent will almost never cause immediate symptoms; a chemical agent almost always will. For a biological agent the goals are to reduce the total number of people exposed and to be sure you can find everyone who was exposed. For a chemical release, the goal is to minimize the concentrations to which people are exposed.

For any indoor release, whether chemical or biological: if evacuation can be done safely, evacuate the building to a meeting point upwind of the building.

Biological release (or unknown):
Shut off HVAC and close outdoor air dampers (or, if this is not possible, put them into full recirculation mode). Local exhausts, such as those serving bathrooms and kitchens, must also be shut off—they are often controlled separately from the HVAC system. These actions will prevent the building from becoming a source of contamination for people outside.

If possible, stairwells should be pressurized with 100% outdoor air to provide an evacuation route. Other HVAC, and bathroom and utility room fans, should be shut off.

Segregate people known to be exposed, to avoid contaminating others via contact with clothes or skin, and tag or mark these people for medical treatment and decontamination.

For a biological release, the goal is to minimize the total number of people exposed, even if this leads to higher or longer exposure for some people.

Chemical release:
It is best to leave the HVAC system operating without alteration, unless a knowledgable building operator is available to perform HVAC manipulation. Under normal operation, the HVAC system will provide some outdoor air and will exhaust some indoor air, so it will help dilute the chemical and exhaust it from the building.

Also, for most buildings the normal operation of the HVAC will tend to isolate areas that are served by different air handling units, thus helping to slow the spread of contamination. The isolation is reduced if the HVAC system is not properly balanced, or if the building's air recirculation system mixes air from different supply zones.

If the building operator has checked system operation and is sure that dampers and fans are working correctly, some HVAC manipulations can be beneficial. The simplest such action is to put the building on 100% outside air (no recirculation), with supply and exhaust fans on full power.

If more sophisticated actions are possible, then do the following:

• The operator should be aware of the possibility of a release into the building air intakes, and should shut off the supply from any intake in which this is thought to have occurred.
• Pressurize stairwells with 100% outdoor air—this will help provide a safe evacuation route.
• Put the air handlers that serve heavily contaminated areas onto full exhaust and shut off supply to those areas—this will force air to flow from safe areas to contaminated areas, rather than the other way around.
• Provide 100% outdoor air to uncontaminated areas and areas with people in them—this will help keep people safe, as long as the chemical isn't being released into those areas' air intakes.

Depending on the HVAC design, some of the items above may be performed by putting the building into a "smoke removal" mode.