Unknown Release

It is imperative to determine whether the release is indoors or outdoors.

It would take a very large or very toxic outdoor release to cause severe effects on people indoors. If this has occurred, there will probably be obvious signs: birds falling out of trees, people collapsing on the street, etc. Also, an outdoor release will usually affect people throughout the building (because all ventilation zones constantly get some outdoor air), whereas an indoor release will often affect some areas of the building much more quickly and more severely than others.

If there are no visible signs of an outdoor release, and if some areas of the building appear to be more severely affected than others, then assume the release is indoors.

A release into one or more of the building's air intakes is also possible. Shutting off the air supply for any area that is known to be contaminated, and putting that area on full exhaust, is good advice whether the release is in the air intake or indoors.