Each year hundreds of chemical incidents occur in the United States that include airborne hazards (such as hazardous fumes, noxious chemicals, or mysterious odors) that permeate buildings and cause illness, injuries, or disruption of activities. Most of these incidents involve accidental releases of chemicals, either inside or outside the building. Some may be the result of malicious acts-vandalism, pranks, or in extreme cases, terrorism. Deliberately releasing chemicals or biological aerosols (fine particles), or a radioactive substance into a building can be life threatening and can create substantial economic impact.