In public areas, an attack or its immediate preparations may be sufficiently unusual as to be noticed by the general public or venue employees. Signs that may indicate a chemical event may be in progress are:
- Unattended or otherwise suspicious packages
- Visible aerosol or gas clouds
- Patron actions, including complaints and spontaneous evacuation
Following the release of some types of chemicals (for example, the nerve agent sarin, as discussed above), near immediate human health effects will be seen, making the recognition that an adverse event is occurring relatively straightforward (even though the specific chemical involved may not be known). However, releases of smaller volumes or exposures to slower-acting substances (e.g., thallium via ingestion), where considerable time could elapse before individuals develop symptoms, may not be immediately obvious. In such cases, it may be the venue employees that are the first to experience symptoms as they remain within the area and are exposed for a much longer duration than typical patrons. Beyond event recognition, delayed effects also create challenges with contamination control: many seriously exposed patrons will likely have departed the area of exposure prior to symptom onset. In this case, reports of widespread adverse health effects may occur.
