These codes generally referred to the immediacy of the chemical threat. Enemy forces have the capability to employ NBC weapons, but attack is not probable at this time. Attack probable, units maintain MOPP-0. Attack is imminent. Units go to MOPP-4.
Attack is probable.
Wear CPE and field gear as directed. Attack is imminent or in progress. NBC contamination is suspected or actual. Wear full chemical protective ensemble and field gear. Personnel were warned to take cover but the alert was called off after a short period of time.