Chemical agents are characterized as non-persistent or persistent.

The non-persistent agents remain effective in the open for a period of ten minutes, or less, at the point of release. Liquid tear gas vapors or fumes generated by burning or combustion-type delivery systems are categorized as non-persistent.

Persistent agents remain effective in the open field for periods in excess of ten minutes at the point of release. CN, CS and micro-pulverized type tear gas are considered persistent agents.