THREE LAW ENFORCEMENT SCENARIOS AND RELATED CHEMICAL MODELS ARE THE ONE-ON-ONE SCENARIO AND THE PROJECTOR MODEL, THE BARRICADE AND HOSTAGE SCENARIO AND THE VENTILATION MODEL, AND THE CROWD DISPERSAL SCENARIO AND THE MODIFIED CLOUD TRAVEL MODEL. A DESIRABLE EFFECT IS ACHIEVED WHEN A DEVICE ACCOMPLISHES THE PURPOSE FOR WHICH LAW ENFORCEMENT OFFICERS USE IT. ANY ONE OF THE FOLLOWING EFFECTS ARE CONSIDERED UNDESIRABLE: LETHALITY, SERIOUS OR IRREVERSIBLE DAMAGE, PROVOCATION OF OBSERVERS TO JOIN A DISTURBANCE, DISCOMFORT OR PAIN AMONG OBSERVERS, AND RETALIATION BY TARGET PERSONNEL. HAND-HELD AND HAND-ACTUATED PRESSURIZED CHEMICAL INCAPACITATING AGENT DISPERSERS ARE SMALL CYLINDRICAL CONTAINERS. THE UNITS DISPERSE THEIR CONTENTS AS HIGHLY DIRECTIONAL SHORT-RANGE BURSTS OF SPRAY. WHEN LAW ENFORCEMENT OFFICERS EMPLOY CHEMICAL AGENT-FILLED GRENADES AND PROJECTILES AGAINST AN INDIVIDUAL WHO HAS BARRICADED HIMSELF IN AN ENCLOSURE, DESIRABLE EFFECTS ARE ACHIEVED WHEN THE SUBJECT IS TAKEN INTO CUSTODY WITHOUT SERIOUS INJURY. ONCE THE CHEMICAL DEVICE IS FIRED INTO AN ENCLOSURE, LAW ENFORCEMENT OFFICIALS ARE FACED WITH THE PROBLEM OF WHEN TO ENTER. THE MODIFIED CLOUD TRAVEL MODEL FOR EVALUATING CHEMICAL WEAPONS IN A CROWD DISPERSAL ROLE IS APPLICABLE TO INSTANTANEOUS POINT SOURCES, I.E., BURSTING-TYPE DEVICES. EXAMPLES ILLUSTRATING THE APPLICATION OF THE MODELS IN SAMPLE ASSESSMENTS ARE DETAILED, AND SUPPORTING MATHEMATICAL EQUATIONS AND DATA ARE GIVEN. REFERENCES ARE INCLUDED, WITH FURTHER DATA ON THE EFFECTIVENESS OF LESS-LETHAL CHEMICAL DEVICES APPENDED. (DEP)