Answer:
A blister agent is a severe contact irritant. They cause severe chemical burns to any exposed tissue, resulting in large water-filled blisters forming on the affected tissue.
Most blister agents are both contact and inhalation hazards. If inhaled, they can cause death shortly after exposure, as the lungs and throat quickly burn and fill with blisters, inhibiting breathing. Alternately, these blister burst, filling the lungs with fluid. Death from inhalation of a blister agent can vary from minutes to several days later, depending on the amount of exposure (the more, the quicker the death).
Contact with the outer skin is much less fatal, though extremely painful. Fatalities are usually the result of infection and sepsis from the burst blister wounds.