What color does the M8 indicator turn when blister agents are present?

• A. Blue
• B. Green
• C. Yellow
• D. Red

Answer: (D)

M8 paper uses a colorimetric reaction to identify liquid chemical agents. When blister agents such as sulfur mustard or related vesicants are present, the reagents in the M8 indicator react with the agent and produce a red color. That red change is chosen because it’s highly visible in field conditions, making it a quick, reliable cue to treat the sample as contaminated with a vesicant and to proceed with protective measures and decon. Other colors on the indicator would indicate different results or no reaction for blister agents, so red specifically signals the presence of blister agents in this test.