When a drop of chemical warfare agent is absorbed by the paper, it dissolves one of the pigments. Mustard agent dissolves a red dye and nerve agent a yellow. Detection paper can thus be used to distinguish between three different types of chemical warfare agents. A disadvantage with the papers is that many other substances can also dissolve the pigments. Consequently, they should not be located in places where drops of substances such as solvent, fat, oil, or fuel can fall on them.
Drops of water produce no reaction.
Depending on the spot diameter and density on the detection paper, it is possible to gauge the original size of the droplets and the degree of contamination. The M256A1 kit is a portable, expendable item capable of detecting and identifying hazardous concentrations of chemical agent. The M256 kit is used after a chemical attack to determine if it is safe to unmask. The M256A1 kit has replaced the M256 kit. The only difference between the two kits is that the M256A1 kit will detect lower levels of nerve agent.