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, e.g., solvent, fat, oil or fuel can fall on them.
Drops of water give no reaction.
On the basis of spot diameter and density on the detection paper, it is possible to obtain an opinion on the original size of the droplets and the degree of contamination. A droplet of 0.5 mm diameter gives a spot sized about 3 mm on the paper. 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.