The M40 Field Protective Mask is one of various gas masks used by the military of the United States and its allies to protect from field concentrations of chemical and biological agents, along with radiological fallout particles. It is not effective in an oxygen deficient environment or against ammonia.
The C2 canister on the M40 mask can protect the user from up to 15 nerve, choking, and blister agent attacks, and two blood agent attacks. However, the original C2 canister contains a radioactive source (Chromium 63) that has prompted the U.S. Army to replace it with a newer, non-radioactive alternative. Original C2 canisters are colored black; the newer, non-radioactive canisters are colored dark green.
The M40 comes with a drinking system that allows the user to drink water when donning the mask for long periods of time in a chemically contaminated environment.
The M40 was phased into service with the US Army in the mid-1990s, with another new design, the MCU-2/P similarly replacing older masks in service with the Air Force. However, both masks suffered from the inadequate protective capabilities of their facepiece, which was of a latex polymer susceptible to corrosion from blister agents. Thus, the MCU-2/P is effectively ill-suited for protecting against much more than riot control agents, and a 'Second Skin' had to be quickly issued for fitting over and reinforcing the M40 facepiece to make it effective in its intended role.
The M42 is a variant of this mask with alterations that make it better suited for use by armored vehicle crews, who have to connect their masks to and draw air from their vehicle's own filtration system.
The M40 field protective mask, along with the older M17 mask, are currently being replaced by the XM50 JSGPM, Joint Service General Purpose Mask.