Modern gas masks are quite safe and do not use asbestos, but it is still important to be careful when using a modern gas mask. Typically, masks using 40 mm connections are a more recent design. Rubber degrades with time, so new in-box "modern type" masks can be cracked and leak. Also, the US C2 canister (black) was shown to contain hexavalent chromium: the studies by the U.S. Army Chemical Corps showed that the levels in the filter were acceptable, but imply caution when using, as it is a carcinogen.