For complete functionality, gas masks must cover the entire face, and must be worn with as much protective clothing as possible covering all of the body. This is because many CBRN agents can be harmful and often even lethal when  vapors, powders, or liquids come in contact with the eyes and skin. Half-mask respirators are not intended for CBRN protection and will not provide NBC protection. For that matter, a mask alone only provides limited protection to CBRN agents.