The Collaborative International Dictionary of English v.0.48:

gasmask \gasmask\ n.
   a mask with a filter which protects the face and lungs
   against poisonous gases. It is used in warfare, and also by
   police to allow them to effectively use tear gas or other
   disabling gas to disperse a crowd or force fugitives to leave
   a building.

   Syn: respirator, gas helmet.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

gasmask
    n 1: a protective mask with a filter; protects the face and
         lungs against poisonous gases [syn: gasmask,
         respirator, gas helmet]