From Longman Dictionary of Contemporary Englishgas maskˈgas mask noun [countable] 
TPMa piece of equipment worn over your face to protect you from poisonous gases
TPMa piece of equipment worn over your face to protect you from poisonous gasesExamples from the Corpus
gas mask• Every child had a gas mask and a suitcase, or paper parcel.• Delaney snapped on his gas mask.• Everywhere I went, I carried the gas mask I had purchased on the black market. 