gasmask vs gas helmet

gasmask and gas helmet both are nouns.

Word	Noun	Adjective	Verb	Adverb
gasmask	Yes	No	No	No
gas helmet	Yes	No	No	No

As nouns, gasmask and gas helmet are synonyms defined as:

gasmask and gas helmet: a protective mask with a filter; protects the face and lungs against poisonous gases

Other synonyms of gasmask include respirator.

gasmask (noun)	gas helmet (noun)
a protective mask with a filter; protects the face and lungs against poisonous gases	a protective mask with a filter; protects the face and lungs against poisonous gases