noxious
NOK-shusadjMeaning harmful, especially to health; also, very unpleasant
Memory hook KNOCKS-ious: fumes that KNOCK you back. The old truck coughs out a black cloud so foul that it nearly knocks the jogger off his feet; its fumes are noxious.
The noxious fumes from the burning tires drove everyone indoors.
In context Unlike the harmless steam from the kettle, the gas leaking from the old furnace was noxious, and the family had to leave the house until it was repaired.
Similar poisonous, harmful, foul, toxic
Opposite harmless, wholesome, beneficial
From Latin noxius, “harmful,” from noxa (harm).