It is a white, sand-like powder used to kill bacteria. It's not combustible, but is described as a "dangerous explosion hazard" when it reacts with other substances, or is heated.
The acid "gives off irritating or toxic fumes (or gases) in a fire," including chlorine gas, according to safety information posted by the Centres for Disease Control and Prevention in the U.S.
A material safety datasheet for the acid said it is harmful if inhaled, and could cause damage to the kidneys, liver, respiratory, eyes and skin.
Cover eyes and mouth to reduce exposure
People living in Vancouver east of Main Street and north of East 1st Avenue are told to close their windows and remain inside, said Vancouver police in a statement.
A spokesperson for Vancouver Coastal Health described the acid as a "respiratory irritant," that irritates eyes and lungs, but is only dangerous in large amounts.
"If you need to go outside, cover your eyes and mouth with a wet cloth," said the Vancouver Coastal Health spokesperson.