Alberta is updating its mask-use guidelines in the light of a recent messaging shift from Canada’s top doctor on the risk of airborne spread of COVID-19, but the province is otherwise continuing to use a year-old document which considers aerosol virus transmission to be a non-dominant form of spread. Dr. Theresa Tam, Canada’s chief public health officer, said in mid-November that respiratory aerosols — microscopic airborne particles which can carry SARS-CoV-2, the virus that causes COVID-19 — can linger in air much like smoke , underlining the importance of mask use and ventilation in indoor spaces as virus mitigation measures.