A dust advisory was issued Thursday morning due to high readings for coarse particulate downtown and at the BCR industriral site.
Readings were 56 and 66 micrograms per cubic metre respectively at the two locations while the reading at College Heights was 34. An advisory is issued when a reading rises above 50 for coarse particulate.
"Based on the quality data, the high levels are mainly due to road dust in the bowl area," Ministry of Environment air pollution meteorologist Dennis Fudge said.
"Light winds at night and dry conditions over the past several days contributed to dust accumulating in the air shed.
"Conditions may improve in the afternoon, but high levels are expected to redevelop overnight and tomorrow morning.
"Air quality is expected to be better at higher elevations."
The advisory is expected to be in effect at least until Saturday morning, with an update to be provided at 4:30 p.m. today.
As a result, free transit will be provided beginning Friday until the advisory ends.
All transit buses will switch their digital signs to display "Free Fare for Clean Air" and will cover the fare boxes so passengers will not be able to deposit money.