A fog advisory has been issued for the Barrie, Orillia, Collingwood and Midland areas.
Environment Canada issued the advisory Monday morning, saying patchy, dense fog is expected in the region.
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The weather agency says the fog may cause visibility to be significantly and suddenly reduced, making travel hazardous.
Officials say if visibility is reduced while driving, residents should turn on their lights and maintain a safe following distance.
Environment Canada says the fog is expected to lift later Monday morning or early afternoon.