Carry oxygen. Or don't. It's not worth your health to train in those conditions. And as for why some smog conditions are worse than others, you'd need complete info on concentrations of O3, NOx, CO, hydrocarbons to really compare. Maybe the Montreal metro area has different industries and therefore a different pollutant mix than Toronto. Are mornings any better than afternoons? Sunlight is required to make smog, but the stuff can sit for a few days if stagnant air persists.