Tainio et al. 2016, Can air pollution negate the benefits of cycling and walking

Comparative risk assessment of physical activity and air pollution. Across the great majority of real-world fine-particulate (PM2.5) levels, the health benefits of walking and cycling outweigh the harm from pollution. At an average urban background of 22 µg/m³, the tipping point where extra activity stops adding benefit is reached only after about 7 hours of cycling a day; the background would need to reach roughly 95 µg/m³ for it to fall to 30 minutes, a level exceeded in under 1% of cities. Strong.