I asked the study's lead scientist, Michael Koehle of the University of British Columbia, about their results. His team was surprised, too. "We were expecting to find more effects from the pollution and bigger effects in high intensity pollution than in low intensity pollution," he says. After all, when you exercise harder, you breathe harder, which means you're taking in more pollution. You're also more likely to breathe through your mouth, bypassing the natural filtering mechanisms inside your nose.