That’s the alleged “sweet spot” to avoid inhaling too much pollution.
If you biked to work today at any speed other than 8 mph (for women) or 9 mph (for men), you did it wrong. To put that in perspective, those are roughly the top speeds of a house mouse and a pig, respectively.
Also, if you walked at a gait under 2.5 mph (both sexes), you messed up majorly.
These are the “sweet spots” of movement for people between 20 and 60 years old traveling on flat urban roads, according to Alexander Bigazzi at the University of British Columbia. Bigazzi, who works in the Department of Civil Engineering and School of Community and Regional Planning, calculated these speeds in an effort to decrease people’s exposure to air pollution. “The faster you move,” he says in a press release, “the harder you breathe and the more pollution you could potentially inhale, but you also are exposed to traffic for a shorter period of time.”
There are plenty of caveats to go along with these numbers, which appear in a study in the International Journal of Sustainable Transportation. Bigazzi says the window of safety for cycling is about 7 to 12 mph, and about 1 to 4 mph for walking. But once you go 6 mph outside of these optimal speeds, he says, your exposure to air pollution can more than double. There are a host of mitigating factors like age (see the above chart for younger/older speeds), road grade, weather, and proximity to pollution hot spots such as highways.
Should you be constantly adjusting your speed to protect your precious lungs? Maybe not. Bigazzi notes: “It appears that pedestrians and bicyclists choose travel speeds that approximately minimize pollution inhalation dose, although pollution is unlikely a primary motivation.”