If you’re walking through your city’s smoggy streets, instinct might tell you it’s best to walk as fast as possible to minimise how much time you’re exposed to exhaust fumes and other air pollution.
But the faster you walk, the more puffed you’ll get — meaning you’ll breathe deeper, potentially inhaling more pollutants.
So you don’t want to walk too slow, but you don’t want to walk too fast. Then what speed should you walk at?
That question has been answered by new research from the University of British Columbia (UBC), which calculated the optimum speed for pedestrians and cyclists on level ground to minimise their air pollution exposure.
"The faster you move, 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,” said UBC transportation expert Alex Bigazzi.
“This analysis shows where the sweet spot is."
According to that analysis, published in the International Journal of Sustainable Transportation, pedestrians aged under 20 should aim to walk at 3km/h, while those aged 20 to 60 should aim for 4km/h.
In practice, those speeds work out to be a little slower than a brisk walk: fast enough to be a determined pace, but not fast enough to break into heavier breathing.
Cyclists aged under 20 should ride at 12.5km/h (if they’re women) and 13.3km/h (if they’re men).
Cyclists aged 20 to 60 should aim for 13km/h (for women) and 15 km/h (for men).
Bigazzi said small variations from his “minimum-dose speeds” (MDS) aren’t anything to worry about: up to 6km/h for walkers and 20km/h for cyclists. However, larger variations might be cause for concern.
"If you move at much faster speeds than the MDS — say, cycling around 10 kilometres faster than the optimal range — your inhalation of air pollution is significantly higher," said Bigazzi.
"The good news is, the MDS numbers align pretty closely with how fast most people actually travel."
For now, Bigazzi’s calculations are speculative —the next step of his research will be to validate his findings with real-world data taken from actual roads.
It’s fair to wonder whether it’s better to just avoid walking or cycling altogether in air-polluted cities, but other studies have found that the benefits of “active travel” outweigh the risks.
University of Cambridge research published earlier in 2016 suggested that exposure to air pollution —even very high air pollution — doesn’t cancel out the improvements to your heart and lungs, and the reduced risk of disease, that you gain from moving more.
“In London, health benefits of active travel always outweigh the risk from pollution,” said Cambridge’s Dr Marko Tainio in May.
“Even in Delhi, one of the most polluted cities in the world — with pollution levels 10 times those in London — people would need to cycle over five hours per week before the pollution risks outweigh the health benefits.”