For a 38-year old professor who cycled to work every day, asthma symptoms tended to be worst in midmorning and early afternoon on weekdays, but improved noticeably on weekends.

Her doctor suspected the air pollution on her commute was provoking her asthma, so he mapped a new route further from major roadways. Within a month, her wheezing and other symptoms had decreased dramatically, according to the authors of a review of her case.

“It has long been suspected that avoiding air pollution is a good idea for people with asthma,” said senior author Dr. Chris Carlsten of Vancouver General Hospital in British Columbia.

But it’s not clear if people with asthma are at greater risk from pollution than people without asthma, or exactly what levels of air pollution should prompt strict avoidance, he told Reuters Health by email.

There’s enough evidence to say that air pollution exacerbates existing asthma, Carlsten and his colleagues write in the Annals of Allergy, Asthma and Immunology. And there’s growing evidence that exposure to pollution can cause new cases of asthma, the researchers say.