LONDON — Every winter, as if on cue, the coughing begins.

As soon as the weather turns cold, Tara Carey, an international aid worker living in London, ritually places cough syrup on her bedside table because she knows her sleep will be punctuated by hacking coughs. She also coughs at work. And she coughs while cycling to her office, on a road so toxic that for a brief period last month the air pollution there was greater than in infamously smoggy Beijing.

With her cough persisting winter after winter, Ms. Carey, 43, became worried she might have contracted tuberculosis during a work trip to Africa and sought medical help. She was shocked by the doctor’s eventual diagnosis: asthma.

In Ms. Carey’s view, she said the only reasonable explanation for her illness was the pollution to which she was exposed over the last six years cycling through thick traffic on Brixton Road, one of London’s busiest and most noxious routes.

“You get a massive mouthful of fumes,” she said, noting that asthma does not run in her family. “But we don’t really realize how much toxic air we breathe in because we’re acclimatized to it. It’s pernicious.”