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.