That list is for carcinogens, which are mostly invisible. Smog affects asthma and emphysema quite strongly, but has a relatively small effect on cancer. So it doesn't seem all that implausible to me. Cities tend to concentrate smog, especially cities with a lot of car traffic, in valleys. Carcinogenic toxins are more likely to come from big factories, and from agricultural toxins like pesticides. It's been a long time since companies wanted to build big factories in a crowded city--land and construction cost so much less away from cities. So many cancer risk factors are about being downwind or downstream of pollution sources, more than whether the source is nearby, or whether the pollutant is mixed with something visible like dust or smoke.