According to D. Vogt, a writer and historical researcher, and manager of the Canadian History channel on Helium, apart from smoking other airborne chemicals are also known carcinogens and account for much of the remaining cases of lung cancer. The first of these is asbestos, which accounts for about one in thirty cancer cases and the second is radon. Radon gas has no colour or odour, and seeps up from the Earth's crust, potentially through household basements. Concerns about radon gas are higher in some areas than others - for instance, Iowa in America and Cornwall in England have large concentrations of radon gas. Overall, a small percentage of homes have excess radon gas. In major cities, particulate matter associated with pollution likely also results in a measurable increase in lung cancer over what would have been the incidence of the disease in a pollution-free environment.