The health effects of air pollution vary with the size and characteristics of the exposed population, the specific pollutant or mix of pollutants and the concentration of pollutants, both in the short term and the long term. Children, the elderly, and people with pre-existing health problems are known to be especially vulnerable. Generally speaking, respiratory and cardiac effects are the most significant, but there is increasing evidence that air pollutants play a role in cancer and genetic mutations, some of which can be inherited. Cumulative and synergistic effects are hard to study: ethical concerns rule out some otherwise desirable experimental procedures, and multi-pollutant studies are complex and expensive, leaving these areas less well understood experimentally. Sometimes ecologic studies can contribute to our understanding of combined effects. In the early 1990s, the province commissioned Dr. Sverre Vedal, then a researcher at the University of British Columbia, to investigate air pollution in BC, and to assess and to rank its health impacts. His report concluded that particulate pollution was the gravest concern, estimating an annual toll of 82 deaths, among other consequences. As of late 2004, the province continues its long-term effort to control sources of particulates. The varying population and topography have given rise to different problems in different areas, necessitating differing approaches. Other important air pollutants are oxides of nitrogen and of sulphur, volatile organic compounds of various kinds, polycyclic aromatic hydrocarbons, dioxins and furans. Exposure and health effects of these vary by area and sometimes by season.