Just knowing is a start. The California Environmental Protection Agency, for example, has compiled a list of the most-polluted census tracts in the state, along with a mapping tool to help locate and compare them. That would be useful here. There are no simple answers, however, to reducing neighborhood or urban pollution levels. Using zoning laws to prevent poor neighborhoods from being burdened with pollution-emitting plants and new highways, however, and the regulation of emissions, are important steps.