The primary focus is on sulfur dioxide (SO2) and nitrogen oxides (NOx), the most important gases which caused acid rain; and particulates which cause visible air pollution, illness and premature deaths. SO2 can be removed by flue-gas desulfurization and NO2 by selective catalytic reduction (SCR). Particulates can be removed with electrostatic precipitators. Although perhaps less efficient, wet scrubbers can remove both gases and particulates. Reducing fly ash reduces emissions of radioactive materials. Mercury emissions can be reduced up to 95%. However capturing carbon dioxide emissions from coal is generally not economically viable.