Together with ozone and NOx, S02 causes damage to forests, fisheries and other ecosystems through "acid rain”. Natural coal has trace amounts of uranium and thorium, and radiation levels become noticeable when these radioactive materials are concentrated in the fly ash (PM10) emissions from combustion. Indeed, a series of studies since the initial report in Science (McBride et al., Disposal sites for coal waste also emit non-negligible levels of radiation, owing to the presence of fly ash.