Quantities of radiation released by coal plants under normal operations are not a health concern. Coal has plenty of other health, and environmental, concerns.

Quanities of radiation released by a coal plant are unlikely to vary much at all, no matter how critically mismanaged the plant might be.

Quantities of radiation emitted by a badly-mismanaged, sabotaged, or attacked nuclear power plant, on the other hand, become a health concern for people across continents and beyond, as with Chernobyl and Fukushima.

(Note that "concern" need not mean "realised risk", but given that one of the characteristics of nuclear incidents to date has been inaccurate or misleading information on precisely the levels of risk presented, prudence and caution strongly recommends presuming a worst case until otherwise conclusively demonstrated.)