An environmental radioactivity monitor comprises a filter (12) through which air is caused to flow, and a detector (13) of radiation emitted from the particles trapped on the filter. The signals from the radiation detector are analyzed into a multiplicity of energy channels, for example with a multi-channel analyzer (22), the counts in at least two energy windows (W1, W2; W3, W4) being used to determine the shapes of the low-energy tails of the peaks of the naturally occurring radio-nuclides ThC' and RaC', so their effects can be eliminated. The shape of the ThC' peak is used to predict and strip out the ThC peak, and the shape of the RaC' peak is then used to predict and strip out the RaA peak, so all background effects are eliminated. Accurate measurements of air-borne radioactivity are hence possible with short counting times.
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