The research tests 7 alternative ways of communicating about the magnitude of risks and the changes in risk that can be achieved by an individual's mitigating actions. The technique uses 4-page information leaflets, accompanied by self-administered mail questionnaires. Two example risks, radon and asbestos, were used to give a context to the expression of risk magnitudes. All formats yielded expected perceptions about the relative risk of readings for that hazard. However, people with the same size readings for asbestos as those receiving radon readings reacted by saying the asbestos risk at that level has about the same degree of seriousness as reported by those receiving the corresponding radon reading. This occurred even though a given radon reading is associated with a risk 25 times larger than the same reading for asbestos. Based on the results, the authors provide tentative recommendations for communicating about environmental risks.