Radioactivity -- bombardment by invisible particles which can split growing cells and the greatest menace life has ever faced -- is the day-to-day study of a group of Canadian health-scientists. Their job: collect facts, isolate dangerous elements, as above, and evaluate health hazards. From a modest start, the program of protection services for industrial radiation workers, plus subsequent analysing of fallout radiation levels in milk, water, air dust and human bone, has been steadily intensified. The Radiation Protection Division in Ottawa obtains data from (1) a constant check on radiation received by 13,000 workers, who use radioactive substances and x-rays in hospitals, industry and research projects, by issuing them with radiation-sensitive badges which are periodically sent to Ottawa for checking; (2) atmospheric fallout observers at 24 stations across Canada who collect samples of dust and rainwater and forward them to Ottawa for radiation count; (3) detection of strontium 90 -- potentially most dangerous because of its high radiation, similarity to calcium and long-lived radioactivity -- by extracting it from the ashes of milk and bones; results indicate trends in strontium 90 levels of dairy milk and its human absorption; (4) building of two special chambers for measuring radioactivity in a living person; their eight-inch-thick steel walls will shield against natural and man-made radioactivity in the area, allowing accurate readings of the person under test.