noun
the property of certain substances to spontaneously emit radiation, often in the form of particles or electromagnetic waves
adjective
relating to or exhibiting radioactivity
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The scientist conducted a study on the effects of radioactivity on living organisms.
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I heard that radioactivity can be harmful if not properly handled.
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Dude, stay away from that area, it's full of radioactivity!
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The tension between them was like radioactivity, always present and potentially harmful.