A form of energy that is transmitted in waves, rays or particles from a natural source, such as the sun and the ground, or an artificial source, such as an x-ray machine. Radiation can be ionising or non-ionising. Ionising radiation includes ultraviolet rays, radon gas and X-rays. Too much exposure to ionising radiation can be harmful, leading to increased risk of cancer. Non-ionising radiation includes visible light, radio waves and microwaves. This type of radiation is less risky to health because it contains less energy, but it can still be harmful at high levels for a long time.

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Source: Environmental Protection Agency