Radiation is energy that travels as a wave or particle. Some types of radiation, called ionizing radiation, can be harmful. Radioactivity is ionizing radiation that is given off by substances, such as uranium, as they decay.
About half of the ionizing radiation we're exposed to comes from nature. It's in rock, soil, and the atmosphere. The other half comes from man-made sources like medical tests and treatments and nuclear power plants.
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There is always a risk of damage to cells or tissue from being exposed to any amount of ionizing radiation. Over time, exposure to radiation may cause cancer and other health problems. But in most cases, the risk of getting cancer from being exposed to small amounts of radiation is small.
The chance of getting cancer varies from person to person. It depends on the source and amount of radiation exposure, the number of exposures over time, and your age at exposure. In general, the younger you are when you are exposed to radiation, the greater the risk of cancer.
Someone who has had many CT scans starting at a young age is more likely to get cancer later in life than someone who hasn't had any or as many of these tests. CT scans generally use more radiation than other X-ray tests. The risk of an adult getting cancer from a CT scan is less than 1 in 1,000. The risk of a child getting cancer from the same CT scan can be much higher.1
A child who was treated with radiation for cancer is more likely to get another cancer later in life.
A person who has been exposed to large amounts of radiation from a nuclear accident is more likely to get cancer than someone who has not been exposed.
Exposure to small amounts of radiation doesn't cause any symptoms. But exposure to large amounts all at once may cause radiation sickness and death.
How do different sources of radiation compare?
Some sources of radiation give off larger amounts than others. For example, when you go through a full-body airport scanner, you're exposed to very small amounts of radiation. But if you live near the site of a nuclear accident, you're exposed to large amounts of radiation.