Ionising radiation comes from unstable atoms spontaneously disintegrating and releasing particles and energy that can ionise other atoms they interact with.
Small sources of radiation are all around us, such as granite rocks and potassium-rich foods. We encounter radiation as x-rays produced artificially and used for medical imaging.
Ionising radiation carries much more energy than other types of radiation, such as visible light, ultra-violet light, and infra-red radiation, and therefore has much more potential to cause harm.
The trefoil warning symbol is used to identify radioactive sources