When radiation collides with molecules in living cells it can damage them. This can cause a mutation. If the DNA in the nucleus of a cell is damaged, the cell may become cancerous. In this case the cell divides rapidly and causes serious health problems.
The greater the dose of radiation a cell gets, the greater the chance that the cell will become cancerous. However, very high doses of radiation can kill the cell completely. We use this property of radiation to kill cancer cells, and also harmful bacteria and other micro-organisms.
The hazard symbol is shown on containers of radioactive substances to warn of the danger.
The degree to which each different type of radiation is most dangerous to the body depends on whether the source is outside or inside the body.
If the radioactive source is inside the body, perhaps after being swallowed or breathed in:
If the radioactive source is outside the body:
Notice these effects are opposites.