Radiation causes harm by ionisation — stripping electrons from atoms in biological molecules. When DNA in a cell nucleus is ionised, the resulting chemical changes can break DNA strands or alter the base sequences.
If DNA damage occurs in a normal body cell: the cell may die (radiation sickness at high doses) or the DNA repair mechanisms may incorrectly repair the break, creating a mutation. If the mutation affects genes that control cell division, the result can be uncontrolled division — cancer.
If DNA damage occurs in a reproductive cell (sperm or egg), the mutation can be passed to offspring — causing heritable genetic mutations in the next generation.
Why alpha is most dangerous inside the body: Although alpha cannot penetrate skin from outside, if an alpha-emitting material is inhaled or swallowed, it deposits all its energy in a tiny volume of tissue nearby. This concentrated ionisation causes severe, localised DNA damage.
This how it works covers How It Works: Why Radiation Is Biologically Harmful within Uses & Hazards of Radiation for GCSE Physics. Revise Uses & Hazards of Radiation in Atomic Structure for GCSE Physics with 18 exam-style questions and 5 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 9 of 17 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Uses & Hazards of Radiation
Which type of radiation is used in smoke detectors?
Explain how a smoke detector uses an alpha source to detect smoke.