According to my radiobiology text (Hall), "mammalian cells on average experience over 100,000 DNA lesions per day as a result of replication errors, chemical de

According to my radiobiology text (Hall), "mammalian cells on average experience over 100,000 DNA lesions per day as a result of replication errors, chemical decay of their bases, attack by reactive oxygen species, or exposure to ionizing radiation. However, the mutation rate in mammalian cells is quite low owing to the development of DNA repair pathways that handle each type of damage." Basically the threshold hypothesis assumes that the body can successfully repair up to a point, beyond which the risk of cancer increases with the increasing number DNA lesions. Alphas (and other charged particles heavier than electrons) act in a slightly different way. They can do more direct DNA damage. They can also occur externally, from cosmic rays or accelerator beams. I only addressed photons because we were talking about the body scanners.