Linear Dose Response in Radiation Protection relates to the zero-threshold model which predicts that every small addition of radiation exposure above background contributes to an increment in the probability of a Probabilistic / Stochastic effect (excess cancer). The response relies on the assumption that even one Photon has the ability to cause an Ionisation event in DNA which may initiate cancer (or other genetic effects). In adopting this model one has to remember that there is no certainty that an ionising radiation event actually leads to biological damage (and thus cancer etc), rather, it it the likelihood of induction which increases. LNT is the basis for Dose Limits, but many argue it exaggerates the risks from ionising radiation.