Linear No-threshold Model

The linear no-threshold model (LNT) is a model used in radiation protection to estimate the long term, biological damage caused by ionizing radiation. It assumes that this damage is directly proportional to the dose at all dose levels. In other words, radiation is always considered harmful with no safety threshold, and the sum of several very small exposures are considered to have the same effect as one larger exposure (response linearity).

It opposes two competing schools of thought: the threshold model, which assumes that very small exposures are harmless, and the radiation hormesis model, which claims that radiation at very small doses can be beneficial. Because the current data are inconclusive, scientists disagree on which model should be used. Pending any definitive answer to these questions and the precautionary principle, the linear no-threshold model is acknowledged worldwide as the appropriate basis for radiation protection regulations.

Since there is no way to contain or dispose of many fission products, politically imposed safety thresholds are often unenforceable. For example, safety thresholds have been exceeded in both non-emergency and emergency situations.

The LNT model is sometimes applied to other cancer hazards such as polychlorinated biphenyls in drinking water.