Linear No-Threshold Model (LNT)

The linear no-threshold model assumes that cancer risk rises in a straight line with dose, all the way down to zero, so that there is no dose below which the risk is zero. Regulators use it to estimate cancer risks from low exposures to carcinogens such as asbestos, where direct measurement of risk is not possible.

Origin

The model was first developed for radiation protection in the mid-20th century and was later adopted by the U.S. EPA and other agencies for cancer-causing chemicals. EPA's cancer risk estimates for asbestos have used linear extrapolation from studies of exposed workers.

Significance in Asbestos Context

The linear no-threshold model is the scientific basis for the regulatory position that there is no known safe level of asbestos exposure. Studies of heavily exposed workers show cancer risk climbing with cumulative dose; the model extends that line down to low exposures, such as those from building materials, consumer products or household contact. Defense experts argue that the body can clear low doses and that a threshold exists, especially for chrysotile; plaintiffs' experts note that mesothelioma has occurred after brief or low exposures and that no threshold has been identified. Courts treat the model differently. Some accept it as support for general causation; others say a regulatory model built to protect the public cannot by itself prove that a particular low exposure caused a particular person's disease, and require more specific evidence of dose. The model also underlies risk estimates in agency documents, such as the expected number of cancers per million people exposed at a given level.

Example Sentence

EPA's risk estimate assumed a linear no-threshold model, so even exposures far below the OSHA limit were assigned some added cancer risk.

Related Terms

No Safe Level of Exposure, Dose-response relationship, Cumulative Exposure, EPA, Permissible Exposure Limit (PEL), Each and Every Exposure Doctrine, General Causation, Specific Causation

Notes

• The model is about population risk estimates; your lawyer and experts will look at your own exposure in detail.
• Even brief or low exposures can be worth recording in your history; write down every source you remember.