Linear No-threshold Model

A Precautionary Principle applied to radioactivity …

Is it possible to predict the effects of a dose of exposure to radiations ? The UNSCEAR and the ICRP, the radioprotection institutions in charge of issuing directives and recommendations on an international level, have proposed a simple rule. In the technical jargon, this rule – considered as a reference – is called the ‘linear no-threshold model ‘..

This important relationship is an application of a ‘precautionary principle‘. The point is to set the maximum possible risks posed by radioactivity exposure. The ICRP rule states in simple terms that the risks are directly proportional to the doses absorbed. This proportionality is represented by a straight line (a linear relationship) on a graph which passes through the origin. Considering for instance the probability that a dose triggers a potentially mortal cancer, the ICRP claims that the proportionality rate is of 5% per  sievert …

The virtue of this linear relationship is its simplicity; and if it does indeed hold then it implies that doses and risks add up independently. This means that a patient can pass through a scanner without having to worry about any previous radiation he may have absorbed.

One of the consequences of this proportionality is that the risk remains even for very weak doses. The idea that doses of even a few millionths of a sievert can pose a threat is one that has divided the scientific community. Many radiobiologists, claim that below a certain level (known as the threshold), the risk is nonexistent. Such for instance, is the point of view in France of the Medicine Academy.

Take as an analogy a rainfall of feathers onto a city. The idea of a threshold comes into play when considering that it takes a certain weight of feathers falling onto an individual to cause any damage. A million feathers landing on a million heads would go unnoticed because a feather weight is below the threshold of being harmful, but a bag with a million feathers falling onto one person’s head could kill her.

If we assume the absence of a threshold and say that even one feather has a small probability of causing a death, then the total number of deaths should be identical whether the feathers are falling individually or in heavy bags. This paradox is at the core of the controversy about the model.

The linear relationship was established based on observations made of comparatively high doses. The lack of reliable data has led to massive speculation with regards to what happens in the range of doses below 100 or 200 mSv. A linear relationship with no threshold clearly overestimates the risks posed by low doses if, in reality, this threshold did exist.

Despite its limitations, the relationship has a useful regulatory role because it provides an easy and effective framework for radiation protection.