Linear no-threshold model

Figure 1: The LNT Model (red line) assumes a straight line relationship between dose and cancer risk. The green solid line represents the natural cancer occurrence without the effects of radiation. Any dosage above the dotted line represents high dosage levels (100 mSv). Dosages below this have never been shown to have the linear relationship; the health response is almost certainly lower.

The Linear no-threshold model (LNT) is the most commonly used model to estimate the biological risks from ionizing radiation. While cancer is only one of the possible problems from radiation (others include acute radiation sickness), it's a common public health concern, and so will be the focus of this discussion. Eventually, Energy Education would like to have a reasonably full treatment of the biological effects of radiation, but doesn't yet. An excellent summary is the BEIR VII report.

The linear no-threshold model is based on biological responses at high radiation doses and dose rates. Of course, the higher the dose and dose rate, the higher the biological response; the lower the dose and dose rate, the lower the response. This model assumes the simplest possible relationship, a straight line (hence linear no-threshold model).

Several difficulties exist with testing this model. Data collection to support or oppose this model (in humans) is challenging, since most people are not willing to expose themselves to harmful (or even close) levels of radiation. The LNT relies on assumptions to extrapolate the biological harm at lower levels to make up for the lack of data.

Another difficulty with determining the biological response to radiation comes from the time delay between exposure to a carcinogen, and when the cancer actually develops. This time period is referred to as the latency period. Using the LNT to predict the risk of a person developing cancer is especially difficult (compared to acute radiation sickness) since a large percentage of the population will develop cancer for reasons other than radiation exposure, so a direct link between the two cannot be made.

Roughly 40% of people living their full lifespan will develop cancer. Many things cause cancer; genetics, environmental risks, and obesity being some examples. Different populations have different cancer risks, but radiation is likely very rarely the only cause of cancer.

Because of the limitations, this model is primarily a policy model in order to set limits on exposure to radiation and is widely used for this purpose. This model is deliberately conservative; it almost certainly overestimates the risk associated with radiation exposure. A model is needed to make policy decisions and this model works sufficiently well that most relevant organizations around the world have adopted it.

The data to make the LNT model came from Japanese survivor data from World War II. The bomb victims of Nagasaki and Hiroshima who received the highest amount of radiation exposure displayed an increased risk response. From this correlation, data were then extrapolated to the origin, assuming a linear no-threshold relationship between dosage and the risk of getting cancer. Yet another challenge of this is that at low dose rates of radiation, it is difficult to differentiate between natural background cancer rates and those caused by radiation.

LNT Assumptions

The linear relationship that exists between cancer risk and dosage has the following assumptions:

1. A linear relationship exists between DNA double-stranded break and the probability of developing into a cancer.
2. Cancer risk is linearly proportionate to dose.
3. There is no threshold and risk is additive.
4. Dosage outweighs any biological variables.
5. Dosage is more important than dose rate.

As with most models, the assumptions are only approximations, but the question becomes: are they good approximations for reality? If these assumptions are 'true enough,' then the LNT is a good scientific model; if the assumptions are inaccurate, then it's a poor scientific model.

In either case, because the model is primarily for setting public policy, the important point is that the model gives an overestimation of risk. Detractors from the LNT claim that this has made the public overly concerned about the biological effects of radiation, leading to unnecessary levels of panic in events like the Fukushima nuclear accident. Detractors of the model also point to the financial cost to making nuclear power very expensive and not actually saving human lives, an idea for which energy education will eventually have a page to explore in depth.

Issues with the LNT model

One of the biggest arguments against the LNT model is that it does not consider human defense mechanisms. The human body produces enzymes that repair DNA damage with an efficiency of 99.99% for single stranded breaks and 90% for double stranded breaks. Some studies have shown that apoptosis (cellular death that occurs naturally within growth cycle of cells) can be stimulated by low-level radiation. Radiation has also been shown to alter cell cycle timing, thereby increasing the time before the next cell division (mitosis). This gives a cell more time to notice the DNA damage and go into apoptosis before mitosis, ultimately preventing the cell from becoming cancerous, by eliminating the problem cell and preventing it from reproducing. The human body suffers DNA damage via corrosive chemicals and thermal processes millions of times per day; however, only about one DNA damage per cell per day remains unrepaired.

There are many models of biological response to radiation that show that the LNT model is unreasonable at low doses. One surprising model is radiation hormesis, which states that there are benefits from low-level ionizing radiation stimulation. Other scientists have noticed that humans often have an adaptive phase response: low-level ionizing radiation seems to be able to condition cells to have better responses to higher amounts of radiation dose. Some models assume a higher risk at lower dose rates (a super-linear model), but this model doesn't seem to mesh well with what's known about this biology, and the data aren't there to support this idea.

Biological effects of radiation remains an active area of research in the scientific community.

For Further Reading

• Ionizing radiation
• Energy from nuclei
• Strong nuclear force
• Half life
• Or explore a random page