The linear no-threshold model (LNT model) is a conservative model used in radiation protection to estimate the health effects of small radiation doses. According to the LNT model, radiation is always considered harmful with no safety threshold. The sum of several very small exposures is considered to have the same biological risk as one larger exposure (linearity). The problem is that, at very low doses, it is practically impossible to correlate any irradiation with certain biological effects. This is because the baseline cancer rate is already very high, and the risk of developing cancer fluctuates by 40% because of individual lifestyle and environmental effects, obscuring the subtle effects of low-level radiation. Therefore it isn’t easy to validate this model. Since data for low doses regions are not available, the biological effects of low doses of radiation must be extrapolated. The question is, how?
The LNT model is recommended by the ICRP and accepted by most radiation protection authorities worldwide. It must be emphasized that the conservativeness of this model has enormous consequences. Many organizations disagree with using the linear no-threshold model to estimate environmental and occupational low-level radiation exposure risk. This principle was introduced in the late 1950s and is still the basis for all dose limits recommended.
According to ICRP:
“A dose-response model which is based on the assumption that, in the low dose range, radiation doses greater than zero will increase the risk of excess cancer and/or heritable disease in a simple proportionate manner. “
Special Reference: ICRP, 2007. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. Ann. ICRP 37 (2-4).
Today the protection system is based on the LNT hypothesis, assuming that all radiation is bad and that the deleterious effect (essentially the cancer risk) increases linearly with dose with no threshold (start at zero dose). Since zero dose is not attainable, the ALARA – principle (As Low As Reasonable Achievable) was introduced. This allows the summation of all radiation exposure by dosimeters without considering dose levels or dose rates. However, the aggregation of very low individual doses over extended periods is inappropriate, and in particular, the calculation of the number of cancer deaths based on collective effective doses from trivial individual doses should be avoided.
See also: Collective Dose
LNT Model – Dose-effect Curve
As can be seen from the LNT dose-effect curve, the risk does not start at 0 because there is some risk of cancer, even with no occupational exposure. Note that radiation is one of the physical carcinogenic agents, while cigarettes are an example of a chemical cancer-causing agent. Viruses are examples of carcinogenic biological agents. The slope of the line means that a person that receives 5 mSv in a year incurs 10 times as much risk as a person that receives 0.5 mSv in a year.
As can be seen, the linear no-threshold model assumes that more exposure means more risk, and there is no dose of radiation so small that it will not have some effect.
Consequences of LNT model
Today the protection system is based on the LNT hypothesis, assuming that all radiation is bad and that the deleterious effect (essentially the cancer risk) increases linearly with dose with no threshold (start at zero dose). The probabilistic nature of stochastic effects and the properties of the LNT model makes it impossible to derive a clear distinction between ‘safe’ and ‘dangerous,’ and this creates some difficulties in explaining the control of radiation risks. The major consequence of the LNT model is that some finite risk, however small, must be assumed and a level of protection established based on what is deemed acceptable. Since zero dose is not attainable, the ICRP (Publication 103) defines the System of Radiological Protection, which is based on the following three principles:
- Justification. “Any decision that alters the radiation exposure situation should do more good than harm.”
- Optimization of Protection. “Doses should all be kept as low as reasonably achievable, taking into account economic and societal factors.” (known as ALARA or ALARP)
- Dose Limitation. “The total dose to any individual … should not exceed the appropriate limits.”
These three principles have bases in the LNT model. The Commission considers that the LNT model remains a prudent basis for radiological protection at low doses and low dose rates.
LNT and Collective Dose
In the case of collective dose, the conservativeness of the LNT model has enormous consequences, and the model is sometimes wrongly (perhaps intentionally) used to quantify the cancerous effect of collective doses of low-level radioactive contamination. A linear dose-effect curve makes it possible to use collective doses to calculate the detrimental effects on an irradiated population. Simply:
But what does it mean? If ten million people receive an effective dose of 0.1 µSv (the equivalent of eating one banana), then the collective dose will be S = 1 Sv. Does it mean there is a 5.5% chance of developing cancer for one person due to eating bananas? Note that, for high doses, one sievert represents a 5.5% chance of developing cancer. Many organizations disagree with this result and use the linear no-threshold model to estimate environmental and occupational low-level radiation exposure risk. There is a question of whether the collective dose is meaningful at all.
The ICRP alone states:
“The collective effective dose quantity is an instrument for optimization, for comparing radiological technologies and protection procedures, predominantly in the context of occupational exposure. A collective effective dose is not intended as a tool for epidemiological risk assessment, and it is inappropriate to use it in risk projections. The aggregation of very low individual doses over extended periods is inappropriate, and in particular, the calculation of the number of cancer deaths based on collective effective doses from trivial individual doses should be avoided.”
Special Reference: ICRP, 2007. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. Ann. ICRP 37 (2-4).
The controversy of the LNT Model
As written today, the protection system is based on the LNT hypothesis, a conservative model used in radiation protection to estimate the health effects of small radiation doses. This model is excellent for setting up a protection system for all use of ionizing radiation. This model assumes that there is no threshold point and risk increases linearly with a dose, i.e., the LNT model implies that there is no safe dose of ionizing radiation. If this linear model is correct, natural background radiation is the most hazardous radiation source to general public health, followed by medical imaging as a close second.
In the case of low doses, its conservativeness has enormous consequences. The model is sometimes wrongly (perhaps intentionally) used to quantify the cancerous effect of collective doses of low-level radioactive contaminations. A linear dose-effect curve makes it possible to use collective doses to calculate the detrimental effects on an irradiated population. It is also argued that the LNT model had caused an irrational fear of radiation since every microsievert can be converted to the probability of cancer induction, however small this probability is. For example, if ten million people receive an effective dose of 0.1 µSv (the equivalent of eating one banana), then the collective dose will be S = 1 Sv. Does it mean there is a 5.5% chance of developing cancer for one person due to eating a banana? Note that, for high doses, one sievert represents a 5.5% chance of developing cancer.
The problem with this model is that it neglects many defense biological processes that may be crucial at low doses. The research during the last two decades is very interesting and shows that small doses of radiation at a low dose rate stimulate the defense mechanisms. Therefore the LNT model is not universally accepted, with some proposing an adaptive dose-response relationship where low doses are protective, and high doses are detrimental. Many studies have contradicted the LNT model, and many have shown adaptive response to low dose radiation resulting in reduced mutations and cancers. This phenomenon is known as radiation hormesis.
