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 physical carcinogenic agents, while cigarettes are an example of a chemical cancer causing agent. Viruses are examples of biological carcinogenic agents. The slope of the line just 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.
The LNT model is recommended by the ICRP and accepted by most radiation protection authorities in the world. It must be emphasized, conservativeness of this model has enormous consequences and a number of organisations disagree with using the linear no-threshold model to estimate risk from environmental and occupational low-level radiation exposure. 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 by dosimeters of all radiation exposure, without taking into consideration dose levels or dose rates. However, the aggregation of very low individual doses over extended time 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.