# Stochastic Effects – Ionizing Radiation

Stochastic effects of ionizing radiation occur by chance, generally occurring without a threshold level of dose. The probability of occurrence of stochastic effects is proportional to the dose, but the severity of the effect is independent of the dose received. The biological effects of radiation on people can be grouped into somatic and hereditary effects. Somatic effects are those suffered by the exposed person, and hereditary effects are those suffered by the offspring of the individual exposed. Cancer risk is usually mentioned as the main stochastic effect of ionizing radiation, but also hereditary disorders are stochastic effects.

According to ICRP:

(83) On the basis of these calculations, the Commission proposes nominal probability coefficients for detriment-adjusted cancer risk as 5.5 x 10-2 Sv-1 for the whole population and 4.1 x 10-2 Sv-1 for adult workers. For heritable effects, the detriment-adjusted nominal risk in the whole population is estimated as 0.2 x 10-2 Sv-1 and in adult workers as 0.1 x 10-2 Sv-1.

Special Reference: ICRP, 2007. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. Ann. ICRP 37 (2-4).

The SI unit for effective dose, the sievert, represents the equivalent biological effect of depositing a joule of gamma rays energy in a kilogram of human tissue. As a result, one sievert represents a 5.5% chance of developing cancer. Note that the effective dose is not intended as a measure of deterministic health effects, which is the severity of acute tissue damage that is certain to happen, measured by the quantity absorbed dose.

There are three general categories of stochastic effects resulting from exposure to low doses of radiation. These are:

• Genetic effects. The genetic effect is suffered by the offspring of the individual exposed. It involves the mutation of specific cells, namely the sperm or egg cells. Radiation is an example of a physical mutagenic agent. Note that there are also many chemical agents as well as biological agents (such as viruses) that cause mutations. One very important fact to remember is that radiation increases the spontaneous mutation rate but does not produce any new mutations.
• Somatic effects. Somatic effects are those suffered by the exposed person. The most common impact of irradiation is the stochastic induction of cancer with a latent period of years or decades after the exposure. Since cancer is the primary result, it is sometimes called the carcinogenic effect. Radiation is an example of a physical carcinogenic, while cigarettes are an example of a chemical cancer-causing agent, and viruses are examples of carcinogenic biological agents.
• In-Utero effects involve the production of malformations in developing embryos. However, this is a special case of the somatic effect since the embryo/fetus is exposed to radiation.

Most thought somatic effects resulting from radiation exposure are thought to occur in a stochastic manner. The most widely accepted model posits that the incidence of cancers due to ionizing radiation increases linearly with effective radiation dose at a rate of 5.5% per sievert. This model is known as the linear no-threshold model (LNT). This model assumes no threshold point, and the risk increases linearly with a dose. 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. The LNT is not universally accepted, with some proposing an adaptive dose-response relationship where low doses are protective, and high doses are detrimental. It must be emphasized that many organizations disagree with using the linear no-threshold model to estimate risk from environmental and occupational low-level radiation exposure.

## Stochastic Effects and Effective Dose

The effective dose is defined as the doubly weighted sum of absorbed doses in all the organs and tissues of the body. It is very important whether a person is exposed partially or completely, and it is very important whether a person is exposed to gamma rays or another type of radiation. Effective dose allows for determining stochastic biological consequences of all types of radiation. Dose limits are set in terms of effective dose and apply to the individual for radiological protection purposes, including the assessment of risk in general terms. Mathematically, the effective dose can be expressed as:

The radiation weighting factor is a dimensionless factor used to determine the equivalent dose from the absorbed dose averaged over a tissue or organ based on the type of radiation absorbed. In the past, a similar factor known as the quality factor was used for this purpose. The radiation weighting factor estimates the effectiveness per unit dose of the given radiation relative to a low-LET standard.

In 2007 ICRP published a new set of radiation weighting factors (ICRP Publ. 103: The 2007 Recommendations of the International Commission on Radiological Protection). These factors are given below.

### Tissue Weighting Factor

The tissue weighting factor, wT, is the factor by which the equivalent dose in a tissue or organ T is weighted to represent the relative contribution of that tissue or organ to the total health detriment resulting from uniform irradiation of the body (ICRP 1991b). It represents a measure of the risk of stochastic effects that might result from exposure to that specific tissue. The tissue weighting factors consider the varying sensitivity of different organs and tissues to radiation.

Various ICRP (International Commission on Radiological Protection) publications list the tissue weighting factors. According to the actual determination of the ICRP, the risk factors are in the following table (from ICRP publication 103 (ICRP 2007)).

Special Reference: ICRP, 2007. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. Ann. ICRP 37 (2-4).

## Examples of Doses in Sieverts

We must note that radiation is all around us. In, around, and above the world we live in. It is a natural energy force that surrounds us, and it is a part of our natural world that has been here since the birth of our planet. In the following points, we try to express enormous ranges of radiation exposure, which can be obtained from various sources.

• 0.05 µSv – Sleeping next to someone
• 0.09 µSv – Living within 30 miles of a nuclear power plant for a year
• 0.1 µSv – Eating one banana
• 0.3 µSv – Living within 50 miles of a coal power plant for a year
• 10 µSv – Average daily dose received from natural background
• 20 µSv – Chest X-ray
• 40 µSv – A 5-hour airplane flight
• 600 µSv – mammogram
• 1 000 µSv – Dose limit for individual members of the public, total effective dose per annum
• 3 650 µSv – Average yearly dose received from natural background
• 5 800 µSv – Chest CT scan
• 10 000 µSv – Average yearly dose received from a natural background in Ramsar, Iran
• 20 000 µSv – single full-body CT scan
• 175 000 µSv – Annual dose from natural radiation on a monazite beach near Guarapari, Brazil.
• 5 000 000 µSv – Dose that kills a human with a 50% risk within 30 days (LD50/30) if the dose is received over a very short duration.

As can be seen, low-level doses are common in everyday life. The previous examples can help illustrate relative magnitudes. From biological consequences point of view, it is very important to distinguish between doses received over short and extended periods. An “acute dose” occurs over a short and finite period, while a “chronic dose” is a dose that continues for an extended period so that a dose rate better describes it. High doses tend to kill cells, while low doses tend to damage or change them. Low doses spread out over long periods don’t cause an immediate problem to any body organ. The effects of low radiation doses occur at the cell level, and the results may not be observed for many years.

## Stochastic Effects and Dose Limits

In radiation protection, dose limits are set to limit stochastic effects to an acceptable level and prevent deterministic effects completely. Note that stochastic effects arise from chance: the greater the dose, the more likely the effect. Deterministic effects normally have a threshold: above this, the severity of the effect increases with the dose. Dose limits are a fundamental component of radiation protection, and breaching these limits is against radiation regulation in most countries. Note that the dose limits described in this article apply to routine operations. They do not apply to an emergency when human life is endangered, and they do not apply in emergency exposure situations where an individual attempts to prevent a catastrophic situation.

The limits are split into two groups, the public and occupationally exposed workers. According to ICRP, occupational exposure refers to all exposure incurred by workers in the course of their work, except for

1. excluded exposures and exposures from exempt activities involving radiation or exempt sources
2. any medical exposure
3. the normal local natural background radiation.

The following table summarizes dose limits for occupationally exposed workers and the public:

According to the recommendation of the ICRP in its statement on tissue reactions of 21. April 2011, the equivalent dose limit for the eye lens for occupational exposure in planned exposure situations was reduced from 150 mSv/year to 20 mSv/year, averaged over defined periods of 5 years, with no annual dose in a single year exceeding 50 mSv.

Limits on effective dose are for the sum of the relevant, effective doses from external exposure in the specified period and the committed effective dose from intakes of radionuclides in the same period. For adults, the committed effective dose is computed for a 50-year period after intake, whereas for children, it is computed for the period up to age 70. The effective whole-body dose limit of 20 mSv is an average value over five years, and the real limit is 100 mSv in 5 years, with not more than 50 mSv in any year.

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