Radiation health risk assessment evaluates how often adverse health effects caused by a given radiation exposure occur in a population (or group). Radiation risk assessment is the process of estimating the type and level of risk to human health from exposure to radiation.
Scientists have learned much about how radiation exposure can harm humans. However, scientists still have to make some key assumptions related to radiation-induced health risks because of limited knowledge in some areas.
Radiation risk assessment is therefore not a precise science. Large uncertainty can be associated with a given radiation risk estimate.
Radiation risks are usually expressed as a probability. A risk of 0.01 would indicate that 1 out of 100 people would be expected to be affected.
Sometimes a range of probabilities is used rather than giving a single estimate of risk to account for uncertainties. A hypothetical example of risk stated as a range of probabilities follows:
Our best estimate of the risk of cancer after exposure of those people to radiation is one additional case in 10,000 people, but the risk could be as high as one additional case in 1,000 people, or as low as one additional case in 1,000,000
people.
5.1 The Steps in Risk Assessment
A radiation risk assessment often consists of four steps.
5.2 Risk Assessment vs. Risk Management
Risk assessment is distinct from risk management. Risk assessment is a scientific process of investigating phenomena to estimate the level of risk. Risk management is an effort to reduce the risk through education, regulation, etc.
Risk managers use the results of risk assessments, plus economic, social, and legal considerations to make regulatory and policy decisions. While economic, social, and legal considerations have a legitimate place in risk management, they have no place in the scientific process of risk assessment.
5.3 Risk Assessments and Judgments
Risk assessment is not completely devoid of judgments. Sometimes the decisions are based on the scientist's best judgment. Such decisions can affect the outcome of the risk assessment.
5.4 Risks Associated with Small Radiation Doses
Cancer is the major risk associated with exposure of humans to low radiation doses. Two types of models are usually used for evaluating the risk of radiation-induced cancer in humans: (1) absolute-risk models and (2) relative-risk models.
Absolute-risk Models
Absolute risks are usually based on the assumption of a linear risk vs. dose relationship that passes through zero excess risk at the origin. This represents what has become known as the linear, no-threshold (LNT) model.
As an example of how absolute risk is applied, if the normal risk over the lifetime is 0.001 for a specific type of cancer, and radiation adds an additional risk of 0.02, then the absolute risk of cancer over the lifetime is 0.001 + 0.02 or 0.021.
Relative-risk Models
The relative risk considers how the normal risk changes with age. For example, if the normal risk of developing a given type of cancer between age 50 and age 51 years is 0.001, and radiation exposure leads to a relative risk of 2, then the relative risk is used to multiply the normal risk, so one has to calculate the product 2 x 0.001 or 0.002. Thus, instead of having a normal risk of 0.001 for cancer in the age interval 50 to 51 years, the risk is increased to 0.002 because of the radiation exposure.
Similar calculations are carried out for other age intervals depending on the age of the person at the time of exposure and the latent period for the cancer type of interest.
The tumor latent period is the time it takes for a tumor to develop. The latent period is different for different types of cancer.
To determine the lifetime risk, the risks for the different age intervals are added. However, no radiation-related risks would be counted during the latent period.
Effects other than cancer that could be caused by low radiation doses include genetic effects and effects on the unborn embryo or fetus carried by a pregnant mother. Small radiation doses could also cause temporary suppression in sperm counts in males.
5.5 Risks Associated with Large Radiation Doses
In addition to cancer and genetic effects, the risk associated with large radiation doses includes a variety of other effects that require killing large numbers of cells in an organ or tissue. For effects that require killing large numbers of cells, a threshold dose occurs below which the effect would not be expected to occur. The shape of the dose-response curve for such effects is generally S-shaped (sigmoidal).
The steepness of the sigmoid curve usually depends on the radiation dose rate and the degree of uniformity of the radiation dose to local tissue. Risk is evaluated based on a sigmoidal dose-response curve that accounts for dose rate effects, the type of radiation, and how radiation dose is distributed over a given target (organ/tissue) in the body.
Separate risks can be evaluated for radiation effects associated with different organs. Examples follow:
Risks can also be influenced by exposure to other agents (biological and/or chemical agents) in addition to radiation.
In risk characterization, scientists combine information from radiation exposure assessments and radiation toxicity assessments to estimate the type and magnitude of risk faced by the exposed population. The risk characterization usually states important uncertainties.
With absolute-risk models, the excess risk due to exposure to radiation does not depend on the normal risk that would arise when there is no radiation exposure. Absolute risks are evaluated on a scale from 0 to 1. A risk of 1 corresponds to 100% of the exposed individuals being affected.
With relative-risk models, the relative risk is a multiple of the normal risk. Unlike absolute risks, relative risk values range from 1 to very large numbers. A value of 1 for the relative risk means that there is no excess risk.
Some risks can be increased by wounds (e.g., risk of death from radiation induced damage).