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2.5.3 Risk assessment approaches

Contents
2.5.3.1 Dose assessment approach
2.5.3.2 Cancer slope factor approach
2.5.3.3 Comparison of radiation risk assessment approaches

Two methods for calculating adverse health effects associated with radiation exposure may be distinguished [ITRC-2002]:

  • Dose assessment – where a dose is calculated by multiplying a dose conversion factor (expressed in terms of unit dose/unit intake) for a given radionuclide by the total intake/exposure to that radionuclide (i.e., ingestion, inhalation, or external exposure). The calculated dose can also be multiplied by a probability coefficient to arrive at a risk value.
    This dose approach originated with the need to protect workers and the public from ongoing nuclear operations. Since dose can be directly measured in the workplace, while cancer risk cannot, it was natural to adopt the dose approach. ICRP methods are based on a “safe dose” below which the exposure to radioactivity is protective of workers and the exposed public. When criteria for license termination have been developed, the dose approach was extended to cover clean-up. Clean-up levels were derived using dose conversion factors to back-calculate radionuclide concentrations (activity per mass) corresponding to a target dose. While ongoing doses can be directly measured, future doses to the public must be modelled.
  • Risk assessment (cancer slope factor approach) – where risk is calculated directly by assigning a unit of risk for every unit of exposure (i.e., probability of adverse effect/µSv), and multiplying by the total exposure.
    The clean-up of radioactively contaminated sites was approached from the perspective of having studied many cancer-causing chemicals. Future risks were expressed in terms of excess cancer probabilities. This method was extended to radionuclides, and an external radiation pathway was added. Low-level exposure to radionuclides can result in non-carcinogenic risk and well as carcinogenic risk. However, in evaluating exposure to radioactive materials at contaminated sites, only carcinogenic risk is considered for most radionuclides. The non-carcinogenic health effects associated with exposure to ionizing radiation include mutagenic, teratogenic, and acute toxicity effects. These effects are generally less significant for doses associated with environmental exposures. Therefore, carcinogenic risk is considered to be a sufficient basis for assessing radiation related to human health risk at sites.

The two methods both require exposures to be modelled. Using site conceptual models and exposure scenarios, the pathways by which radiation can affect the body are determined. These are external exposure, inhalation, direct ingestion of soil, ingestion of contaminated food (plant, meat, milk, or aquatic), and ingestion of drinking water. Using appropriate transfer equations, the quantity of external gamma exposure or intake of internal radionuclides is calculated over a period of time.

2.5.3.1 Dose assessment approach

The dose approach is based on an annual exposure to radiation. “Dose” generally refers to the Effective Dose Equivalent (EDE), a unit of measure to normalize radiation doses by considering the adverse effects on a total body basis for the purpose of regulation of occupational exposure. The Effective Dose Equivalent is derived by multiplying a Dose Conversion Factor (DCF) for a given radionuclide by the unit intake of exposure to that radionuclide (i.e., ingestion, inhalation, or external exposure). For instance, the standard equation for an inhalation pathway is:

Annual Dose (inhalation pathway) = (DCF) x (radionuclide concentration in air) x (breathing rate) x (exposure duration)

Dose Conversion Factors are defined by the International Commission on Radiological Protection (ICRP) and expressed as dose per unit exposure. Most workplace standards are based on DCFs in ICRP Publication 30 [ICRP-1979]. The newer DCFs in ICRP Publication 72 [ICRP-1996] are based on additional scientific data 20. They are more applicable to the general public, correspond to current cancer slope factors and put more emphasis on the ingestion pathway at the expense of the inhalation pathway.

Each radionuclide has a unique DCF and therefore produces different doses. A total dose is the sum of doses from all applicable pathways (ingestion of contaminated soil, water, and plants; inhalation; and external exposure).

Most health physicists are concerned with radiological doses and do not calculate the risk associated with a given dose. They compare the dose to an appropriate dose-based standard, e.g., 1 mSv/year for public exposure or 50 mSv/year for occupational exposure.

The risk associated with a given dose can be calculated using a probability coefficient. According to the 1990 Recommendations of the ICRP, the probability coefficient from fatal cancers, non-fatal cancers, and severe hereditary effects is 7.3 × 10-2/Sv [ICRP-1991]. This risk coefficient is based on low, linear energy transfer (LET) (gamma) radiation (clearly not appropriate for some radionuclides) and considers all cancers. As a result, the risk from a given dose may be calculated as:

Risk = (total dose) x (probability coefficient in risk/unit dose)

2.5.3.2 Cancer slope factor approach

The evaluation of risks to human health and the environment from exposure to radioactive substances at sites has been documented in Risk Assessment Guidance for Superfund (RAGS): Part A [EPA-1989]. The RAGS methodology provides the framework for assessing baseline risks, developing and refining preliminary remediation goals, and evaluating risks associated with various remedial action alternatives. Only cancer risks are considered for most radionuclides; for uranium, non-cancer toxicity hazards are also considered. These methods are confirmed and extended in the document Soil Screening Guidance for Radionuclides [EPA-2000]. The soil screening levels are not clean-up goals but are risk-based concentrations associated with 10-6 risk level, below which the sites do not require further attention.

The risks to potentially exposed human receptors is computed as the product of the estimated lifetime intake or external exposure for a contaminant of concern times a measure of the likelihood of incremental cancer induction per unit exposure for that contaminant, termed the “slope factor.” A slope factor is similar to a dose conversion factor, but instead of assigning a unit dose for every unit of exposure (i.e., μSv/Bq), a unit of risk is assigned for every unit of exposure (i.e., probability of adverse effect/Bq). The slope factor is an estimate of the probability of a response, i.e., the probability of an individual developing cancer per unit intake of, or external exposure to, a carcinogen over a lifetime. The slope factor multiplied by an estimate of the total lifetime exposure is used to estimate the probability of an individual developing cancer as a result of that exposure. For instance, the standard equation for an inhalation pathway is

Risk (inhalation pathway) = (inhalation slope factor) x (radionuclide concentration in air) x (breathing rate) x (exposure duration)

Calculating risk directly in this way yields a lower result than calculating risk using the dose conversion method.

Slope factors have been calculated for most radionuclides, and – just as different radionuclides have different DCFs – different radionuclides generally have different slope factors. The slope factors also vary depending on the exposure route. Therefore, risk associated with inhaling 37 Bq of uranium is different from that of inhaling 37 Bq of caesium. Also, the risk associated with inhaling 37 Bq of radium is different from that of ingesting 37 Bq of radium via drinking water.

Federal Guidance Report No. 13 provides updated and improved radiation risk coefficients for cancer incidence and mortality [EPA-1999]. These updated risk coefficients are the basis for new slope factors in the Health Effects Assessment Summary Tables (HEAST) [EPA-2001].

2.5.3.3 Comparison of radiation risk assessment approaches

Traditionally, impacts from exposure to radioactive materials have been expressed in terms of dose. Most radiation protection standards and requirements are specified in terms of a radiation dose limit (e.g., mSv/year).

Parameter Risk Assessment Dose Assessment

Competing risks Persons dying from competing causes of death (e.g., disease, accident) are not considered susceptible to radiation-induced cancer. Probability of dying at a particular age from competing risks is considered based on the mortality rate from all causes at that age in the 1979-81 U.S. population.

Competing risks are not considered explicitly.
Risk models Age-dependent and sex-dependent risk models for 14 cancer sites are considered individually and integrated into the slope factor estimate. Separate dose conversion factors for infants, children, and adults. Annual dose requires that infants and children be considered separately.

Genetic risk Genetic risk is not considered in the slope factor estimate. Effective dose equivalent value includes genetic risk component.

Dose estimate Low-LET and high-LET dose estimates considered separately for each target organ. Dose equivalent includes both low-LET and high-LET radiation multiplied by appropriate relative biological effectiveness (RBE) factors (see below).

Relative biological effectiveness (RBE) for alpha radiation 20 for most sites (8 prior to 1994) 10 for breast (8 prior to 1994) 1 for leukemia (1.117 prior to 1994).

20 (all sites).
Organs considered Estimates of absorbed dose to 16 target organs/tissues considered for 13 specific cancer sites plus residual cancers. Effective dose equivalent ICRP 1979 considers dose estimates to 6 specified target organs plus remainder (weighted average of 5 other organs). Effective dose ICRP 1991 considers dose estimates to 12 specified target organs plus remainder (average of 10 other organs).

Lung dose definition Absorbed dose used to estimate lung cancer risk computed as weighted sum of dose to tracheobronchial region (80%) and pulmonary lung (20%). Average dose to total lung (mass-weighted sum of nasopharyngeal, tracheobronchial, and pulmonary regions).

Integration period Variable length (depending on organ-specific risk models and considerations of competing risks) not to exceed 110 years.

Fixed integration period of 50 years typically considered.
Domestic/metabolic models Metabolic model parameters for dose estimates generally follow ICRP 1979 recommendations; exceptions include transuranic radionuclides. Typically employ ICRP 1979 and ICRP 1991 models and parameters for radionuclide uptake, distribution, and retention.

Standards Expressed as a target risk of lifetime excess cancer incidence. Generally expressed as an annual dose limit.

Table 2.4 Comparison of radiation risk estimation methodologies

Prior to the development of radionuclide slope factors, cancer risk from radiation exposure was traditionally estimated by multiplying the radiation dose, computed using the DCFs, by an estimate of the cancer risk per unit dose, which is averaged over all organs and tissues. The magnitude of discrepancy in the two methods depends on the particular radionuclide and exposure pathways for the site-specific conditions. These differences may be attributed to factors such as the consideration of competing mortality risks and age-dependent radiation risk models in the development of slope factors, different distribution of relative weights assigned to individual organ risks in the two methods, and differences in dosimetric and toxicological assumptions. A comparison between the bases of the two methods is summarized in Table 2.4 [EPA_1996c], [USNRC-1999].

Considering the foregoing evaluations, it should be recognised that there are uncertainties in the dose to risk relationship [CIRIA-2005a]. Therefore, risks should be calculated on the basis of best current information, using central values, with no bias towards conservatism or pessimism.

In assessing potential risks from implementing alternative options, differing views of the best current information may be taken into account when forming a preliminary view on the significance of the source term and should be examined in sensitivity analyses. Alternative views may in some cases lead to results that differ by orders of magnitude. However, a complete assessment of options would include, for example, assessment of the impacts of disposing of soil removed from a site as an element to balance against the reduction of risk on-site. Since the same views on radiation risks would apply to the assessment of all risks on- and off-site, the range of final decisions might not be so large.