ENDNOTES:
a EPA classifies all radionuclides as Group A (known human) carcinogens. Radionuclide risk coefficients, or slope factors, are calculated by EPA's Office of Radiation and Indoor Air (ORIA) to assist HEAST users with risk-related evaluations and decision-making at various stages of the remediation process. Most values presented in this tabulation are taken from Federal Guidance Report No. 13 (EPA 402-R-99-001). (Risk estimates for the soil ingestion pathway are not addressed in Federal Guidance Report No. 13, but have been computed using similar methods.) Ingestion and inhalation slope factors are central estimates in a linear model of the age-averaged, lifetime attributable radiation cancer incidence (fatal and nonfatal cancer) risk per unit of activity inhaled or ingested, expressed as risk/picocurie (pCi). Ingestion values are tabulated separately for ingestion of tap water, dietary intakes (food), and incidental soil ingestion. Inhalation values for particulates represent the indicated ICRP lung absorption type, based on the guidelines described in the User's Guide; where appropriate, separate values are tabulated for inhalation of the radionuclide in vapor or gas form. External exposure slope factors are central estimates of the lifetime attributable radiation cancer incidence risk for each year of exposure to external radiation from photon-emitting radionuclides uniformly distributed in a thick layer of soil (expressed as risk/year per pCi/g of soil). If required, slope factors in Table 4 can be converted into the International System (SI) units of becquerels (1 Bq = 1 nuclear transformation per second) by dividing each inhalation, ingestion, or external exposure value by 0.037. Users can calculate cancer risks using slope factors expressed in either customary units or SI units with equivalent results,provided that they also use air, water and soil concentration values in the same system of units. For a discussion on the derivation of radionuclide slope factors and guidance on their use, refer to the User's Guide section on radionuclide carcinogenicity.
b The curie (Ci), the customary unit of activity is equal to 3.7 x 1010 nuclear transformations per second. 1 picocurie (pCi) = 10-12 Ci. The International System (SI) unit of activity is the becquerel (1 Bq = 1 nuclear transformation per second).
c For each radionuclide listed, slope factors correspond to the risks per unit intake or exposure for that radionuclide only, except when marked with a "+D" to indicate that the risks from associated short-lived radioactive decay products (i.e., those decay products with radioactive half-lives less than or equal to 6 months) are also included. Refer to Exhibit 1 in the User's Guide section on radionuclide carcinogenicity for guidance on determining slope factors for partial or complete radioactive decay chains.
d Chemical Abstract Service Reference Number (CASRN). For risk calculations involving decay chains, a CASRN should be reported for the parent radionuclide and each chain member. For radionuclides not included in the 1995 HEAST tabulation, CASRN numbers have not yet been assigned and are designated as "NA" (except metastable forms of isotopes with previously assigned numbers, which are designated with a trailing "m").
e Radioactive half-life: s = second, m = minute, d = day, y = year. For those radionuclides with decay products (+D), half-lives are listed for the parent radionuclide.
f Lung absorption type recommended by the International Commission on Radiological Protection (ICRP): F = fast (particulate), M = medium (particulate), S = slow (particulate), V = vapor, G = gas.
g Gastrointestinal (GI) absorption fractions (f1) are the fractional amounts of each radionuclide absorbed across the GI tract into the bloodstream. Lung absorption types and GI absorption fractions are provided for reference only. Do not use these factors to adjust inhalation or ingestion slope factors. See the User's Guide for instructions.
h For isotopes of carbon, two inhalation entries are provided: the value in the first line represents inhalation of carbon in particulate aerosols (with default ICRP lung absorption type M), while the value in the next line represents inhalation of carbon dioxide gas. Values for inhalation of carbon monoxide gas would be lower than the corresponding carbon dioxide values by a factor of 2 for C-11 and 6 for C-14.
i For tritium, two sets of values are provided for ingestion and inhalation pathways. The values in the first line represent ingestion of H-3 in the form of tritiated water and inhalation of tritiated water vapor, while values in the second line represent ingestion of organically bound tritium and inhalation of H-3 in particulate form (with default ICRP lung absorption type M). The corresponding value for inhalation of H-3 in organically bound gas would be greater than the value for tritiated water vapor by a factor of 2.3, while the value for inhalation of elemental hydrogen gas would be lower by a factor of 10,000.
j For radioisotopes of iodine, the values listed for food ingestion represent ingestion of milk; corresponding values for ingestion of nondairy foods would be lower by a factor of approximately 2. Inhalation values are provided both for inhalation of particulate aerosols (with default ICRP lung absorption type F) and inhalation of vapor. Corresponding values for inhalation of methyl iodide vapor are also provided in Federal Guidance Report 13, and are slightly lower than the vapor entries in each case.
k For radioisotopes of mercury, Federal Guidance Report No. 13 provides values for inhalation of organic particulates, inorganic particulates and vapors. The inhalation value presented in the first line represents the maximum of the organic or inorganic particulate values (these values are very similar in most cases), and the value in the second line represents inhalation of the radionuclide in vapor form. For ingestion, Federal Guidance Report No. 13 provides values for inorganic compounds, organic compounds, and methyl mercury for each isotope. The ingestion values reported in Table 4 for each isotope represent the maximum of the these three forms.
l For radioisotopes of polonium, Federal Guidance Report No. 13 provides values for ingestion of organic (f1=0.5) and inorganic (f1=0.1) compounds. For purposes of this tabulation, polonium is assumed to be ingested in organic form in foods and in inorganic form in water and soil. The value f1=0.1 also applies to the inhalation entries in Table 4 (Type M).
m To derive the inhalation slope factor for Rn-222+D, EPA's Office of Radiation and Indoor Air (ORIA) uses a risk model based on radon decay product exposure and the following exposure assumptions: inhalation rate of 2.2E+04 L/day; 50% equilibrium for decay products; and a risk coefficient of 2.2E-4 fatal lung cancers per working level month (WLM)(see EPA 402-R-93-076, Estimating Radiogenic Cancer Risk). [NOTE: EPA expects to publish revised risk estimates for inhalation of radon decay products in the near future, based on a revised equilibrium assumption of 40%.]
n For radioisotopes of sulfur, Federal Guidance Report No. 13 provides values for ingestion of both organic and inorganic compounds. For the purpose of this tabulation, sulfur is assumed to be ingested in organic form in foods and in inorganic form for ingestion of water and soil. The first inhalation value represents inhalation of inorganic particulates (with default ICRP lung absorption type M), while the second value represents inhalation of carbon disulfide vapor; Federal Guidance Report No. 13 also provides a value for inhalation of sulfur dioxide vapor, at approximately 17% of the value for carbon disulfide.