14.2 Cancer Risk Characterization: Excess Lifetime Cancer Risk & Acceptable Risk Tolerances
Key Takeaways
- Under low-dose linear (mutagenic) cancer assessment, extra lifetime risk ≈ CSF × LADD, with CSF in (mg/kg-day)^−1 and LADD in mg/kg-day. Using ADD instead of LADD inflates risk by mixing averaging times.
- Inhalation extra risk ≈ IUR × lifetime-equivalent concentration when units match (for example IUR per µg/m³ times µg/m³). Do not multiply an oral CSF by an air µg/m³ without dosimetry.
- EPA Superfund decision context for known or suspected carcinogens is generally an excess upper-bound lifetime individual risk range of 10^−6 to 10^−4; 10^−6 is often the screening point of departure for remediation goals.
- Occupational cancer-risk tolerances differ from Superfund community values; OSHA significant-risk discussion historically includes lifetime risks on the order of 10^−3. Different statutes and populations, not a single number.
- ABT does not publish a universal acceptable-cancer-risk constant. Named programs (Superfund, OSHA, Proposition 65, some drinking-water goals) each state their own decision context.
Extra lifetime risk is a characterization, not a tumor count
Handbook III.D.1 still applies when the apical effect is cancer. Hazard identification may have classified the agent; dose–response may have produced a cancer slope factor (CSF) or inhalation unit risk (IUR) (Chapter 12). Characterization asks what extra lifetime cancer risk that slope implies at this exposure, and which named decision context—not a folkloric “safe number”—you are using to interpret it. Independent OpenExamPrep teaching in this section covers Risk ≈ CSF × LADD, IUR × concentration, the EPA Superfund 10^−6 to 10^−4 range, occupational tolerances that are not Superfund clones, and why ABT has not published a universal acceptable-risk constant. It is not an ABT, EPA, or OSHA product and does not claim official approval, review, or partnership with those bodies.
EPA slope factors used in Superfund-style work are typically upper-bound (often a 95% upper confidence limit on the slope). The product is therefore an upper-bound extra lifetime individual risk, not a prediction of next year’s case count in a census tract and not a proof that the central-estimate risk equals that number.
CSF × LADD (oral, low-dose linear)
For a linear no-threshold oral assessment (the EPA 2005 mutagenic default):
Extra lifetime risk ≈ CSF × LADD
CSF has units of (mg/kg-day)^−1. LADD is the lifetime average daily dose in mg/kg-day, with averaging time 70 years × 365 days (Chapter 11). ADD (averaging time = exposure duration) is the noncancer input. Putting ADD into a lifetime CSF overstates risk whenever exposure lasts less than a lifetime, because the same ingested milligrams are not spread over 70 years.
Worked residential water example. Concentration C = 0.002 mg/L, adult intake 2 L/day, body weight 70 kg, exposure duration 30 years, every day, cancer averaging time 70 years.
LADD = (C × IR × ED) / (BW × 70) when EF = 365 (the 365 cancels).
C × IR = 0.002 × 2 = 0.004 mg/day.
LADD = 0.004 / 70 × (30 / 70) = 0.00005714 × 0.42857 = 2.45 × 10^−5 mg/kg-day.
Check with totals: numerator 0.004 × 365 × 30 = 43.8 mg; denominator 70 × 25,550 = 1,788,500 kg·day; 43.8 / 1,788,500 = 2.45 × 10^−5 mg/kg-day.
If CSF = 0.41 (mg/kg-day)^−1, extra risk = 0.41 × 2.45 × 10^−5 = 1.0 × 10^−5 (one extra case in 100,000 upper-bound individual lifetime risk, as a teaching product).
If a reviewer instead used ADD = 0.004 / 70 = 5.71 × 10^−5 mg/kg-day (30-year average, no 70-year stretch), risk would be 0.41 × 5.71 × 10^−5 = 2.3 × 10^−5—about 70/30 = 2.33 times higher, which is exactly ADD/LADD. That is an averaging-time error, not a more conservative statute.
Unit risk × concentration (inhalation)
Inhalation unit risk (IUR) is extra lifetime risk per unit air concentration, commonly per µg/m³. When the exposure metric is already a lifetime-equivalent concentration:
Extra risk ≈ IUR × C
Worked IUR. IUR = 2.5 × 10^−6 per µg/m³; lifetime-equivalent concentration C = 8 µg/m³. Risk = 2.5 × 10^−6 × 8 = 2.0 × 10^−5.
Do not multiply an oral CSF by an air concentration in µg/m³. The units are incommensurate until you convert air to a mg/kg-day intake with a defensible inhalation rate and bioavailability—and Superfund RAGS Part F prefers to keep inhalation as a concentration compared with an IUR or RfC. An 8-hour occupational TWA is not a lifetime-equivalent concentration until you apply the hours/24 and days/7 (and working-year versus 70-year) adjustments the scenario actually needs.
Oral drinking-water unit risk is the cousin: extra risk per µg/L (or per mg/L) in water under specified consumption and body-weight assumptions. It is still unit risk × concentration, with the unit risk already folding in those assumptions. Mixing a water unit risk with an air µg/m³ is the same unit error.
Superfund’s 10^−6 to 10^−4 decision context
Under the National Contingency Plan, EPA Superfund generally discusses acceptable exposure levels for known or suspected carcinogens as concentrations corresponding to an excess upper-bound lifetime individual risk between 10^−4 and 10^−6 (1 in 10,000 to 1 in 1,000,000). 10^−6 is often the point of departure for setting preliminary remediation goals when other applicable or relevant and appropriate requirements do not control: below about 10^−6, cancer risk commonly drops out of the action discussion; between 10^−6 and 10^−4, risk managers weigh feasibility, other health endpoints, and community factors; above 10^−4, action is more often expected. That range is a Superfund decision context. It is not a biological threshold, not a drinking-water MCL by itself, and not an occupational PEL.
The water example at 1.0 × 10^−5 sits inside that range: above the 10^−6 screening departure, below the 10^−4 upper end. The IUR example at 2.0 × 10^−5 sits in the same band. Neither number is “acceptable” or “unacceptable” until the program is named.
The bar chart in this section plots those magnitudes as extra cases per million people for teaching: 1 (10^−6), 10 (10^−5), 100 (10^−4), and 1,000 (10^−3, the occupational-order figure in the next heading). It is a scale diagram, not a regulatory table of approved limits.
Occupational cancer-risk tolerances differ
Workplace statutes protect adult workers, usually over a working lifetime, not 24-hour residential children. In Industrial Union Department v. American Petroleum Institute (the 1980 benzene decision), the U.S. Supreme Court discussed significant risk in occupational health in terms that OSHA has long treated as including lifetime risks on the order of 10^−3 (1 in 1,000). OSHA then sets feasible standards; a PEL is not a Superfund 10^−6 cleanup goal rewritten for factories. ACGIH TLVs are health-based recommendations with a different method and no claim to a 10^−6 individual-risk calculation. Comparing a factory TWA with a Superfund PRG without restating population, duration, and statute is a characterization error.
Other named programs use other lines. California Proposition 65 no-significant-risk levels are often built around 10^−5 extra lifetime risk. Some drinking-water cancer discussions use 10^−6 or 10^−5 as a goal while MCLs remain technology- and feasibility-constrained. WHO/EFSA food work may stay in MOE language (section 14.1) rather than CSF × LADD. Learn to name the program.
| Named program / context | Typical extra-lifetime-risk discussion | Whose exposure |
|---|---|---|
| EPA Superfund (NCP) | Generally 10^−6 to 10^−4 upper-bound individual lifetime extra risk; 10^−6 often the screening point of departure | Residents and other site receptors, including children |
| OSHA significant-risk discussion (benzene decision lineage) | Order of 10^−3; PELs then constrained by feasibility | Adult workers, working lifetime |
| California Proposition 65 NSRL | Often built around 10^−5 | General public under that statute |
| EFSA food genotoxic-carcinogen MOE | MOE from BMDL10, with 10,000 as a low-concern discussion point (section 14.1)—not a CSF × LADD line | Dietary consumers |
| ABT / DABT handbook | No published universal acceptable-cancer-risk constant | Examination tests method, not a secret cutoff |
What ABT does not publish
The DABT examination tests whether you can compute extra risk and place it in a stated framework. ABT has not published a single acceptable-cancer-risk number that applies to Superfund, OSHA, food contaminants, and drugs. Inventing “the DABT limit is 10^−6” is the same class of error as inventing a numeric passing score. Report the calculated risk, the averaging time, the upper-bound versus central-estimate status, and the decision context you actually invoked.
Scenario
A Superfund risk assessor reports CSF × LADD = 1.0 × 10^−5 for residents and IUR × C = 2.0 × 10^−5 for indoor air after vapor-intrusion adjustment. Both sit in the 10^−6–10^−4 Superfund band; neither is automatically “safe” or “close the site.” A plant industrial hygienist notes that the same chemical’s OSHA discussion historically tolerated ~10^−3 theoretical lifetime risk at the PEL and concludes Superfund must therefore use 10^−3 for the neighboring houses. That conclusion imports the wrong statute and the wrong receptor. A third reviewer multiplies the oral CSF of 0.41 (mg/kg-day)^−1 by 8 µg/m³ and reports risk = 3.28, which is unit-free nonsense. Air belongs with an IUR, or with a converted intake that is then multiplied by a CSF after documenting the conversion.
Traps
- Using ADD in CSF × dose, or LADD against an RfD.
- Treating 10^−6 as a biological zero, or 10^−4 as a license to ignore risk.
- Declaring a universal “acceptable risk” as if ABT or a single EPA office issued one number for all statutes.
- Multiplying oral CSF by air µg/m³, or IUR by mg/kg-day.
- Equating OSHA significant-risk language (~10^−3) with Superfund residential PRGs.
- Reporting extra risk as next year’s observed cases without an exposed population and a central estimate.
Residential water is 0.002 mg/L, 2 L/day, 70 kg, 30 years every day. CSF = 0.41 per (mg/kg-day). What is LADD, and what extra lifetime risk does CSF × LADD give?
Which statement correctly describes EPA Superfund cancer-risk decision context and what ABT publishes?
IUR = 2.5 × 10^−6 per µg/m³ and lifetime-equivalent air concentration = 8 µg/m³. Which extra-risk statement is correct, including occupational contrast and unit discipline?