Cancer Risk Assessment
Learning Objectives
- Describe the multistep model of carcinogenesis
- Identify major carcinogenic air pollutants and their mechanisms
- Apply unit risk factors to calculate lifetime cancer risk
- Interpret IARC carcinogen classifications
- Evaluate risk-benefit tradeoffs in regulatory decision-making
Carcinogenesis: A Multistep Process
DNA damage
Clonal expansion
Additional mutations
Spread
Key mutations: Oncogene activation (ras, myc) + Tumor suppressor inactivation (p53, Rb)
Latency period: 10-40 years between exposure and clinical cancer
Carcinogenic Air Pollutants
| Agent | IARC Group | Sources | Primary Cancer |
|---|---|---|---|
| Outdoor air pollution | 1 (carcinogenic) | Vehicles, industry, power | Lung |
| Diesel exhaust | 1 | Trucks, buses, generators | Lung |
| Benzene | 1 | Gasoline, solvents, tobacco | Leukemia |
| Formaldehyde | 1 | Building materials, combustion | Nasopharyngeal, leukemia |
| Radon | 1 | Soil gas, building materials | Lung |
| Asbestos | 1 | Insulation, brake pads | Mesothelioma, lung |
| PAHs | Various | Incomplete combustion | Lung, skin, bladder |
IARC Classification System
- Group 1: Carcinogenic to humans (sufficient evidence)
- Group 2A: Probably carcinogenic (limited human, sufficient animal evidence)
- Group 2B: Possibly carcinogenic (limited evidence overall)
- Group 3: Not classifiable as to carcinogenicity
Important distinction: Classification indicates hazard (can it cause cancer?) not risk (how likely under real exposures?).
Quantitative Cancer Risk Assessment
Unit Risk Factor (URF)
The probability of developing cancer from continuous lifetime exposure to 1 ug/m3 of a carcinogen.
Lifetime Cancer Risk = Concentration x URF
Selected Unit Risk Factors
| Pollutant | URF (per ug/m3) |
|---|---|
| Benzene | 2.2 x 10-6 to 7.8 x 10-6 |
| Formaldehyde | 1.3 x 10-5 |
| Diesel exhaust PM | 3 x 10-4 |
| Acetaldehyde | 2.7 x 10-6 |
Acceptable risk levels: Regulatory agencies typically aim for lifetime risks below 10-6 (1 in a million) to 10-4 (1 in 10,000).
Radon: An Indoor Cancer Risk
Radon-222: Radioactive decay product of uranium in soil
- Enters buildings through cracks, sump pits, foundation joints
- Decays to polonium-218 and polonium-214 (alpha emitters)
- Alpha particles damage bronchial epithelial DNA
- Second leading cause of lung cancer (after smoking)
EPA action level: 4 pCi/L (148 Bq/m3)
Risk at action level: ~7 excess lung cancer deaths per 1,000 persons over lifetime (for never-smokers); ~62 per 1,000 for current smokers due to synergy.
Activity: Cancer Risk Calculation
Scenario: A school built in 1970 contains building materials that release formaldehyde. Indoor concentrations average 40 ppb (49 ug/m3).
- Using URF = 1.3 x 10-5 per ug/m3, calculate the lifetime cancer risk for a student exposed during school hours (assume 1/3 of time exposed)
- How does this compare to the EPA's acceptable risk range?
- If remediation costs $500,000 and reduces concentrations to 10 ppb, calculate the risk reduction
- Discuss: How should we weigh cancer risk against remediation costs?
Note: Adjust for exposure duration: Risk = Concentration x URF x (hours exposed per day / 24) x (days exposed per year / 365)
Key Takeaway
Cancer risk assessment provides a quantitative framework for evaluating carcinogenic air pollutants. Unit risk factors allow us to estimate lifetime cancer probability from specific exposures, enabling evidence-based regulatory decisions. Understanding these tools reveals how indoor air quality decisions - from building materials to ventilation - directly affect long-term cancer risk.