Cancer risk refers to the probability of contracting cancer associated with exposure to a substance.  It is expressed as the chance per million of a cancer case occurring.  A risk of one per million, for example, would mean that in a population of one million individuals exposed over a 70 year lifetime, one additional cancer case would be expected.  

Cancer risks were calculated assuming a 70 year lifetime exposure to the annual average levels estimated by the model.  This is consistent with guidelines for risk assessment established by the California EPA Office of Environmental Health and Hazard assessment (OEHHA).  The unit risk potency factors used in these calculations are listed in Appendix I - List of Substances and Their Associated Risk Factor (PDF, 17kb) of the MATES IV Draft Report.   

Lifetime risk can be calculated by multiplying the concentration of a substance by its unit risk factor.  Additional details of the modeling can be found in Chapter 4 - Regional Modeling and Evaluation (PDF, 3,909kb) and Appendix IX - Regional Modeling Analyses (PDF, 3,309kb) of the MATES IV Draft Report. 

Additional information on risk assessment from OEHHA can be found on the OEHHA's Risk Assessment for the Layperson webpage.