A slope factor is an upper bound, approximating a 95% confidence limit, on the increased cancer risk from a lifetime exposure to an agent by ingestion or inhalation.
This estimate, usually expressed in units of proportion (of a population) affected per mg of substance/kg body weight-day, is generally reserved for use in the low-dose region of the dose-response relationship, that is, for exposures corresponding to risks less than 1 in 100.
For carcinogens, it is commonly assumed that a small number of molecular events may evoke changes in a single cell that can lead to uncontrolled cellular proliferation and eventually to a clinical diagnosis of cancer.
For chemicals that are carcinogens, a two-part evaluation to quantify risk is often employed in which the substance first is assigned a weight-of-evidence classification, and then a slope factor is calculated.
[2] When the chemical is a known or probable human carcinogen, a toxicity value that defines quantitatively the relationship between dose and response (i.e., the slope factor) is calculated.
The slope factor is used in risk assessments to estimate an upper-bound lifetime probability of an individual developing cancer as a result of exposure to a particular level of a potential carcinogen.
Slope factors should always be accompanied by the weight of-evidence classification to indicate the strength of the evidence that the agent is a human carcinogen.
Where: The Cancer Slope Factor is used to derive the Risk Specific Dose (RSD)(mg/kg-day) for direct-acting carcinogenic agents, those that cause chemical changes (mutations) in DNA.