Linear dose-response assessment
If the "mode of action" information (discussed above) suggests that the toxicity does not have a threshold, then this type of assessment is referred to by the Agency as a "linear" dose-response assessment. In the case of carcinogens, if "mode of action" information is insufficient, then linear extrapolation is typically used as the default approach for dose-response assessment (for more detailed information, please see EPA’s Guidelines for Carcinogen Risk Assessment). In this type of assessment, there is theoretically no level of exposure for such a chemical that does not pose a small, but finite, probability of generating a carcinogenic response.
So how does this mumbo jumbo work? Suppose we have a 20 foot ladder that ten people have fallen off and five of them die. From this it is determined that for every 200 feet a population is to fall 5 will die.
For example if the population was 1000 people and each of them were to fall off a ladder a little over 2 inches the prediction is 5 of them will die from this fall.