Before revision: The toxicity of carcinogens is referred to as being "non-threshold" because there is believed to be essentially no level of exposure that does not pose some probability of producing a carcinogenic response, therefore, there is no dose that can be considered to be risk-free.
After revision: This toxicity of carcinogens is referred to as being "non-threshold" because there is believed to be essentially no level of exposure that does not pose some probability of producing a carcinogenic response, therefore, there is no dose that can be considered to be risk-free.