A threshold response is characterized by a toxic effect occurring above an exposure concentration. Most environmental contaminants are threshold contaminants.
The maximum allowable exposure concentrations, called the Exposure Limits (or Toxicity Reference Values) are based on the threshold determined from toxicity experiments. Usually the Exposure Limit incorporates an Uncertainty Factor (or Safety Factor) to account for uncertainties in the estimate. The Exposure Limit for a Threshold Contaminant is generally presented as a Tolerable Daily Intake value (TDI; mg chemical/kg body weight/day) (Health Canada 1994) or a Reference Dose (RfD; US EPA).
You can calculate Hazard Qutotient for a threshold contaminant using the Risk Calculation Tools of the toolkit.