For threshold contaminants, the risk to a human receptor from being exposed to a chemical via a single pathway can be expressed as an Exposure Ratio, commonly called a Hazard Quotient (HQ).
|HQ =||Dose Rate||=||Exposure Concentration||=||Estimated Dose (μg/kg/day)|
|Reference Dose||Reference Concentration||Tolerable Daily Intake (μg/kg/day)|
The reference dose is interpreted as the Tolerable Daily Intake (TDI; mg/kg/day).
A Hazard Index (HI) is the sum of HQ’s for all pathways and similar toxic effects. A HQ of <0.2 for any given pathway is often considered acceptable; while an HI of <1.0 is considered acceptable (Health Canada 2004).
For purposes of preliminary quantitative risk assessment, exposures associated with a HQ = 0.2 will be deemed negligible. This is consistent with the CCME (1996) and the OMEE (1996a), and has become accepted as common practice (Health Canada 2004). If the HQ is greater than 0.2, or the HI is greater than 1, the risk assessment should either be refined and/or risk management measures should be taken.
You can calculate a Hazard Quotient using the Risk Calculation Tools.