How to Calculate Hazard Quotient (HQ) and Risk Quotient (RQ)

Little Pro on 2018-06-13 Views:  Update:2018-06-13

Both hazard quotient (HQ) and risk quotient (RQ) are very important concepts in chemical risk assessment. They are used by regulatory authorities such as US EPA to describe the risk category of a chemical substance. In this article, we will summarize the definition of  hazard quotient (HQ) and risk quotient (RQ) and show you how to calculate them. We will also compare their differences. 

What Is Hazard Quotient (HQ)?

A hazard quotient is the ratio of the potential exposure to a substance and the level at which no adverse effects are expected. It is primarily used by US EPA to assess the health risks of air toxics. 

A hazard quotient less than or equal to 1 indicates that adverse effects are not likely to occur, and thus can be considered to have negligible hazard. HQs greater than 1 are not statistical probabilities of harm occurring. Instead, they are a simple statement of whether (and by how much) an exposure concentration exceeds the reference concentration (RfC).

Another very important concept related to HQ is Hazard Index(HI). It is the sum of hazard quotients for substances that affect the same target organ or organ system. As with the hazard quotient, aggregate exposures below an HI of 1.0 derived using target organ specific hazard quotients likely will not result in adverse non-cancer health effects over a lifetime of exposure.

How to Calculate Hazard Quotient (HQ)

Item Description

Hazard Quotient (HQ) = Exposure Concentration/Reference Concentration (RfC)

  • Exposure concentration: in mg/L or ppm
  • Reference Concentration(RfC): An estimate of a continuous inhalation exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime. Its unit is also mg/L or ppm.

Read more: How to claculate reference dose (RfD) or reference concentration (RfC).

  • Exposure concentration: 0.005mg/L
  • Reference Concentration(RfC): 0.01mg/L (rat sub-chronic NOAEC = 1mg/L, UF=1000)
  • Hazard quotient (HQ): 0.5
  • Concluation: HQ<1, negligible hazard

What Is Risk Quotient (RQ)

A risk quotient (RQ) is the ratio of a point estimate of exposure and a point estimate of effects. It is primarily used by US EPA to assess the ecological risk of pesticides.


In above equation, exposure refers to estimated environmental concentration (EEC). Toxicity refers to an effect level obtained from eco-toxicity testing, such as an LC50 or NOEC.

After the risk quotient(s) is calculated, it is compared to EPA's Level of Concern (LOC). If RQ is less than LOC, it is generally regarded that the risk is acceptable.

The table below shows you how to cauculate different types of risk quotients for aquatic species and EPA's level of concern. An LOC is a policy tool that the Agency uses to interpret the risk quotient and to analyze potential risk to non-target organisms and the need to consider regulatory action.

Risk Presumtions Risk Quotient(RQ) LOC
Acute High Risk EEC/(lowest LC50 or EC50) 0.5
Acute Restricted Use EEC/(lowest LC50 or EC50) 0.1
Acute Endangered Species EEC/(lowest LC50 or EC50) 0.05
Chronic Risk EEC/(lowest NOAEC or NOEC) 1.0

Read more: Ecotoxicity testing and endpoints(LC50, EC50, NOEC)

How to Calculate Risk Quotient (RQ)

In order to calculate Risk Quotient (RQ), you need to have access to both estimated environmental concentration (EEC) and toxicity endpoints obtained from ecotoxicity studies.  EEC is usually obtained through modelling by taking into account of the use scenario, physio-chemical properties and environmental fate of a chemical substance.

RQ can be calculated for various routes (i.e, dietary, dermal) and exposure durations (i.e, acute or chronic). You need to choose appropriate ecotoxcity endpoints to calculate the RQ. 

Let's assume that we have obtained EEC from modelling (EEC = 5mg/L). The RQ for aquatic organisms can be calculated as follows:

Risk Available Endpoints RQ Conclusion
Chronic Risk
  • NOEC(Algae growth inhibition): 30mg/L;
  • NOEC(Daphnia reproduction):10mg/L;
  • NOEC (Fish):20mg/L.
EEC/Lowest NOEC = 0.5

RQ (0.5) < LOC (1)

No chronic risk concern.

Acute Risk
  • ErC50(Algae growth inhibition): 50mg/L;
  • LC50 (Acute toxicity to fish Fish): 60mg/L.
EEC/(Lowest EC50 or LC50) = 0.1

RQ (0.1) < LOC (0.5)

No high acute risk concern

Differences Between Hazard Quotient (HQ) and Risk Quotient (RQ)

Item Hazard Quotient (HQ) Risk Quotient (RQ)
Assessment Target
  •  Air toxics or industrial chemical agent
  • Pesticide
Type of Risk Assessment
  • Health risk assessment
  • Ecological risk assessment
  • Hazard Quotient (HQ) = Exposure Concentration/Reference Concentration (RfC)
  • Risk Quotient(RQ) = Esitmated Environmental Concentration (EEC)/Ecotoxicity Endpoints
Risk description
  • Whether HQ is > 1 or <1
  • Whether RQ is > LOC or <LOC


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 Tags: Topics - CRAToxicology and Health Risk Assessment

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