Chemical Exposure Dose & Hazard Quotient

Understanding the chemical exposure dose and hazard quotient calculator — concept and principle

This calculator turns a contaminant concentration in drinking water into an average daily dose per kilogram of body weight, then compares that dose with toxicity values to give a hazard quotient (non-cancer) and an estimated lifetime cancer risk. It follows the US EPA RAGS screening approach used in site assessments and well-water screening.

How it is calculated

• Average daily dose: ADD = C × IR × EF × ED ÷ (BW × AT), with AT = ED × 365 days. C is concentration (mg/L), IR intake (L/day), EF exposure frequency (days/yr), ED duration (years), BW body weight (kg). The result is in mg/kg·day.
• Hazard quotient: HQ = ADD ÷ RfD, where RfD is the reference dose (mg/kg·day).
• Lifetime cancer risk: the dose is re-averaged over a 70-year lifetime, LADD = ADD × ED ÷ 70, and multiplied by the slope factor: Risk = LADD × SF. Enter SF = 0 for a non-carcinogen and this line is skipped.

The principle

The RfD estimates a daily dose that people, including sensitive groups, can take for a lifetime without appreciable risk of harm, so dose ÷ RfD tells you whether exposure is under (below 1) or over that estimate. For carcinogens EPA assumes no threshold and a linear low-dose response, so risk is the slope times the dose averaged over a whole lifetime. Hence the two averaging times.

Worked example

1. Defaults: C = 0.01 mg/L, IR = 2 L/day, EF = 350 days/yr, ED = 30 yr, BW = 70 kg, RfD = 0.0003, SF = 1.5. These values match the EPA IRIS figures for inorganic arsenic at a concentration of 10 µg/L.
2. Numerator: 0.01 × 2 × 350 × 30 = 210 mg. Denominator: 70 × 30 × 365 = 766,500 kg·days.
3. ADD = 210 ÷ 766,500 = 0.0002739726 mg/kg·d.
4. HQ = 0.0002739726 ÷ 0.0003 = 0.9132420091.
5. LADD = 0.0002739726 × 30 ÷ 70 = 0.0001174168; Risk = 0.0001174168 × 1.5 = 0.0001761252, about 1.8 in 10,000.

How to read the result

An HQ at or below 1 suggests adverse non-cancer effects are unlikely; above 1 means the exposure deserves a closer look, not that harm will occur. EPA generally treats lifetime cancer risks between 1 in 1,000,000 (1E-6) and 1 in 10,000 (1E-4) as the range for risk management decisions; the example sits just above 1E-4, so it would usually prompt action even though the HQ is below 1.

Common mistakes

• Mixing µg/L and mg/L (a factor of 1,000).
• Using an inhalation or dermal toxicity value for an oral water dose.
• Comparing the lifetime-averaged dose with the RfD instead of the ADD.

Frequently asked questions

Where do I find RfD and slope factor values?

The US EPA IRIS database is the primary source; check the route (oral) and assessment date.

Why 350 days per year instead of 365?

The EPA residential default assumes about two weeks a year spent away from home.

Is a hazard quotient a probability?

No. It is a ratio to a reference level, not the chance of getting sick.

Sources: US EPA, Risk Assessment Guidance for Superfund, Volume I, Part A (EPA/540/1-89/002, 1989) · US EPA Integrated Risk Information System (IRIS) · US EPA, Exposure Factors Handbook, 2011 Edition