Running a Prop 65 exposure assessment

Proposition 65 only requires a warning if a product can expose someone in California to a listed chemical above the 'safe-harbor' level. The exposure assessment is the calculation that decides whether you need the warning at all — and it's a defense if you're ever sued by a private enforcer.

Two safe-harbor thresholds:

• NSRL (No Significant Risk Level) — for carcinogens. Calibrated so lifetime exposure leads to fewer than 1 case of cancer per 100,000 people. Below this level, no warning is required.
• MADL (Maximum Allowable Dose Level) — for reproductive toxicants. Set at 1/1000 of the No Observed Effect Level.

Four inputs to every assessment:

1. Concentration — how much of the listed chemical is in the product (ppm, μg/g). From lab test or supplier disclosure.
2. Use rate — how much of the product is consumed, applied, or contacted in a typical use scenario
3. Route of exposure — oral (food, mouthing toys), dermal (skin contact), inhalation
4. Frequency — uses per day, days per year, years of use

The calculation:

Daily exposure (μg/day) = concentration × use rate × bioavailability factor

If daily exposure is below the NSRL or MADL for the chemical, no warning is required. OEHHA publishes the official safe-harbor levels at oehha.ca.gov.

TIP: Document the assessment even when no warning is required. If a Prop 65 plaintiff sues, your assessment is the evidence that exposure was below safe-harbor — without it, the case becomes much harder to defend.

WARNING: Listed chemicals below safe-harbor still need vigilance. OEHHA updates the list quarterly, and the safe-harbor level for lead (0.5 μg/day MADL) is so low that any detectable amount typically triggers a warning.