How Can a Manufacturer Respond to Requests for Assurances that a Prop 65 Warning is not Required?

January 15, 2008

California Prop 65 NSRLs and MADLs are stated in terms of exposure levels (µg/day) but suppliers of raw materials (resins, inks, etc.) used in manufacturing plastic goods use concentration levels (ppm) when responding to inquiries concerning substances controlled by Prop 65. A telephone conversation and email exchange with a representative of the Cal EPA resulted in little more than confirmation of the above discrepancy. Thus, letters from raw material suppliers that refer to a ppm level cannot be translated into an equivalent µg/day that does or does not require a warning notice. Of course, any such translation would require certain assumptions concerning the sum of exposures of a theoretical subject person but California provides no equation or table for this purpose. Question: In light of the above, how can a manufacturer of finished plastic goods safely respond to requests from its customers for written assurances that the finished goods do not require a warning under Prop 65?


Your question asks how to convert the thresholds set under Proposition 65, provided in micrograms per day, to dietary intake. As you note in your inquiry, there are many variables that figure into such calculations to arrive at the most accurate measure of migration. However, it is possible to make general, conservative assumptions to arrive at an estimate. The example below demonstrates most simply how to translate a concentration in a packaging component to a microgram per day exposure.

To illustrate an example of a "close call," let us assume that the manufacturer produces a single-serving package for a food and that the package uses a component containing lead at a level of 0.001% or 10 ppm, meaning that there would be 10 micrograms (µg) of lead per one gram of the packaging material. If we know that this packaging component is used at a level of 0.04 g in the finished article, we would have a level of 0.4 µg of lead in the finished article (i.e., 0.04 g of component times (10 µg of lead per 1 g of component)).

The Maximum Allowable Dose Level (MADL) for lead is 0.5 µg/day. (See, Proposition 65 Safe Harbor Levels: No Significant Risk Levels for Carcinogens and Maximum Allowable Dose Levels for Chemicals Causing Reproductive Toxicity (October 2007); by clicking here.) The most conservative assumption would be to assume that all (i.e., 100%) of the lead in the component would migrate into the food—in this case 0.4 µg of lead. If the manufacturer is aware that a consumer eats only one serving of the food per day, the inquiry is done—the exposure from the single-serving package being 0.4 µg/day, less than the MADL. However, if the manufacturer is aware that the average consumer eats four servings of the food per day, the exposure from the product would be 1.6 µg/day (i.e., 4 times 0.4 µg/day). The product would contribute more than 0.5 µg/day and would need to be labeled under Proposition 65.

If a manufacturer found that its product would not meet the 0.5 µg/day limit using 100% migration calculations, the manufacturer could perform migration testing to simulate the food-type, temperature, and time conditions of the intended application to accurately estimate the amount of lead that would to migrate to food.

This response does not address the many variables that should be considered when making an actual determination of Proposition 65 compliance. Proposition 65 is a very complex regulation, and as you are aware, determining compliance can be equally if not more complex. Therefore, we recommend consulting an attorney to answer any actual questions about Proposition 65 compliance.