California Proposition 65
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California Proposition 65, the Safe Drinking Water and Toxic Enforcement Act of 1986, constitutes the basis for a legislation on transparency of exposure for consumers before they purchase a product - not limited at all to water safety as the titel might pretend. Prop 65 often is used as reference for specifying carcinogenic compounds in other regulations and specifications as well, e.g. in CDPH Section 01350 regarding emissions from products into indoor air.
Any consumer product, if manufactured, handled or sold in California by a business with more than 9 employees, is covered by this regulation.
Chemicals involved are those that can cause cancer or reproductive harm such as birth defects (CMR).
A warning must be placed on the product if any CMR chemical is contained in the product and can lead to health risk by exposure during its use. This is the case if the exposure level is assumed to cause a risk of 1 additional case of any such harm per 100.000 people.
Exposure can be calculated from composition with the worst-case assumption that the whole amount of the chemical is released. Another option is to determine average release rates, such as emission into indoor air or migration into drinking water or food. Exposure is calculated over 70 years life duration.
Risk is estimated from exposure pathways and quantity in comparison with limit values (if available). As an example, release of particles from painted wall can occur in very small quantities and the particles can be dispersed in indoor air. If e.g. lead is contained in that paint then this small release can already make a Proposition 65 warning necessary.