
Proposition 65 provides that "the governor shall cause to be published a list of those chemicals known to the state to cause cancer or reproductive toxicity." It is failure to provide a "clear and reasonable warning" before exposure to those chemicals that subjects a company to Proposition 65 liability. While there are more than 900 chemicals on the list, the vast majority are never the subject of a Proposition 65 claim.
From time to time, Cal Biz Lit surveys the years' notices to determine which chemicals are most often the subject of notices. And now, we've done our full review for 2018. And the result is this:

In other words, 92.8% of the noticed exposures involved one or more of ten chemicals, and 64.5% involved DEHP and lead or lead compounds.
Of the other chemicals noticed, the following were noticed ten or more times: styrene (23), wood dust (22) ethylbenzene (21), benzene (19), nickel (18) benzofuron (17), quinolone and its strong acid salts (17) and polychlorinated biphenyls (10).
And the following were noticed between two and five times: Marijuana smoke, Di-isodecyl phthalate (DIDP), benzophenone, Chromium (hexavalent compounds), formaldehyde, Tetrochloroethylene (Perchloroethylene) and toluene.
And finally, twenty-seven chemicals were noticed only once: flame retardants Tris(1,3-dichloro-2-propyl) phosphate (TDCPP) and Tris(2-chloroethyl) phosphate, 1,1-Dichloro-2.2-bis(p-chloropheny)ethylene (DDE), 1,4-Dioxane, Bromodichloromethane, Carbon disulfide, Chloroethane (Ethyl chloride), Chloroform, DDE (Dichlorodiphenyl-dichloroethylene), Trichloroethylene, Vinyl chloride, 1,3-butadiene, acetaldehyde, benz[a]anthracene, benzo[a]pyrene, soot, benzo[b]fluoranthene, dibenzo(a,h)pyrene, benzo[k]fluoranthene, carbazole, chrysene, chloroform, furan, napthalene, dichloromethane, and indeno[1,2,3-cd]pyrene.
Note that many of the infrequently noticed chemicals have been noticed only in connection with airborne exposure, the majority allegedly caused by charcoal and “steam stones” in hookah bars.
There have been some changes in frequency, but not that many. When we analyzed the 2016 notices, the ten most frequently noticed chemicals were Lead or lead compounds, DEHP, DINP, Cadmium, Carbon Monoxide, Acrylamide, DBP, Benzophenone, Coal tar, DBP and BPA.
We previously analyzed notices for the first five months of 2017, in the same post as the 2016 ones. We noted then that the most frequently noticed chemicals were DEHP, Lead or lead compounds, DINP, Marijuana Smoke, Acrylamide, Cadmium, DIDP, BPA, Myclobutanil and Carbaryl. The latter two are pesticide / fungicides that were allegedly being found in cannabis edibles sold at medical marijuana dispensaries. For unknown reasons, the attorney general strongly objected to the many notices of these pesticide /fungicides. The private party enforcer did not proceed with them, and they haven’t been noticed since.
The fact that the most frequently noticed chemicals are those shown in our 2016, 2017 and 2018 analysis is not a guaranty that other chemicals won’t be the subjects of Proposition 65 notices and litigation. Ideally, companies doing business in California should be aware of the risk that their activities or products in California may be alleged to cause exposures to any of the listed chemicals without required warnings. However, the “top ten chemicals” for each of the past three years certainly constitute a starting point for any risk management efforts.