Toxic Avenger or Label Overlord? Unmasking the Real Impact of California’s Prop 65

California’s Proposition 65 (Prop 65) labels are an invasive species across all products and geographies across the US. Design in 1986 as a right-to-know law, it aims to warm the public when products, processes, or emissions may expose individuals to listed chemicals above a certain risk threshold. Those risk thresholds are set automatically through designation by “authoritative bodies” or by scientific review by state experts. Out of a potential 42,000 or more chemicals that we are exposed to, Prop 65 now has 950 entries involving 850 unique chemicals. 

Approaching a law in effect for nearly 4 decades, researchers ask the question, 

“Have Prop 65 listings reduced population-level toxic chemical exposures in California or nationwide?”

To begin a search for their answer, they turned to our old friend, the CDC's National Health and Nutrition Examination Survey (NHANES), which captures biomonitoring data in blood or urine samples of the participants. The biannual nature of the survey allows for a look at the trends from 1999 when NHANES began to 2016 and involves 15 nationwide locations. The final dataset included blood or urine metabolite concentrations for 37 chemicals: 11 listed by Prop 65 before 1999, 11 listed during the study interval, 4 listed subsequently, and 11 not listed by Prop 65 but acting as proxies for similarly structured chemicals that were Prop 65 listed. [1]

The chemicals considered included those with both toxicity and high-volume use:

• Phthalates
• Phenols commonly used in consumer products, such as parabens, bisphenols, ultraviolet filters, triclosan, and chlorinated phenols
• Volatile Organic Compounds (VOCs), including gasoline-related chemicals such as benzene, vehicle exhaust constituents, chlorinated and non-chlorinated solvents, chloroform (a byproduct of water disinfection), and polymer building blocks such as acrylamide and acrylonitrile.
• Polycyclic Aromatic Hydrocarbons (PAHs) products of combustion.
• Per- and Polyfluoroalkyl Substances (PFAS), the dreaded forever chemicals
• Heavy Metals, e.g.,  lead, cadmium, mercury

 The presence of a chemical in the NHANES study was driven, in part, by health concerns and subsequent regulation. This led to an expectation that the chemicals considered by NHANES would decline over time due to those health concerns and regulations, as well as voluntary removal by producers or avoidance by users.

The graphic shows the changes in the chemicals listed by Prop 65 during the monitoring period, with those trending towards the left demonstrating decreasing exposures, those to the right increasing exposures, and the dotted line showing the changes for those NHANES participants living outside California, the solid line for those living within California.

Simply put, it is a mixed bag of results. While 14 of 22 (63%) chemicals on the Prop 65 list showed decreased exposure, several did increase. At the same time, 11 of 15 (73%) unlisted chemicals decreased. 

The researchers note that isolating the effect of Prop 65 in these changes is challenging. Among the patterns they noted was chemical substitution by manufacturers. Bisphenol A (BPA) concentrations, used in polycarbonate plastic bottles, thermal receipts, and food can linings, declined after Prop 65 listing, simultaneously with increases in Bisphenol S (BPS), an unlisted chemical. However, scientists are already voiced concerns about the similar toxicities of BPS. In this instance, one can ask whether Prop 65 improves our risk profile or simply creates unrecognized harm.  

In other instances, listing a chemical resulted in reductions that might better be attributed to new regulatory limits and enforcement litigation in the case of DEHP (Di(2-Ethylhexyl)Phthalate), an agent making plastic more pliable. There were also declines for chemicals not explicitly listed by Prop 65 but within what amounts to a “halo of concern.” For example, methyl and propyl parabens, antimicrobials used as preservatives in many cosmetics and personal care products, declined, reflecting the impact of consumer-driven campaigns – a market force.    

Naphthalene exposures comes from diverse sources, from diesel combustion to household goods like mothballs and vinyl materials. 2-naphthol (a naphthalene metabolite) rose nationally post-listing by Prop 65. While indoor sources have declined, it seems that outdoor sources have resulted in overall increased exposure. 

Even for Californians, the biological impact of Prop 65 was mixed. Californians had significantly lower concentrations of about half the chemicals studied, including many diesel-related chemicals, most phthalates, BPA, and all PFAS. Only four chemicals—styrene, acrylonitrile, cadmium, and mercury—were higher in California. Those diesel-related emissions were linked to strict regulatory action following their Prop 65 listing (a 78% decrease over the interval). The researchers attribute the rising mercury to “greater fish consumption.”

While these examples highlight the complex interplay of regulatory and non-regulatory forces driving changes in population-level chemical exposures, some take-home points exist.

• The listing on California’s Prop 65, reflecting the concerns of a significant market share, has resulted in manufacturers deselecting “specific toxic chemicals, whether by substituting other chemicals, reducing the concentration of the listed chemical, or redesigning products to obviate the need for the toxic chemical’s function.” Because manufacturers found it easier to make these changes across all of their geographical markets, unlike Vegas, what is decided in California does not remain in California. 
• Some product redesigns resulted in the avoidance of Prop 65 listings, but the substitutions were considered equally concerning. 
• For some chemicals listed by Prop 65, those that say increases, the researchers write,  “the information-forcing policy effect of Prop 65 chemical listings sends too weak a market signal for its effects to be detected by current surveillance-level biomonitoring,” and “we are confident in asserting that a right-to-know law [Prop 65] on its own is insufficient to eliminate population-level exposure to the toxics it targets.”

So, what does Prop 65 provide? A legacy of manufacturers reformulating products to avoid California's wrath, often exporting those changes nationwide. Sometimes, this improves public health; other times, it swaps one toxin for another. While Prop 65 has undeniably driven awareness, the researchers were blunt: a warning label alone won’t eliminate exposure to toxic chemicals. California’s right-to-know chemical crackdown seems to demonstrate that progress is as complex as the chemicals we’re trying to avoid.

[1] Urine metabolites were adjusted for dilution, and chemicals that were related to smoking were adjusted by the presence of cotinine, a smoking biomarker. 

Source: Trends in NHANES Biomonitored Exposures in California and the United States following Enactment of California’s Proposition 65 Environmental Health Perspectives. DOI:10.1289/EHP13956