A Wide Reach: California’s Prop 65 and Reduced Chemical Exposures Across the United States

Short abstract

Levels of certain chemicals listed under California’s law have declined in biosamples from people across the nation.

In 1986, California passed Proposition 65 (Prop 65),¹ a “right to know” law that requires companies doing business in the state to warn the public when their products or processes could expose people to chemicals known to cause cancer or reproductive or developmental harm. Under the law, the state maintains a list of chemicals and mixtures—currently numbering more than 850—that may be present in consumer products, processed foods, and even structures. Written warnings must be provided if exposures to any listed substance exceed a certain risk threshold.² Text warnings may be posted at the point of sale or in such locations as parking garages and gas stations, as well as printed on product packaging.

From its inception, the law spawned public and scientific debate about its effectiveness,^3,4 including the potential for “warning fatigue”⁵—when people become desensitized to warnings due to their widespread presence. Although the law did not require changes to product formulations or processes, interviews with representatives of affected companies have indicated that many businesses did alter formulations to avoid having to post warnings or manufacture special products just for California.⁵

The authors’ research strategy revealed that as body levels of several Prop 65 chemicals decreased for NHANES participants across the United States—an unexpected result—levels of certain replacements increased. Note: NHANES, National Health and Nutrition Examination Survey: Prop 65, Proposition 65. Image: Epolk, used under CC BY-4.0 license (https://creativecommons.org/licenses/by-sa/4.0/deed.en).

Determining whether the law changed chemical exposures throughout the state and beyond has proven challenging—until now. A study recently published in Environmental Health Perspectives⁶ used National Health and Nutrition Examination Survey (NHANES) data from 1999 through 2016 to assess whether and how US population–level exposures to some of those chemicals changed after they were added to California’s list. NHANES, the most comprehensive biomonitoring project across the United States, releases data in 2-year cycles from varied locations in the country.⁷ Certain data (such as state of residence) are available only through a special request that must be approved by the US Centers for Disease Control and Prevention, which administers NHANES.⁸

The team studied 22 listed chemicals, 11 unlisted chemicals that were structurally similar to listed chemicals, and 4 chemicals that were added to the list after the biomonitoring period. These 37 chemicals included phthalates, phenols, volatile organic compounds, metals, per- and polyfluoroalkyl substances (PFAS), and more.

The researchers looked for associations between the timing of each chemical’s Prop 65 listing (before, during, or after the biomonitoring period) and changes in blood or urinary levels in Californians vs. people from all other states. If the researchers had data from before and after a chemical’s listing, they used a difference-in-differences model, which “lets you look at whether the change in response to the chemical being listed on Prop 65 was different for Californians compared with non-Californians,” explains first author Kristin Knox, a research scientist at the Silent Spring Institute.

The results surprised them. “Although chemical exposure levels in Californians and non-Californians both went down after Prop 65 listing, we did not see a differential effect for Californians,” says Knox. In other words, the levels did not decrease disproportionately for Californians. Concentrations declined for all biomonitored, Prop 65-listed chemicals except a very small group (a metabolite of naphthalene and a metabolite of 1,3-butadiene), which increased. The reasons for these increases were unclear, the authors noted.

Knox, left, is with the Silent Spring Institute. Coauthor Claudia Polsky, right, directs the Environmental Law Clinic at the University of California, Berkeley, School of Law, where she is a clinical professor of law. Images: Silent Spring Institute, left, and David Page, right.

Other chemicals declined across the US population regardless of their listing under Prop 65, including certain PFAS that were listed after the biomonitoring window of this study. Overall, Californians had lower average levels of biomonitored chemicals in blood and urine than did other US residents for most chemicals in the study, regardless of whether the chemical was listed on Prop 65.

The study also revealed unlisted chemicals that had replaced similar listed ones. For example, after bisphenol A was listed in 2013, its levels declined, whereas concentrations of the replacement chemical bisphenol S increased. Similarly, concentrations of di-$n$-butyl phthalate metabolites decreased across the nation after the parent chemical’s listing in 2005; concurrently, metabolites of the closely related and unlisted replacement, diisobutyl phthalate, increased. In both cases the replacement chemical has proven to raise as many concerns as the chemical it replaced.^9,10

“This is an impressive body of work. The authors creatively use difficult-to-access data to answer some important public health policy questions,” says Gina Solomon, chief of the Division of Occupational, Environmental, and Climate Medicine at the University of California, San Francisco, who was not involved in the study. “Kudos to this team.”

Solomon explains that NHANES is not designed to be a representative sample at the state level, making variability an additional challenge. “It might focus on Latinos in Los Angeles in one cycle and Asians in [California’s] Central Valley in another cycle,” she says. “These issues make it even harder to discern a … trend, so the fact that some clear trends emerged in this study is even more impressive.”

“I was surprised how clear the data are that California’s prioritization of environmental and public health has paid off,” says Veena Singla, an adjunct assistant professor of environmental health sciences at Columbia University, who was not involved in the study. That said, the data do not allow researchers to find trends for specific communities, Singla adds. “We know that [many] communities remain exposed disproportionately to toxic chemicals, including lead,” she says, “due to structural factors like systemic racism.”¹¹

Knox notes that despite the hundreds of unique chemicals on California’s list, the authors found enough biomonitoring data to analyze Prop 65 effects for only 22 of the listed chemicals. As the authors pointed out, NHANES data collection was not designed to test the efficacy of Prop 65—or any law. Knox suggests designing state and federal policies in ways that require before-and-after biomonitoring. “When you’re putting policies into place,” she says, “you want to think about being able to evaluate [them].”

“Biomonitoring is a key tool to understand chemicals in people’s bodies and health impacts,” notes Singla, “but if we have toxic chemicals in our bodies, our policies have already failed in some respects. We should ensure that chemicals, materials, and products are safe and sustainable by design before producing them.”

Biography

Wendee Nicole is an award-winning science writer based in San Diego. She is a regular contributor to Environmental Health Perspectives.