California Prop 65: What's Listed and Why

Proposition 65 is a California law. It keeps a public list of chemicals known to cause cancer (or reproductive harm). It also requires a warning before people are knowingly exposed to them. The list holds roughly 900 chemicals and grows most years.

A chemical earns its place through set legal pathways. What decides the matter is whether it can cause cancer — a hazard judgment. It is not a judgment about how much risk any one product poses to you. That single fact explains almost everything people find confusing about the warnings, including why they seem to be on everything.

It is easy to mix up the numbers. Proposition 8 was California's 2008 ballot measure about same-sex marriage. It has nothing to do with cancer. The cancer law is Proposition 65, officially the Safe Drinking Water and Toxic Enforcement Act of 1986. When you see a sign reading "this product can expose you to chemicals known to the State of California to cause cancer," that is Prop 65 at work.

Prop 65 does two things. First, it requires California to publish and maintain a list of chemicals known to cause cancer or reproductive toxicity (harm to fertility, pregnancy, or a developing baby). That list must be updated at least once a year. Second, it requires businesses to give a clear warning before they knowingly and intentionally expose people to a listed chemical above certain levels. Those warnings appear on product labels, at building entrances, in workplaces, and so on. The list and the exposure levels are maintained by California's Office of Environmental Health Hazard Assessment (OEHHA).

One point matters a great deal. Prop 65 does not ban the chemicals. It is a right-to-know law. Its goal is disclosure, so people can make their own choices.

The list now contains roughly 900 chemicals. Some are listed because they can cause cancer. Some are listed because they can cause birth defects or other reproductive harm. Some are listed for both. They span an enormous range: industrial solvents, pesticides, metals like lead and arsenic, byproducts of burning such as vehicle exhaust, some food and cooking byproducts, ethyl alcohol in alcoholic beverages, tobacco smoke, and a number of prescription and over-the-counter drugs.

The list is genuinely dynamic. Recent additions and pending listings show the process in motion. OEHHA added N-methyl-N-formylhydrazine as a carcinogen — something known to be able to cause cancer — effective December 8, 2025. In 2026 it issued notices of intent to list several more cancer-linked substances. Those include the medicines hydrochlorothiazide, voriconazole, and tacrolimus, as well as welding fumes. Because the list changes, the authoritative, always-current version is OEHHA's official Proposition 65 List page.

A chemical does not land on the list by opinion. Prop 65 defines specific legal pathways. The most common one is the "authoritative bodies" listing. Under it, a chemical is added because a respected scientific body has already concluded that it can cause cancer. Those bodies include the International Agency for Research on Cancer (IARC), the cancer arm of the World Health Organization, and the U.S. National Toxicology Program (NTP), which publishes the Report on Carcinogens. Other pathways include listing by California's own qualified state experts, or listing because a federal agency already requires a cancer warning.

This is why Prop 65 is best understood as a hazard list. It gathers up the conclusions of hazard-focused science, which asks one question: can this substance cause cancer under some conditions? It does not calculate your personal risk from a given cup of coffee or a specific phone case. For the mechanics of each pathway, see our companion guide on how chemicals get listed under Prop 65.

Two things combine. First, the list is hazard-based. So a warning is triggered by the mere presence of a listed chemical above a threshold. It is not triggered by proof that a particular product will harm you. Second, the penalties for failing to warn are large. So many businesses post warnings broadly and defensively, even when the actual exposure is tiny. The result is warning fatigue. The signs appear on parking garages, hardware, furniture, and food, and people stop reading them.

The antidote is to remember what a warning means and doesn't mean. It means: a listed chemical may be present here. It does not mean: this will give you cancer. OEHHA sets "safe harbor" levels, including a No Significant Risk Level for cancer. Below those levels no warning is required, and most everyday consumer exposures fall well under the level of real concern. Our guide to understanding Prop 65 warnings walks through how to read a specific sign.

Prop 65 lists many substances, and they are not interchangeable. Each is tied to particular cancers by the science behind its listing. A few well-established examples help make the list concrete:

• Benzene (in gasoline, vehicle exhaust, and tobacco smoke) — linked to leukemia and other blood cancers.
• Formaldehyde (in some building materials, resins, and smoke) — linked to nasal/nasopharyngeal cancers and myeloid leukemia.
• Tobacco smoke — linked to lung cancer and many others, including mouth, throat, esophagus, bladder, and pancreas.
• Asbestos — linked to mesothelioma and lung cancer.
• Arsenic (natural in some water and soil) — linked to skin, bladder, and lung cancers.
• Vinyl chloride (plastics manufacturing) — linked to a rare liver cancer (angiosarcoma).
• Benzo[a]pyrene, a PAH (in smoke, char, and exhaust) — linked in studies to lung and digestive-tract cancers.
• Radon — linked to lung cancer.

These examples share a pattern. The strongest, clearest links usually come from high, sustained, often occupational exposures. The same chemical at a trace level in a consumer product may pose little measurable risk. There is a gap between what a substance can do at a high dose and what it does at the dose in front of you. That gap is exactly the hazard-versus-risk distinction, and a Prop 65 warning cannot capture it on its own. For where these substances actually show up day to day, see our guide to the common chemicals behind Prop 65 warnings.

Treat a warning as a prompt to think, not a reason to panic. Ask whether the exposure is large and repeated, which is worth reducing. Or whether it is trace and occasional, which is rarely worth worrying about. Aim your energy at the exposures that carry real, well-established risk: tobacco smoke, radon in the home, heavy occupational exposures. Aim it also at the biggest, most controllable cancer-risk factors in your life. Put any single warning in context with our overview of what raises cancer risk.

Cancer Explained is a free, ad-free educational project. If this plain-language explanation helped, you can keep guides like it free for everyone by supporting our work.

Proposition 65 is a California right-to-know law. It is not a ban, and it is not the same-sex-marriage measure people sometimes confuse it with. Its roughly 900 chemicals are listed because authoritative science says they can cause cancer or reproductive harm. That is a hazard judgment. It explains why the warnings are broad, and why they cannot tell you your personal risk. Different chemicals are tied to different cancers, and the real danger almost always tracks the amount and length of exposure. Read the warnings, understand what they mean, and aim your attention at the exposures that matter most.

• National Toxicology Program — Report on Carcinogens
• IARC — Agents Classified by the IARC Monographs
• National Cancer Institute — Cancer-Causing Substances in the Environment
• California OEHHA — The Proposition 65 List