My first encounter with a Prop 65 warning was on the back of my TV.

I was selling it on Facebook Marketplace, and when the buyer arrived to pick it up, he spotted a warning I had never noticed. “California Proposition 65 Warning: This product contains chemicals known to the state of California to cause cancer and birth defects or reproductive harm.” Neither of us had seen such a label before, and for a moment, he didn’t want the TV anymore. We did what anyone would do in this situation: We Googled it. He ended up taking the TV. I kept Googling.

California’s Proposition 65, officially called the Safe Drinking Water and Toxic Enforcement Act, applies to a dizzying range of products, from food and cosmetics to electronics and household goods. Passed in 1986, the law turns 40 this year, yet that ominous little label still confuses the average consumer.

So, should you steer clear of all products with a Prop 65 warning? Not necessarily. Here’s what you need to know before you panic-buy a new TV.

Courtesy of Stephen Osman/Getty Images

Proposition 65, Explained

In November 1986, California voters approved Proposition 65 by a landslide margin of about 63 percent to 37 percent. It’s a right-to-know law, meant to inform Californians about potential exposure to chemicals that may cause cancer or reproductive harm over a lifetime.

It doesn’t ban those chemicals; instead, it requires businesses to provide a warning when their products could expose people to a listed chemical at levels above specific thresholds.

“At that time, it was part of a broader effort to address a growing concern that the public had about toxic chemical exposure,” says Megan Schwarzman, environmental health scientist at UC Berkeley’s School of Public Health.

The idea is that consumers can use that information to make their own decisions. A warning might prompt someone to choose an alternative, use a product less frequently, or take other steps to reduce their exposure. Prop 65 also gives businesses an incentive to reformulate products or change manufacturing practices to avoid issuing a warning that may deter customers.

And because California is such a massive consumer market, products often need to comply with Prop 65 requirements even when they’re manufactured or sold by companies elsewhere. Some large retailers, such as Target and Amazon, also require their vendors to comply with Prop 65 warning requirements.

Prop 65 Is Everywhere

Prop 65 covers a broad range of products and potential exposures. You can find warnings on fortified foods, smartphones, and appliances. The law is concerned with exposure, not simply whether the chemical is detectable. California’s Office of Environmental Health Hazard Assessment (OEHHA) establishes the so-called safe-harbor levels for many listed chemicals. Generally speaking, a warning is required when anticipated exposure exceeds these levels.

The Prop 65 list contains both naturally occurring and synthetic chemicals that California has determined are known to cause cancer or reproductive harm. Supplements, for example, can contain trace amounts of naturally occurring or environmental contaminants, including lead, cadmium, and arsenic. The list is updated at least annually, and chemicals can also be removed from the list as scientific evidence evolves.