Prop 65 Safe-Harbor Levels (NSRL and MADL)

The risk-based part of a hazard-based lawExplain▾

Proposition 65 lists chemicals based on hazard — whether they can cause cancer or reproductive harm. But the law also recognizes that dose matters. It does this through "safe harbor" levels: exposure thresholds below which a business does not have to provide a warning.

These levels are where Prop 65 quietly borrows the logic of risk assessment — acknowledging that a tiny exposure to a hazard may not warrant a warning at all.

The NSRL: the cancer thresholdExplain▾

For carcinogens, the safe-harbor level is the No Significant Risk Level (NSRL).

OEHHA defines the NSRL as the level of exposure calculated to result in no more than one extra case of cancer per 100,000 people exposed over a 70-year lifetime. In other words, if 100,000 people were exposed at the NSRL every day for 70 years, the model predicts at most one additional cancer among them.

This is a deliberately conservative, protective standard. It assumes long, lifetime exposure and errs on the side of caution. If a product's exposure stays below the NSRL, no cancer warning is required.

The MADL: the reproductive-harm thresholdExplain▾

For reproductive toxicants, the safe-harbor level is the Maximum Allowable Dose Level (MADL).

The MADL is set at 1/1,000th of the level shown to produce no observable reproductive effect in studies. That thousand-fold margin builds in a large cushion of safety. Below the MADL, no reproductive-harm warning is required.

Why the levels matter — and why warnings still appearExplain▾

In principle, these safe-harbor levels mean warnings should appear only when exposure exceeds a protective threshold. That would make a warning more meaningful: it would suggest exposure above the (very cautious) safe level.

In practice, warnings appear far more widely than that logic implies. Two reasons:

• For many chemicals, no official safe-harbor level has been set, so businesses can't easily show they're below it.
• Businesses often warn defensively — it's legally safer to include a warning than to risk a lawsuit arguing one was needed.

This gap between the protective intent of the NSRL and the reality of ubiquitous warnings is central to the criticisms of Prop 65.

The bottom lineExplain▾

Safe-harbor levels — the NSRL for cancer and the MADL for reproductive harm — are Prop 65's built-in acknowledgment that dose makes the poison. They are set with large safety margins. Understanding them helps explain both what a warning is supposed to mean and why, in practice, the warnings are so common.