The short answer
Prop 65 sets 'safe harbor' levels below which no warning is needed. For cancer, the No Significant Risk Level (NSRL) is the exposure calculated to cause no more than one extra cancer per 100,000 people over a lifetime. These levels add a risk-based layer to a hazard-based law.
'Safe harbor' levels define exposures below which no warning is required.
The cancer level is the No Significant Risk Level (NSRL).
The NSRL is set at no more than 1 extra cancer per 100,000 people over 70 years.
For reproductive harm, the level is the Maximum Allowable Dose Level (MADL).
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The full explanation.
The risk-based part of a hazard-based law
Proposition 65 lists chemicals based on hazard. That is, whether they can cause cancer or reproductive harm. But the law also knows that dose matters. It shows this through "safe harbor" levels. These are exposure limits. Below them, a business does not have to give a warning.
These levels are where Prop 65 quietly borrows the logic of risk assessment. A tiny exposure to a hazard may not warrant a warning at all.
The NSRL: the cancer threshold
For carcinogens, the safe-harbor level is the No Significant Risk Level (NSRL).
OEHHA sets the NSRL as the daily amount figured to cause no more than one extra case of cancer per 100,000 people. The count runs over a 70-year lifetime. Put another way: if 100,000 people took in that amount every day for 70 years, the model predicts at most one extra cancer.
This is a very cautious standard. It assumes long, lifetime exposure. It errs on the safe side. If a product stays below the NSRL, no cancer warning is required.
The MADL: the reproductive-harm threshold
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 appear
In principle, safe-harbor levels mean warnings should appear only when exposure passes a protective threshold. That would make a warning more meaningful. It would signal exposure above the very cautious safe level.
In practice, warnings appear far more widely than that logic implies. There are two reasons.
- For many chemicals, no official safe-harbor level has been set. Businesses cannot easily show they are below it.
- Businesses often warn defensively. It is legally safer to include a warning than to risk a lawsuit arguing one was needed.
That gap between the protective intent of the NSRL and the reality of warnings everywhere is central to the criticisms of Prop 65.
The bottom line
Safe-harbor levels are Prop 65's built-in acknowledgment that dose makes the poison. The NSRL covers cancer and the MADL covers reproductive harm. Both are set with large safety margins. Understanding them helps explain what a warning is supposed to mean, and why the warnings are so common in practice.
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Words to know
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Common questions
What is a safe-harbor level?
An exposure level set by OEHHA below which a business does not need to provide a Prop 65 warning. If exposure stays under this level, the law considers it low enough not to require a warning.
What is the NSRL?
The No Significant Risk Level, used for carcinogens. It is the exposure calculated to result in no more than one extra case of cancer per 100,000 people exposed over a 70-year lifetime — a very conservative, protective standard.
What is the MADL?
The Maximum Allowable Dose Level, used for reproductive toxicants. It is set at 1/1,000th of the level shown to have no observable effect, another highly conservative margin.
Why do these levels matter?
They add a risk-based layer to a hazard-based law. In theory, warnings should appear only when exposure exceeds these protective thresholds — though businesses often warn anyway to avoid legal risk.
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Last updated: 2026-08-11Next planned review: 2028-07-05
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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