The National Toxicology Program and the Report on Carcinogens

Most people meet the National Toxicology Program sideways. A news story says a substance has been "added to the government's list of carcinogens," and the list in question is theirs.

NTP was founded in 1978. Its stated role is to provide the scientific basis for programs, activities, and policies that promote health or prevent disease. It generates, interprets, and shares toxicological information about potentially hazardous substances in the environment, and it says it has evaluated almost 3,000 environmental substances for potential human health effects.

Hold on to the phrase "scientific basis." NTP produces evidence for others to act on. It does not write rules, set legal exposure limits, or take products off shelves.

The Report on Carcinogens, often shortened to RoC, is the program's most visible product. It is a congressionally mandated, science-based public health document that NTP prepares on behalf of the Secretary of Health and Human Services.

It is cumulative. Each edition carries forward what came before, so the report is a running list rather than a fresh snapshot. The 15th Report on Carcinogens was released on December 21, 2021, and the report now includes 256 listings.

Those listings are broader than chemicals alone. They cover chemical, physical, and biological agents, mixtures, and exposure circumstances. The three new listings in the 15th edition were antimony trioxide, haloacetic acids found as water disinfection by-products, and Helicobacter pylori.

The report uses only two listing categories, and the difference between them is the strength of the evidence.

Known to be a human carcinogen means there is sufficient evidence of carcinogenicity from studies in humans, indicating a causal relationship between the exposure and human cancer.

Reasonably anticipated to be a human carcinogen can be reached three ways. There may be limited evidence from human studies, where a causal interpretation is credible but chance, bias, or confounding could not adequately be ruled out. Or there may be sufficient evidence from studies in experimental animals, meaning increased malignant tumors in multiple species, at multiple tissue sites, by multiple routes of exposure, or to an unusual degree. Or the evidence may be less than sufficient in both humans and animals, while the substance belongs to a well-defined, structurally related class whose members are already listed, or there is convincing information that it acts through mechanisms likely to cause cancer in humans.

Those conclusions rest on scientific judgment considering all relevant information: dose response, route of exposure, chemical structure, metabolism, sensitive subpopulations, genetic effects, and mechanism. The criteria also say the reverse can happen. If animal evidence exists but compelling data show the mechanism does not operate in humans, the substance would not reasonably be anticipated to cause cancer in humans.

The process has four parts. Substances are selected for evaluation. Draft RoC monographs are prepared for them. Those monographs are peer reviewed and finalized. Then the report is released.

RoC monographs use systematic review methods that integrate evidence across many different types of studies, including human epidemiology, animal studies, and mechanistic research. The multi-step process includes several opportunities for scientific and public input.

This is slow work, and the slowness is the point. A listing is meant to survive scrutiny.

It can give you a source you can name. If you want to know whether a substance is formally recognized as a cancer hazard in the United States, the report is the document to check, and it is public.

It can give you detail rather than a headline. Each listed substance has a profile describing the evidence behind it, and there is a data exploration dashboard offering a visual breakdown of listed substances and the cancers associated with them.

It can help you evaluate a scary claim. If something is not in the report, that is worth knowing. If it is, you can read why.

And it can support other people's decisions on your behalf. NTP's work has been examined for its influence on public health policy, which is exactly the chain it is designed to feed.

It cannot tell you what caused your cancer. The report identifies hazards across populations. It cannot trace one person's diagnosis back to one exposure.

It cannot tell you how much exposure is too much. Hazard identification asks whether something can cause cancer, not at what dose in what setting. Exposure limits come from regulators, not from this report.

It cannot ban or restrict anything. NTP has no regulatory authority.

It cannot promise a substance not listed is safe. A substance may simply not have been evaluated yet.

And it cannot advise you medically. There is no clinical service here, and nothing in the report tells your oncologist what to do.

Many people find the report while trying to answer a question that has no answer: why me. That search can eat a great deal of energy at a time when you have little to spare.

If a past exposure feels important, the useful step is a small one. Write down what it was, when, and for how long, and mention it to your care team. Then let the document be what it is, a record of what science has concluded about substances in general, and not a verdict about you.