Human carcinogens so far identified

Our plain-English summary of Human carcinogens so far identified, by L. Tomatis, A. Aitio, J. Wilbourn, et al (Japanese Journal of Cancer Research, 1989).

The study

• Authors: L. Tomatis, A. Aitio, J. Wilbourn, L. Shuker
• Journal: Japanese Journal of Cancer Research
• Published: 1989;80(9):795-807
• Type of article: Review
• Cited by: 110 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 003646).

Abstract

The massive exploitation of natural resources, of which tobacco and asbestos are two conspicuous, though very different examples, and the synthesis of industrial chemicals have generated new hazards and new carcinogens which have been added to older ones. The majority of the over 50 agents that have been firmly identified so far as being human carcinogens belong to the relatively new hazards, that is environmental chemicals or chemical mixtures to which humans have been exposed only during the last century and a half. They are of more importance for cancer occurring in men than in women, and there is no evidence so far that they are related to cancers occurring at some of the most common target sites in either sex. It would be mistaken to believe that complete cancer prevention could be achieved solely by controlling these new, or relatively new, carcinogenic agents, but it would be similarly wrong to deny the importance of trying to control them and of continuing to do so. The experimental approach for the identification of carcinogens has an irreplaceable role to play in preventing the dispersal into our environment of new hazards and in identifying among the chemicals already in use, those that are carcinogenic. That a closer integration between the epidemiological and the experimental approaches may succeed in substantially reducing the size of the unknown region within the spectrum of cancer-causing factors, is today's hope that awaits confirmation. At the same time, advances in the understanding of the mechanisms underlying the different steps of the process leading to the clinical manifestation of cancer may help in the uncovering of agents and risk factors that the approaches used, at least in the way they have been used until now, may not have been apt to identify.

In plain English

This article was a review by authors at the International Agency for Research on Cancer (IARC). It reported no new study of people. Instead it drew mostly on the IARC Monographs database (volumes 1 to 48) to list agents judged to cause cancer in humans and to compare human and animal evidence. The authors counted 57 exposures recognized as carcinogenic to humans, 53 of them chemicals or chemical mixtures. Nineteen were linked to lung cancer. Asbestos was listed among chemicals with mostly occupational exposure, with human target organs including lung, pleura and peritoneum (mesothelioma). Talc containing asbestiform fibres was also listed. The authors noted 36 adequate long-term animal studies on asbestos, involving 8900 rats and about 1000 mice. Of 33 human carcinogens tested in animals, 22 (67%) had sufficient animal evidence. Counting limited evidence as well raised the sensitivity of animal tests to 91%. The authors concluded that over 50 agents cause human cancer, that controlling them matters but would not prevent all cancer, and that animal tests still have an irreplaceable role. They noted that the lists favour cancers in men.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Review of agents evaluated as human carcinogens, based mostly on the IARC Monographs database; no new data were collected
• Who was studied: Agents (chemicals, mixtures, industrial processes, drugs, environmental and cultural factors) evaluated as carcinogenic to humans in volumes 1–48 of the IARC Monographs, plus some agents not yet evaluated there; the review compared human and animal evidence for them
• Exposure: Many agents; asbestos was listed among chemicals with mostly occupational exposure, with human target organs including lung, pleura and peritoneum (mesothelioma); talc containing asbestiform fibres and erionite were also listed
• Main results:
  • 53 of the 57 exposures recognized as carcinogenic to humans were environmental chemicals or complex chemical mixtures
  • Of the 57 factors causally associated with human cancer, 19 induce lung cancer, 12 bladder cancer, 12 leukemia/lymphoma and 10 skin cancer
  • For asbestos, Table II listed human target organs including lung, pleura, peritoneum (mesothelioma), gastrointestinal tract and larynx. Animal target sites were lung, pleura and peritoneum in rats, peritoneum in mice, and pleura and peritoneum in hamsters
  • According to the IARC Monographs database there were 36 adequate long-term animal carcinogenicity studies on asbestos, involving a total of 8900 rats and about 1000 mice. The authors stated that fewer studies would have sufficed and that most aimed at understanding the carcinogenic process
  • Of 33 human carcinogens assessable in animal tests, 22 (67%) had sufficient experimental evidence of carcinogenicity
  • Talc containing asbestiform fibers was one of three chemicals with inadequate evidence of carcinogenicity in animals, the others being ethanol and soots
  • Pooling sufficient and limited animal evidence, the sensitivity of long-term carcinogenicity studies to detect human carcinogens increased to 91%
  • Citing other work, the authors described documented low-level general-population asbestos exposure in Japan: asbestos imports began in the 1950s, and the frequency of asbestiform bodies in the lungs of the general population rose sharply soon after
• The authors' conclusion: The authors concluded that over 50 agents have a demonstrated causal relationship with human cancer, most of them environmental chemicals to which humans have been exposed for no more than about 150 years. In their view, controlling these agents is important but would not by itself achieve complete cancer prevention, and long-term carcinogenicity tests still have an irreplaceable role in identifying carcinogens
• Limitations the authors state:
  • The lists of recognized carcinogens are imbalanced toward cancers in men, and many common cancer sites (cervix, breast, colon-rectum, stomach, ovary, brain, prostate) are largely absent
  • Because IARC Group 1 requires sufficient human evidence, the availability of epidemiological data may bias the list of recognized human carcinogens
  • The specificity of animal tests is difficult to assess because non-carcinogenicity is very difficult to prove

Topics

Medical terms in this study, explained in our Lexicon: Carcinogenic.

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