Cancer: how scientists determine whether a chemical is carcinogenic

Cancer-causing substances are called carcinogens. Familiar examples include tobacco smoke, radon, asbestos, and diesel engine exhaust.

To protect the public’s health, national and international health agencies evaluate many new and existing chemicals to determine whether they are likely to be carcinogens in a process called cancer risk identification.

If agencies consider chemicals to be carcinogenic, they conduct further assessments to determine the level of risk, and legislators can create regulations to limit, or completely prevent, the production and use of these chemicals.

I am a scientist who studies how the human body processes foreign chemicals, such as environmental pollutants and drugs, and the effects of these chemicals on health.

As part of my work, I have participated in chemical and cancer risk identification for several agencies, including the World Health Organization’s International Agency for Research on Cancer. Here’s how chemicals can cause cancer, and how we classify chemicals based on whether they are carcinogenic — sometimes with controversial results.

How do chemicals cause cancer?

The mechanisms behind how toxic chemicals can cause cancer are complex.

After a person is exposed to a carcinogen, the chemical is usually absorbed into the body and distributed to various tissues. Once a chemical moves into cells, it often undergoes chemical reactions that convert it into other forms.

The products of these reactions can directly or indirectly affect the genes of the cell. Altering genes, which contain instructions for the cell to produce specific molecules, or the processes that regulate them can eventually result in dysfunctional cells if the genetic damage is not repaired. These cells do not normally respond to cellular signals and may grow and divide at abnormal rates, which are characteristic of cancer cells.

How are chemicals classified for carcinogenesis?

To help protect the public and reduce the incidence of cancer, several agencies have developed procedures to classify and categorize chemicals based on their potential to be carcinogenic.

These include the International Agency for Research on Cancer, or IARC monograph; National Toxicology Program, or NTP; and the US Environmental Protection Agency, or EPA. In general, these agencies examine one important question: How strong is the evidence that a substance causes cancer or biological changes that might be related to cancer in people? Understanding the processes used to answer this question can help explain the decisions these agencies make.

The procedures used by IARC – because of its long history, credibility and strong international reputation – provide a good example of how this process works. It is designed to be transparent and reduce bias, taking more than a year from the selection of a chemical for evaluation to its final classification.

In this process, IARC selects and invites a panel of scientific experts on the chemical to be evaluated. The panel does not conduct new research on its own, but carefully reviews all available papers in the scientific literature on the carcinogenicity of the chemical in cell and bacterial cultures, animals, and people.

To assess the strength of the evidence, the panel carefully considers the number of studies available and the consistency of results, as well as the scientific quality and relevance of each study to cancer in people.

After discussing and deliberating on the results, the panel makes a final consensus classification. This classification places a chemical into one of four groups: Group 1 indicates that the chemical is carcinogenic to people, Group 2A that it is probably carcinogenic to people, Group 2B that it is possibly carcinogenic to people, and Group 3 that it is not classified.

A Group 3 classification does not indicate that the compound is not carcinogenic, but rather that the panel could not conclude about whether there is a causal relationship between the chemical and cancer from the available studies. For example, exposure to multiple chemicals may make it unclear which chemicals are responsible for a later diagnosis of cancer.

During its 50-year history, IARC has evaluated and classified more than 1,000 chemicals and other hazards. Many of these classifications have wider social implications, such as for tobacco smoke, ambient air pollution, diesel engine exhaust, and processed meat.

All were classified as Group 1, or confirmed to be carcinogenic to humans. Electromagnetic radiation emitted by mobile phones was classified as Group 2B, or possibly carcinogenic, and red meat was classified as Group 2A, or probably carcinogenic.

Although they do not lead directly to regulation, these classifications have inspired additional scientific studies. While IARC can advise regulators, it is up to countries to implement policies.

It is important to note that the classifications do not indicate the size of the risk but are important in supporting health agencies around the world as they take action to limit exposure to known, probable and probable carcinogens. In 2020, when the IARC classified opium consumption as Group 1, or carcinogenic to humans, it led the Iranian government to implement policies to reduce opium addiction in the country.

Controversy in Carcinogenicity Classification

Although IARC’s classifications are based on strong scientific evidence, some have proved controversial.

For example, in 2015, the IARC evaluated the carcinogenicity of glyphosate, a widely used weedkiller found in products such as Roundup, which is manufactured by Monsanto. A panel of 17 experts from 11 countries systematically reviewed results from more than 1,000 scientific studies and classified glyphosate as “probably carcinogenic to humans,” or Group 2A.

Because of its widespread use and multibillion-dollar market value, a cancer classification decision for glyphosate has significant potential financial and legal consequences. Following its evaluation, IARC received support from several regulatory and scientific bodies but was criticized by others. Other agencies, including the EPA, have seen similar controversies and the politicization of their identification of hazards and regulatory decisions.

I believe that agencies such as the IARC play an important role in evaluating the health effects of certain chemicals and reducing exposure to potential carcinogens. Helping the public better understand how these agencies evaluate chemicals can help ensure transparency and protect the environment and public health.

The text of this story is published from a wire agency feed without any modification.