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Why should two substances or agents classified in the same Group not be compared

Posted Date:2025/6/10

Why should two substances or agents classified in the same Group not be compared?

The classifications reflect the strength of the scientific evidence as to whether an agent can cause cancer in humans, but they do not reflect how high the risk of developing cancer is at a given exposure level. The types of exposures, the extent of risk, the people who may be at risk, and the cancer types linked with the agent can be very different across agents. Therefore, comparisons within a category can be misleading.

First, exposures may vary widely. For example, there is widespread exposure to the Group 1 agent air pollution, whereas far fewer people would be exposed to certain Group 1 chemicals, such as 1,2- dichloropropane. Second, the magnitude of risk associated with exposure to two agents may be very different. Active smoking carries a much higher risk of lung cancer than do second-hand smoke or air pollution, although all three are classified in Group 1. Third, the number of resulting cancers can be different; for example, tobacco smoking causes some common cancer types, whereas 1,2- dichloropropane causes a rare cancer of the bile duct. This also applies to Group 2 agents. For example, radiofrequency electromagnetic fields and the prescription drug digoxin are both classified in Group 2B.

In summary, because the Groups indicate the strength of the evidence regarding a cancer hazard and not the cancer risk at a given level of exposure, the cancer risk associated with two agents classified in the same Group may be very different.