Exposures to some substances are associated with high cancer risks, while exposures to other substances carry very little risk: exposure to a chemical agent alone is not a guarantee of cancer. The process that determines the relationship between exposure to a substance and the likelihood of developing disease from that exposureis known as risk assessment. It is a critical tool public health agencies use in deciding whether to reduce or eliminate exposure to certain substances.

Some exposures cause cancer only among susceptible individuals. Factors such as age, sex, general health, state of the immune system, smoking history, diet, childhood exposures, and patterns of genetic alterations may play a role in susceptibility. Unless a person has one or more factors that allow the chemical to be changed in the body to a more hazardous form, a chemical may be harmless. At its basic level, risk assessment involves understanding the interactions of three principal susceptibility factors:

• Potency,
• Type of exposure, and
• Dose response.

Potency measures the capacity of a given amount of a substance to cause cancer. In some cases, exposures to small amounts are sufficient—for others much higher exposures are needed. The solvent benzene is a potent carcinogen that from small amounts in the air increases the risk for leukemia. DDT and chloroform require higher exposures to increase the cancer risk by the same amount.

Using evidence of cancer from at least one type of exposure, public health agencies classify substances as known or suspected human carcinogens, Such exposures include

• Either high short-term or long-term workplace exposures,
• Continuous low-level exposure or occasional exposure to carcinogens in air, food, water, drugs, or consumer products, and
• Single, acute exposures following industrial accidents or similar incidents.

Whether the cancer risk increases as the exposure levels increase is important information. This is known as a dose-response trend. Some dose-response trends are linear, which is often considered strong evidence for cancer risk. For example, if 10 units of a substance cause cancer in 1 out of 1,000 people, then 1 unit of exposure would cause cancer in 1 out of 10,000 people. In a linear dose response, the risk would continue to decrease as the exposure decreased all the way to zero. This means that a minuscule risk of cancer is predicted for any exposure, no matter how small.

But for some carcinogens, an exposure level may so low that below it, no detectable increase in risk can be determined. This is sometimes referred to as threshold dose response.

The risk level considered acceptable by regulatory agencies for a linear dose response ranges from 1 cancer in every million people exposed to 1 in every 1,000 people exposed. Acceptable risks are generally higher for exposure in the workplace than in the general environment: allowable air levels of benzene in the workplace are approximately 40 times higher than those allowed in the general environment.

Key Points

• Knowing that exposure to a chemical agent alone is not a guarantee of cancer is important.
• Risk assessment is the determination of the relationship between exposure to a substance and the likelihood of developing disease from that exposure.
• Three factors are important to consider in risk assessment:
  • Potency
    • Potency is a measure of the capacity of a given amount of the substance to cause cancer.
  • Type of exposure
    • Either high short-term or long-term workplace exposures.
    • Continuous low-level exposure or occasional exposure to carcinogens in the environment.
    • Single, acute exposures following industrial accidents or similar incidents.
  • Dose Response
    • In a linear dose response, the risk would continue to decrease as the exposure decreased all the way to zero.
    • In a threshold dose response, an exposure level may be determined that below which no detectable increase in risk is found.
• The risk level considered acceptable by regulatory agencies for a linear dose response ranges from 1 cancer in every million people exposed to 1 in every 1,000 persons exposed. Acceptable risks are generally higher for exposure in the workplace than in the general environment.