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What factors do scientists consider in determining the risk associated with different cancer-causing substances?

Learning Objective

Upon completion of this section, you will be able to

  • Identify the factors scientists consider in determining the risk associated with various cancer-causing substances.

Introduction

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

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.

Type of Exposure

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.
Dose Response

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.

Acceptable Risk Levels

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.
Progress Check

Choose the best answer.

1. Public health agencies classify substances as known or suspected human carcinogens based on evidence of cancer from at least one type of exposure, EXCEPT FOR

A. Either high short- or long-term workplace exposures.
B. Continuous low-level exposure or occasional exposure to carcinogens in the environment.
C. Continuous low-level exposure or occasional exposure to carcinogens in the workplace.
D. Single, acute exposures following industrial accidents or similar incidents.

Answer:

To review relevant content, see Type of Exposure in this section.

2. Which of the following statements concerning acceptable risk levels by regulatory agencies is INCORRECT?

A. Acceptable risks are generally LOWER for exposure in the workplace than for exposure in the general environment.
B. Acceptable risks are generally HIGHER for exposure in the workplace than for exposure in the general environment.
C. The range of the risk level considered acceptable by regulatory agencies for a linear dose response starts at one cancer in every million persons exposed.
D. The risk level range considered acceptable by regulatory agencies for a linear dose response ends at one cancer in every 1,000 persons exposed.

Answer:

To review relevant content, see Acceptable Risk Levels in this section.

3. Which of the following statements concerning dose-response levels and risk of cancer is INCORRECT?

A. In a linear dose response, the risk decreases as the exposure decreases—all the way to zero.
B. In a linear dose response, a minuscule risk of cancer is predicted for any exposure, no matter how limited the exposure might be.
C. In a threshold-dose response, a minuscule risk of cancer is predicted for any exposure, no matter how limited the exposure might be.
D. A threshold dose response may include an exposure level below which an increase in risk is not detectable.

Answer:

To review relevant content, see Dose Response in this section.


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