Environmental factors are potential hazards in a person’s everyday surroundings that may cause changes in cell structure, which may eventually result in cancer. Environmental factors are responsible for up to two-thirds of a person’s cancer risk, according to the National Cancer Institute.

Unrepaired abnormalities in a cell’s DNA (genetic blueprint) can cause cancer. In some cases, these abnormalities are linked to characteristics inherited from parents. In other cases environmental factors are the source of the abnormality. This can occur after exposure to chemicals, radiation or infectious agents. It can also result from lifestyle choices, such as diet and use of alcohol and tobacco.

A substance that causes changes that lead to cancer is known as a carcinogen. In some cases, carcinogens directly damage DNA. In other cases, the effect is more indirect. For example, some carcinogens cause cells to divide more quickly. This increased number of cells also boosts the odds that a cell will appear with a DNA abnormality that might eventually lead to cancer. 

Environmental carcinogens do not cause cancer with every exposure. Some carcinogens are more likely to cause cancer than others, and the cancer risk after exposure to a carcinogen depends on several factors. These factors include the length and intensity of exposure, an individual’s genetic predisposition to cancer and other health related elements.

Before a substance is labeled as a carcinogen, it must undergo rigorous testing and analysis. Laboratory and animal testing helps determine whether or not the substance is likely to cause cancer in humans. There are far too many natural and manmade substances to test for all carcinogens. As a result, scientists select and test chemicals based on the following:

• Knowledge of the substance’s chemical structure
• Results of other lab tests
• Information about the extent of human exposure to the chemical

Carcinogens are also identified through a process known as epidemiologic studies. These investigations look at human populations and search for links between various factors and cancer. 

Scientists combine data from these various studies when deciding whether or not to classify a substance as a carcinogen. Several organizations have their own methods for classifying substances, but the most widely used system is maintained by the International Agency for Research on Cancer (IARC). This branch of the World Health Organization focuses on human cancer and the relationship to it of individuals and the environment. The IARC has identified around 900 substances and assigned them to one of the following categories:

• Group 1: Carcinogenic to humans
• Group 2A: Probably carcinogenic to humans
• Group 2B: Possibly carcinogenic to humans
• Group 3: Unclassifiable as to carcinogenicity in humans
• Group 4: Probably not carcinogenic to humans

The process of identifying carcinogens is not perfect. To ensure human safety, most potential carcinogens are tested in high dosages on animals. The information gleaned from these experiments is then extrapolated and applied to humans in establishing levels of the substance that are safe. Because animals and humans differ, and because of other mitigating factors, these standards are less precise than experts would like.

Epidemiologic studies have their shortcomings. It is very difficult to link a single substance to cancer because humans do not live in a controlled environment and are exposed to many different substances at the same time. In addition, cancer often does not manifest itself until years or decades have passed between the time a person was exposed to a carcinogen and the onset of cancer.

As a result, experts generally err on the side of safety and make conservative assumptions about what constitutes a safe level of human exposure to suspected or known carcinogens. Many federal regulating agencies define acceptable cancer safety risk as being met if the substance does not increase cancer risk by more than one case per million persons over a lifetime. These safety standards are incorporated into laws and procedures established by the Food and Drug Administration (FDA), the Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA). Such regulations govern chemical and radiation exposures in the United States.