Human Health and Toxicology

Reprinted with permission from Washington Toxics Coalition

Answers to questions about human health and toxicology:

When a chemical is toxic, it means that it can adversely affect your health. Because some level of exposure to any chemical can be harmful, we use the term “toxicity” to describe how much exposure is needed to cause health effects. The more toxic a substance is, the smaller the amount that can be harmful. (NOTE: this does NOT mean that small amounts of all substances are harmless! See the FastFacts topic below, Isn’t a small amount of any chemical safe?)

One common benchmark (acute oral toxicity) says that a substance is toxic if ingesting more than 5 milligrams for each kilogram of body mass can be fatal. (Different thresholds exist for inhalation or skin exposure.) However, nowadays it is increasingly realized that toxicity should not just consider death as an outcome. Substances that can cause cancer, birth defects, developmental effects, and hormone disruption are also considered toxic, and research shows that even tiny exposures can sometimes result in serious but subtle or delayed effects. For example, lead is harmful to children even in the smallest amounts.

When you see the words “non-toxic” on a product label, you should be wary. There is very little restriction or supervision of the use of such language on labels (except on pesticides), and marketing claims are sometimes misleading or inaccurate. Some companies say that a product is non-toxic if it meets the oral toxicity standard of 5 mg/kg mentioned above, but this is misleading if ingredients in the product are carcinogens or can cause other health effects from repeated or prolonged exposure.

Determining which chemicals can cause cancer is difficult, and most chemicals have never been tested. Because chemicals are tested on laboratory animals at high doses rather than on humans at realistic doses, the results often are difficult to interpret. Various government agencies publish lists of chemicals that have been tested, with conclusions on how strong the evidence is that each chemical could cause cancer in humans. Such lists characterize carcinogens with adjectives such as known, likely, probable, possible, etc. In addition to the strength of evidence, scientists also consider the potency of a carcinogen: how many cancer cases result from a given exposure level.

Quite a few chemicals used in household products are on government lists of carcinogens. Examples include formaldehyde (manufactured wood products such as plywood and particle board), methylene chloride (some paint strippers), paradichlorobenze (“para” mothballs), and carbaryl (insecticide). Other examples are shown in the table below. It is important to know that carcinogens are not necessarily listed on product labels. Products sold in California that contain carcinogens are labeled with warning statements, but the chemical itself is not identified.

Your risk from using a carcinogen depends on many factors and is difficult to estimate. With many carcinogens, there is no exposure level guaranteed to be safe: in other words, there is no exposure level with zero risk. The surest way to minimize risk is to seek an alternative and avoid exposure.

Examples of carcinogens in common products:

Compound name
Found in
Strength of evidence
arsenic CCA-treated lumber known1
benzene gasoline, cigarette smoke known1
captan fungicides probable2
carbaryl SevinTM insecticides likely2
p-dichlorobenzene mothballs reasonably anticipated3
formaldehyde some manufactured wood products (e.g. particleboard and plywood) reasonably anticipated3
lead acetate progressive hair dyes reasonably anticipated3
methylene chloride paint strippers reasonably anticipated3
permethrin insecticides likely2
PFOA ingredient used in making of non-stick cookware, stain- and grease-repellant coatings likely4

1 National Toxicology Program, Eleventh Report on Carcinogens.
2 U.S. EPA. Office of Pesticide Programs. List of Chemicals Evaluated for Carcinogenic Potential.
3 National Toxicology Program, Eleventh Report on Carcinogens.
4 U.S. EPA Science Advisory Board. Draft report June 27, 2005.

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