Human Health and Toxicology
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.
- U.S. EPA Office of Pesticide Programs: List of Chemicals Evaluated for Carcinogenic Potential (available via e-mail or mail).
- National Toxicology Program: Eleventh Report on Carcinogens.
- International Agency for Research on Cancer: IARC Monographs on the Evaluation of Carcinogen Risks to Humans.
- California Proposition 65 List of Chemicals Known to the State to Cause Cancer or Reproductive Toxicity.
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:
||Strength of evidence
|benzene||gasoline, cigarette smoke
|formaldehyde||some manufactured wood products (e.g. particleboard and plywood)
||progressive hair dyes
|PFOA||ingredient used in making of non-stick cookware, stain- and grease-repellant coatings
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.
The most widely known and easily accessed list of chemicals that cause birth defects is the so-called “Proposition 65” list maintained by the State of California and named after the voter initiative that mandated its creation. This list includes chemicals that the state considers carcinogens, as well as those that cause male or female reproductive effects and developmental effects (birth defects). The most recent Proposition 65 list can be found here. It is important to know that most substances have NOT been tested.
Examples of some chemicals that can cause reproductive or developmental effects include beverage alcohol, phthalates (DEHP, DBP), mercury, lead, and some pesticides.
Every exposure to a reproductive or developmental toxicant does not cause health problems. It depends on both the amount and timing of exposure, as well as each individual’s own personal sensitivity to the substance. People contemplating having a child, as well as pregnant women, should try to avoid contact with toxic chemicals in general, especially those thought to cause reproductive or developmental effects.
The realization that many synthetic chemicals (and some naturally occurring ones) can interfere with proper functioning of the hormone system is fairly recent. Based on a considerable body of research on animals and some human data, a group of eminent scientists issued a consensus statement in 1992 laying out broad concerns that endocrine disruption is real and potentially dangerous. The general public became aware of the subject as a result of the book Our Stolen Future, written by Dr. Theo Colborn, Dr. J. Peterson Myers, and Diane Dumanowski. Although widely attacked by the chemical industry, this book and the research it cites were instrumental in mandating that all pesticides be tested for their potential to affect the endocrine system. Unfortunately, as of early 2007, that testing has not begun. Colborn’s book cited a list of endocrine disrupting substances, and other lists have been compiled, but there is currently no ongoing, systematic program to identify and publish a comprehensive, authoritative list of such chemicals.
There is general agreement that some chemicals are endocrine disruptors and less certainty about others. Chemicals known to be endocrine disruptors include atrazine (herbicide), DDT (insecticide), dioxins, lindane (insecticide), nonylphenol (detergent breakdown product), PCBs, and bisphenol A (ingredient in polycarbonate plastic).
Several groups of chemicals affect the nervous system and can cause nervous system symptoms or even damage if exposures are high enough. Many are pesticides. These include organophosphate insecticides such as diazinon, chlorpyrifos, and malathion; carbamate insecticides such as carbaryl and bendiocarb; and pyrethroid insecticides such as permethrin, resmethrin, and cyfluthrin. Other pesticides that target the nervous system include organochlorine insecticides such as lindane, the herbicide 2,4-D, and the slug/snail killer metaldehyde. Solvents such as hexane, toluene, benzene, xylene, and alcohols cause nervous system affects as well. There are also metals, such as lead and mercury, that can have devastating neurological effects, not just on the nerves, but on the brain as well, even at very low levels of exposure.
It’s true that the amount of chemical we are exposed to is important in determining the risk of harm, but it’s an oversimplification and incorrect to conclude that small exposures are always safe. Other factors affecting risk are timing of the exposure and the sensitivity of the exposed individual. A small exposure of a mother to certain neurotoxicants or developmental toxicants during pregnancy can cause harm to the offspring without obvious affects to the mother at all. While it is often, perhaps usually, true that risk decreases with lower exposure, there are some known examples where, at least at some dose ranges, risk increases as exposure decreases. This may seem strange, but scientists have proposed logical explanations. Another reason to be cautious about even small doses of toxic chemicals is that, unlike laboratory animals, we are exposed to a host of different chemicals all the time. Sometimes there are interactions between different chemicals in our bodies, increasing the risk beyond what laboratory tests would predict.
Take lead, for example. Scientists have known for years that lead is toxic, but only more recently has it become clear that there is no safe dose of lead. Even small amounts of lead interfere with a child’s ability to learn, and lead provides no benefits to the body at any dose. The amount of lead considered “acceptable” in a child’s blood has decreased steadily from 40 micrograms per deciliter (ug/dl) in 1973 to only 10 today. With each 10 ug/dl increase in blood lead, a child’s IQ will drop by 2 to 4 points. Even at 10 ug/dl, studies have now shown adverse effects, and some scientists have argued to lower the “acceptable” level to 2 ug/dl. With time, even this number will probably go lower.
You may have heard the phrase “the dose makes the poison.” This is a simplified version of the first statement of the dose/response principle by the father of modern toxicology in the 16th century. Known by the pseudonym Paracelsus*, Phillippus Aureolus recognized that substances can be toxic at one dose and beneficial at another, such as many medicines. He failed to take account, however, of the unique sensitivity of the individual, which can make a substance toxic to one person but not another at the same dose. He also did not have the benefit of modern research on how chemicals affect both humans and other species. While Paracelsus’ famous statement was a huge step forward at the time and remains useful today in its broadest sense, it is an oversimplification that has unfortunately been used by many people in a misleading way to argue that only large doses of toxic chemicals are ever dangerous.
* He also called himself Theophrastus Bombastus von Hohenheim, an invented name.
Children have elevated risks because they are generally more sensitive than adults and have greater exposures because of their behaviors. Children are not little adults – they breathe and eat more than adults on a body-weight basis, which increases their exposure. Children absorb more lead than adults because lead substitutes for calcium and their growing bodies need calcium. Other high risk groups include pregnant women, the elderly, the chronically ill, and those who have been sensitized by previous exposures. In addition, people in certain professions are more highly exposed to chemicals used in those workplaces. Risks are also increased by geographical pollution patterns such as industrial facilities, waste sites, and transportation corridors.
If you read the newspapers or watch TV, you may get the impression that everything does cause cancer or some dire health problem. That is certainly not the case, yet the truth is still sobering. Very few of the approximately 80,000 chemicals is use today have been tested to see if they cause cancer or other health effects. Most product labels do not list ingredients, and most ingredients are kept confidential by manufacturers. At the same time, human testing has shown that potentially harmful levels of some chemicals are commonly found in our bodies, sometimes at or near levels that cause harm in test animals. Add to this the fact that some diseases that can be caused or aggravated by chemicals, such as asthma, certain cancers, and autism, are rapidly increasing in frequency.
Don’t despair! Harness your concern to make changes in your home and in the way our government regulates chemicals. See our FastFacts below, How can I have a safe home?
It’s harder than you may think, and for many products you won’t be able to find out all, or perhaps any, of the ingredients. The good news is that the Internet makes it easier to find out about products now that in the past; the bad news is that many ingredients are trade secrets.
The first place to look is the product label. If you don’t have one, try searching on the Internet. If your product is a pesticide (including disinfectants or wood preservatives), the label will list the active ingredients but everything else will be lumped together as “inert” or “other” ingredients. Next try to get a Material Safety Data Sheet, again by searching the Internet. Try searching with the product name and MSDS as search terms. The MSDS may list other ingredients in the product that are considered hazardous, but don’t be too surprised if you come up blank.
If your product is a food, drug, or cosmetic (including personal care products), most ingredients may be on the label, although some ingredients such as those in fragrances do not have to be disclosed.
Other kinds of products are regulated rather loosely, and you are likely to find little or no information on labels for paints, cleaning products, etc. Again, try to find an MSDS online. This is about as far as you can go. If you are frustrated by the lack of information, get in touch with the manufacturer.
The nation’s poison centers exist to provide emergency first-aid advice when a person appears to have been exposed to a substance that may be toxic. The vast majority of calls to poison centers come as a result of exposures in the home to everyday products such as cleaners, pharmaceuticals, pesticides, and even poisonous mushrooms or animals. The poison centers have access via computers to the ingredients lists of products, through agreements with manufacturers. They can give product-specific advice and they can release ingredient information to a treating physician, but they cannot generally give out ingredient information to callers. In other words, you cannot just call up the poison center and run down a list of the products in your home, asking for ingredient lists. If the ingredient is not listed on the product label, it may well be considered confidential.
The Pesticide Data Program of the U.S. Department of Agriculture gathers data and publishes annual reports on pesticide residues in food. The reports are available here. These reports contain the information you want, but they are lengthy documents and difficult to read. The Environmental Working Group has analyzed the data and come up with a simplified guide to the most and least polluted fruits and vegetables, available here.
You can do much to make your home safer by reducing toxic chemicals in the air, the dust, and the products you buy. For example:
• Install high-quality door mats at every entrance.
• Institute a shoes-off policy in the home.
• Open windows when safe and practical to allow less-polluted outdoor air to enter.
• Make sure all combustion appliances (stoves, furnaces, fireplaces) are working well and are properly vented.
• Clean regularly and thoroughly with least-toxic cleaning products.
• Reduce or eliminate pesticide use.
• Avoid products made of vinyl (PVC) plastic.
• Buy organically grown food.
• Use the fan in your bathroom and clean up any mold.
• Wash bed linens regularly in hot water to control dust mites.
• For more information, see these tips from the Pollution in People website.
Unfortunately, some sources of toxic chemicals are difficult to control with these measures. Everyday products such as furniture, clothing, electronics, and even children’s and baby products can contain hazardous chemicals not listed on labels. The long-term solution is a change in how toxic chemicals are regulated.