The content below is adapted from A Small Dose of Toxicology by Steven G. Gilbert.

Overview


Persistent bioaccumulative toxicants (PBTs), also known as persistent environmental contaminants, persistent pollutants, or persistent toxic chemicals, are resistant to environmental degradation through chemical, biological, and photolytic processes. They have been observed to persist in the environment, be capable of long-range transport, bioaccumulate in human and animal tissue, biomagnify in food chains, and have potential significant impacts on human health and the environment.

PBT Chemicals


Persistent Environmental Contaminants: Quick Facts

Terminology: has various names depending on agency, e.g. US EPA: Persistent Bioaccumulative and Toxic (PBT) or United Nations: Persistent Organic Pollutant (POP)

Uses: varies, often restricted or banned (but still present in the environment)

Source: industry, waste sites, food chain, and environment

Recommended daily intake: none (not essential)

Absorption: varies

Sensitive individuals: fetus, children, elderly, all species accumulate PBTs

Toxicity/symptoms: range of toxic effects: developmental, learning and memory, cancer, etc.

Regulatory facts: various local, national, and international agencies working to eliminate or greatly reduce

General facts: long history of use, bioaccumulates

Environmental concerns: global environmental contaminants

Recommendations: avoid, work towards phaseouts

Introduction and History


During the 1950s and 1960s there was an enormous increase in the use of chemicals in agriculture, industrial manufacturing, and around the home. DDT was used to remove lice and control mosquitoes, and other pesticides were used to kill insects and control weeds in an effort to improve crop yields. Lead was added to gasoline to make cars run better and added to house paint to make it last longer. It was also combined with arsenic to spray on fruit trees to control pests. Pulp and paper mills used mercury to control fungi and molds to ensure that paper remained white. Seeds were coated with mercury to stop soil fungi. Thermometers, thermostats, and switches brought mercury into homes and schools; many remember playing with a small silver ball of liquid mercury. Expansion of the electrical power system required chemicals that could withstand heat. For this purpose PCBs seemed to be the answer. All these chemicals appeared to be safe. A small dose did not seem harmful.

During the 1970s we began to appreciate that even a small dose can harm sensitive individuals. In Silent Spring, Rachel Carson sounded one of the first alarms about the effects of environmental contaminants. Evidence accumulated that a pesticide like DDT can cause very unexpected effects. The first and most obvious was the thinning of birds' eggshells, which caused a sharp decline in predator bird populations. Predatory birds are at the top of the food chain, where they accumulate and concentrate DDT. We then became aware of the potential of low-level exposures to persistent chemicals to cause diseases like cancer that appear only after many years. Humans, being at the top of the food chain, accumulate DDT in fat. Fat is mobilized during lactation, and mothers who breastfeed pass along the DDT to their infants, who receive a large dose because of their low weight. We also learned that mercury and lead cause developmental effects, harming the developing nervous system for a lifetime. (Also see Biomonitoring.)

It turns out that most of these chemicals have similar characteristics that contribute to their toxicity to both humans and other species. First, the substances are environmentally persistent. Many of the early pesticides, and certainly the metals, do not break down in the environment, or do so only very slowly. If persistent chemicals are released continually to the environment, the levels tend to rise ever higher. Second, the early pesticides were toxic to many species, not just the target species, and often killed beneficial insects or plants. Third, many of these compounds bioaccumulate or concentrate in species as they move up the food chain. The chlorinated pesticides accumulate in the fat of animals, with animals higher in the food chain accumulating more and more of these pesticides. Most species cannot metabolize or break down the compounds: lead accumulates in bone and methylmercury in muscle. And finally, because of their persistence in the environment and accumulation in various species, the persistent toxicants spread around the world, even to places that never used them. Animals at the top of the food chain, such as polar bears and beluga whales, routinely have fat PCB levels greater than 6 ppm even though these animals live far from where PCBs were used or produced.

To address the public and environmental health concerns caused by these and other compounds, government agencies have initiated various programs and regulations to control or restrict the use of the offending substances. Laws were passed to ensure more rigorous testing of compounds before widespread use, although this was not entirely effective. For example, the US Toxic Substances Control Act (TSCA) was passed in 1976 but has been largely ineffective for chemical management. The US Food Quality Protection Act (FQPA) of 1996 was more effective in implementing pesticide testing requirements. Researchers worked to develop new pesticides and other agents that were more specific in their toxicity and much less persistent. The use of many of the persistent chemical pesticides was restricted or even banned in some places. Individual countries are responsible for regulations, so there are some countries that still use pesticides that are banned elsewhere (see Pesticide Use in Developing Nations). Two global treaties that work to address PBTs are the Stockholm Convention on Persistent Organic Pollutants and the Rotterdam Convention on the Prior Informed Consent Procedure for Certain Hazardous Chemicals and Pesticides in International Trade (PIC).

Lists of persistent chemical pollutants are created to help prioritize efforts to reduce exposure. There are many lists, and even lists of lists, that are often revised as new data become available. The United Nations Environment Programme (UNEP) created a list called Persistent Organic Pollutants (POPs) that focuses on "chemical substances that persist in the environment, bioaccumulate through the food web, and pose a risk of causing adverse effects to human health and the environment." The UNEP also created a list of Persistent Toxic Substances. The US EPA created a list of agents called Persistent Bioaccumulative and Toxic (PBT). Both lists included organic chemicals and metals.

Regional groups are also beginning to create lists of persistent chemical pollutants to emphasize and prioritize local issues. For example, Washington State Department of Ecology, in the United States, has created a list of Persistent, Bioaccumulative Toxins (PBTs), with 27 chemicals to be phased out in the state. It is instructive to look at the overlap of these lists. View our table comparing the lists of persistent chemical pollutants from these agencies. Overall, there is considerable agreement on what chemicals are considered a priority. It is also obvious that pesticides are a major class of persistent chemicals, as are flame retardants.

Health Effects


The table below provides a very brief description of the chemicals and their associated toxicity. Profiles of many of the chemicals are available on Toxipedia (click links). For a general description of organochlorine pesicides, see Biological Properties of Insecticides.

PBT Chemicals, Use, and Toxicity

Chemical

Description

Aldrin/Dieldrin

Pesticide; organochlorine; bioaccumulates; used to control mosquitoes and termites; importation and manufacture prohibited in the US in 1987

Benzo(a)pyrene

Polycyclic aromatic hydrocarbon (see below)

Cadmium

Metal; naturally occurring; used in steel, plastics, batteries; in cigarette smoke; lung carcinogen

Chlordane

Pesticide; organochlorine; bioaccumulates; used to control mosquitoes and termites; importation and manufacture prohibited in the US; use banned in 1988

DDT, DDD, DDE

Pesticide (DDT); breakdown product (DDD, DDE); organochlorine; bioaccumulates; used to control mosquitoes; importation and manufacture prohibited in the US in 1972; affects wildlife; found in breast milk and fat

Dicofol

Pesticide; organochlorine; bioaccumulates; insecticide on fruits; analog of DDT; degrades but very toxic to aquatic wildlife including fish

Dioxins (TCDD) & Furans

Byproduct of combustion in municipal and medical waste incinerators; bioaccumulates; human carcinogen

Endrin

Pesticide; organochlorine; bioaccumulates; insecticide used on many crops; most uses canceled in 1980

Endosulfan

Pesticide; organochlorine; bioaccumulates; currently used as an insecticide; US EPA ban to take effect by 2016, efforts to ban globally in progress

Heptachlor epoxide

Pesticide; organochlorine; bioaccumulates; breakdown product of heptachlor, an insecticide used from 1953 to 1974 in the US on a wide range of insects; most uses canceled in 1974; importation and manufacture prohibited in the US; all uses banned in 1988

Hexachlorobenzene

Pesticide; organochlorine; bioaccumulates; fungicide used on seeds; most uses ended in 1965 but is a byproduct of solvent manufacture

Lead

Metal; widely distributed in environment when used as a gasoline additive and in paint; now banned from use in gasoline and paint; potent child neurotoxicant

Lindane

Pesticide; organochlorine; bioaccumulates; insecticide widely used prior to 1983; regulated as drinking water contaminant by US EPA

Mercury (includes organic and inorganic)

Metal; persistent; bioaccumulates; contaminates many species of fish; widely used in industrial processes; causes developmental neurotoxicity; children most susceptible

Methoxychlor

Pesticide; organochlorine; bioaccumulates; used as a replacement for DDT; in the US, 3.7 million pounds manufactured in 1978; use has declined significantly; regulated as a water contaminant

Mirex

Pesticide; organochlorine; bioaccumulates; extensively used in US from 1962-1978 to control fire ants; all uses canceled in US in 1978

Octachlorostyrene

Byproduct of electrolytic production of magnesium; listed by US EPA as persistent and bioaccumulative

Pendimethalin

Pesticide; herbicide used to control grasses and broadleaf weeds in cropfields and turf

Pentabromo diphenyl ether (see PBDEs)

Formerly used as flame retardant

Pentachlorobenzene

Pesticide; fungicide used for treatment of seeds and soil

Polybrominated hydrocarbons

Used in the manufacture of plastic products; bioaccumulate; highly persistent in the environment

Polychlorinated biphenyls (PCBs)

Heat and fire resistant; extensively used from 1929 to 1977 in electrical transformers; all manufacture banned; extensively regulated; very widespread global contaminant

Polycyclic aromatic hydrocarbons (PAHs)

Combustion byproducts; class of 100 chemicals; some of the first known carcinogens

Tin (organotins)

Used in a number of consumer products including paint; bioaccumulates and persistent; affects nervous system

Toxaphene

Pesticide; organochlorine; bioaccumulates; extensively used on US cotton crops from 1947 to 1980; manufacture and use prohibited in the US

Trifluralin

Pesticide; herbicide used to prevent emergence of weeds in cropfields and landscapes

1,2,4,5-tetrachlorobenzene

Pesticide; insecticide; intermediate in herbicide production; related to dioxin (TCDD)

Reducing Exposure


Exposure depends on various factors, such as location, diet, housing, occupation, and socioeconomic issues. For example, methylmercury bioaccumulates in certain fish and is particularly toxic to the developing fetus. Many government agencies advise that children and women of childbearing age reduce their consumption of certain species of fish known to bioaccumulate methylmercury, but this may be difficult for those dependent on high-fish diets. Reducing exposure to persistent chemical pollutants is difficult because they are so pervasive and continue to build up over time. While individuals can sometimes reduce exposure to particular PBTs, such as mercury, by regulating their diet (guidelines available from various sources including Pollution in People), in general government agencies have found that the most effective way of reducing exposure is by phasing out the uses of the products or processes that create these chemicals.

Many of the chemicals identified as persistent chemical pollutants are pesticides. Integrated Pest Management (IPM, see definition below) is an approach to pest control that can significantly reduce pesticide use while still providing adequate or even improved results. IPM programs are used in agriculture, landscaping, and indoor pest control. Typically, IPM programs maximize prevention of pest problems through non-chemical methods, and chemicals, when used, are selected for minimum risk to non-target species. Many institutions, such as schools, are adopting IPM protocols for pest management.

Relevant Pages on Toxipedia


Teaching Resources


 

External Links


European, Asian, and International Agencies

• European Commission. Environment. Website deals with the registration, evaluation, authorization and restriction of chemical substances. [accessed May 11, 2009]

• United Nations Environment Programme (UNEP). Persistent Organic Pollutants. Information on international efforts to reduce persistent pollutants. [accessed May 11, 2009]

Stockholm Convention on Persistent Organic Pollutants. "The Stockholm Convention on Persistent Organic Pollutants is a global treaty to protect human health and the environment from chemicals that remain intact in the environment for long periods, become widely distributed geographically, and accumulate in the fatty tissue of humans and wildlife." [accessed May 11, 2009]

North American Agencies

• Health Canada. Chemical Substances Online. Health Canada provides information on the health effects and environmental distribution of chemical substances in Canada. [accessed May 11, 2009]

• US Centers for Disease Control and Prevention (CDC). National Biomonitoring Program. [accessed May 9, 2009]

• US Environmental Protection Agency (EPA). Persistent Bioaccumulative and Toxic (PBT) Chemical Program. Information on the efforts of EPA to reduce PBT chemicals. [accessed May 9, 2009]

US Geological Survey. This site contains information and maps on the use of pesticides across the US both as contaminants and crop use. [accessed May 11, 2009]

• Washington State Department of Ecology. Persistent, Bioaccumulative Toxics Initiative. Information on Washington's approach to persistent, bioaccumulative toxins, and includes several chemical action plans. [accessed May 11, 2009]

• US Department of Agriculture National Institute of Food and Agriculture. Integrated Pest Management. Site provides information and other links on IPM. [accessed May 11, 2009]

Non-Government Organizations

• Environmental Health Research Foundation (EHRF). Biomonitoring Info. "A resource for policymakers, scientists, educators, workers, journalists and the public on the nature and promise of biomonitoring." [accessed May 11, 2009]

• International POPs Elimination Network. "The International POPs Elimination Network (IPEN) is a global network of NGOs dedicated to the common aim of eliminating persistent organic pollutants."

• Health Care Without Harm. "Health Care Without Harm is an international coalition of more than 470 organizations in 52 countries working to transform the health care sector so it is no longer a source of harm to people and the environment."

Pesticide Action Network UK. PAN UK works to eliminate the dangers of toxic pesticides, exposure to them, and their presence in the environment in Europe. [accessed May 11, 2009]

Pesticide Action Network North America (PANNA). "PANNA works to replace pesticide use with ecologically sound and socially just alternatives." [accessed May 11, 2009]

Washington Toxics Coalition (WTC). WTC provides information on model pesticide policies, alternatives to home pesticides, information on persistent chemical pollutants, and much more. [accessed May 11, 2009]

Beyond Pesticides. "Beyond Pesticides is a national network committed to pesticide safety and the adoption of alternative pest management strategies which reduce or eliminate a dependency on toxic chemicals." [accessed May 11, 2009]

Northwest Coalition for Alternatives to Pesticides (NCAP). "NCAP works to protect people and the environment by advancing healthy solutions to pest problems." [accessed May 11, 2009]

• University of California. Statewide Integrated Pest Management Program (UC IPM). "UC - IPM develops and promotes the use of integrated, ecologically sound pest management programs in California." [accessed May 12, 2009]

• Environmental Defense Fund. The Arctic at Risk: A Circumpolar Atlas of Environmental Concerns. Site has maps and information on contaminants in the arctic. [accessed May 12, 2009]

• IPMopedia. Integrated Pest Management information. IPMopedia offers free and up-to-date integrated pest management advice from green gardening experts. [accessed May 21, 2009]

References


Wargo, John. Our Children's Toxic Legacy: How Science and Law Fail to Protect Us from Pesticides 2nd edition. New Haven: Yale University Press, 1998.

Carson, Rachel. Silent Spring. Boston: Houghton Mifflin, 1994.

Atkin, J. and Klaus M. Leisinger (Editors). Safe and Effective Use of Crop Protection Products in Developing Countries. Wallingford: CABI Publishing, 2000.

Sexton, K., Needham, L., and J. Pirkle. "Human Biomonitoring of Environmental Chemicals". American Scientist Classics 92, 1 (2004): 38.

National Research Council. Human Biomonitoring for Environmental Chemicals. Washington, D.C.: National Academy Press, 2006.