In 1998 researchers in Sweden reported something alarming: concentrations of a class of compounds known as polybrominated diphenyl ethers (PBDEs) have been increasing exponentially in human breast milk. PBDE concentrations in breast milk have doubled every five years for the past 25 years, though their concentrations are still an order of magnitude lower than polychlorinated biphenyls (PCBs). PBDEs are widely used as fire retardants for plastics and textiles. PCBs are a now-banned toxin originally used in many electrical transformers.

PBDEs are so-called persistent organic pollutants (POPs), of concern to health officials worldwide because of their toxicity, tendency to accumulate in human and animal tissue, and their long lifetime in the environment. DDT and chlorinated dioxins are two better-known examples of POPs whose toxic properties have long been recognized. In contrast little is known about PBDEs, particularly in the U.S. Based on what little research has been done, PBDE may cause thyroid disease or neurological problems in children — but these links are far from proven. As of 2000, however, Denmark and Austria have imposed strict regulation on the substance’s use, and other European nations are considering doing likewise. “PBDE may be the PCB of the future,” write Kim Hooper and Thomas McDonald, researchers at the California Environmental Protection Agency.

Cancer, neurological development of children, thyroid disease — these are the types of health effects that make headlines. That puts PBDEs and similar chemicals on the top of any list of emerging contaminants: potentially toxic substances whose effects or presence are poorly known, often because these chemicals have only begun to enter the human water or food supply.

Keeping Up with the Joneses

One of the greatest challenges of keeping U.S. water supplies clean and safe to drink is that the mix of chemicals used by society is continually changing. New compounds, or their by-products, eventually find their way into the U.S. waste stream, and ultimately in some form they become part of what we drink. How do local and national agencies keep up with perpetual change? What emerging contaminants, unknown or virtually unnoticed just a few years ago, are just cropping up today?

Ever since the 1970s, U.S. clean water regulations have been attuned to mitigating the harmful effects of new chemicals. In fact, the Clean Water Act initially required the EPA to identify, monitor, and regulate 25 new toxins every three years, a process that led to an ever-lengthening list of contaminants subject to federal controls. In 1996 Congress modified this system by amending the Safe Drinking Water Act to require a more rational approach to emerging contaminants. In addition to requiring a cost-benefit approach to new contaminant regulation, the amendments focused on developing a prioritized list of new contaminants based on scientific studies of health effects. The result has been new research on emerging contaminants and an increased emphasis on methods of analyzing health effects of contaminants.

One area in which several advances have recently been made is related to long-term health effects of chemical exposure. Humans are constantly exposed to a variety of contaminants present at low levels. These include both new chemicals, with previously unknown effects and those with well-known acute (short-term exposure) human and ecological health effects. Arsenic and radionuclides are two examples where recent long-term health studies have led to increased concern about low-level exposure. As a consequence, regulators are now looking into appropriate responses. Other studies are now examining the impacts of organic compounds that may interfere with the endocrine systems of living organisms. These so-called endocrine disruptors can also be active at very low levels. The EPA’s Endocrine Disruptor Screening Program is helping identify endocrine disruptors from the estimated 87,000 chemicals used today. The consequences of these in humans are just now being studied. In animals, endocrine disruption has been shown to hinder sexual function, complicate pregnancy, and cause thyroid problems leading to metabolic disorders.

Another active area of research is focused on how chemicals interact with each other and the natural environment. In the past, studies have focused on the effects of single chemicals because chemicals are usually regulated singly. However, chemicals are always present as complex mixtures, thus some might say the regulation approach is na├»ve. Thus scientists are increasingly focusing on the toxicity of mixtures of chemicals, acknowledging that the toxicity expressed may be a result of additive or multiplicative effects, depending on interactions with other chemicals present in the environment. In addition, chemicals can have indirect impacts on humans. For example, antibiotics are considered an emerging contaminant because they may influence the development of resistance in microbes. Antibiotics enter the environment through human wastes. In addition, these products are increasingly being used for veterinary purposes and thus may enter from farm and feeding operations — ranging from poultry to hog farming operations.

Finally, researchers are continuing to find new chemicals that bioaccumulate in the food chain. Such chemicals can be present in water at very low levels, yet accumulate to higher concentrations in living tissue, substantially magnifying any health effects. Methylmercury, the most toxic form of mercury, is an example of such a bioaccumulating toxin. With methylmercury, public health officials have in some cases recommended against the consumption of fish by pregnant women.

Mercury emissions from fossil fuel power plants bioaccumulate in the fatty tissue of lake and river fish; its concentrations are biomagnified yet again in fetal tissue and can lead to neurological problems in children, though these neurological studies are somewhat controversial.

DDT is another example of a chemical that bioaccumulates in aquatic organisms. In the 1960s DDT was found to be endangering the bald eagle, peregrine falcon, osprey, brown pelican, and other bird populations reliant on fish diets. DDT and its metabolites reduced the thickness of eggshells, making them more susceptible to breaking during incubation. Since the banning of DDT in 1972 populations of these birds in the U.S. have rebounded dramatically — proof that the environmental levels of the chemicals were reduced, and thus their effects were reduced.

Another challenge faced by researchers is uncovering how widespread emerging contaminants are in the environment. After all, many of the contaminants being examined today are not released in easily measured quantities, like those initially targeted by environmental regulations. For example, pesticides are intentionally released in easily measured quantities. In contrast, household chemicals, drugs, animal feed, viruses, and parasites in fecal matter are all released into the environment after passing through municipal water treatment or solid waste systems, and they can also be released directly. The multiplicity of paths of entry complicates the study of such substances.

To help with this problem, the U.S. Geological Survey, in cooperation with the EPA, has launched the National Emerging Contaminants Reconnaissance, a project targeted at finding and monitoring locations where certain suspect chemicals are highly concentrated. USGS is focusing on four groups of compounds: veterinary and human antibiotics, human drugs, industrial and household products (examples include insecticides, detergents, fire retardants, and fuels), and sex and steroidal hormones. Many of the substances under study are familiar to everyday life: caffeine and ibuprofen, for example.

An Evolving Problem

Contamination of water supplies is an evolving problem and will remain an issue as long as technological change continues. Some of the contaminants now being targeted by researchers may come out with a clean slate, while others will require additional scrutiny. One of the hopes of today’s researchers is that more sophisticated science will help speed the process of identifying and remedying the problems, before damage to either human health or the environment can occur. In any case, science and regulation must continue to evolve and change, as it has in the past few years, to respond to new needs presented by chemicals and our increasing knowledge of them.


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