Energy and the Emerging PBT Focus

When President George W. Bush announced his Clear Skies Initiative in February 2002, the White House hailed it as the most aggressive initiative in American history to cut power plant emissions, as well as a bold new strategy for addressing global climate change. The president spoke of expected reductions of billions of pounds of emissions of sulfur dioxide, nitrogen oxides and greenhouse gases.

In light of the quantities, it may seem surprising that both the Clear Skies Initiative, as well as other national programs, have given an equal amount of attention to releases of pollutants that are only one-millionth of that amount, that are more appropriately measured in pounds, or in some cases, even grams. Yet there are a group of pollutants, recently classified as Persistent Bioaccumulative Toxics (PBTs), that have gained national attention as a significant component of the risk equation when assessing the environmental and health impacts of energy emissions. PBT chemicals are those that partition primarily to water, sediment or soil and are not removed at rates adequate to prevent their bioaccumulation in aquatic or terrestrial species. Chemicals are classified as persistent bioaccumulators based on accepted test methods although there remains controversy over the application of these test methods to certain substances such as heavy metals (see Toxic Release Inventory (TRI) PBT Chemicals Rule Web site). Follow-on toxicity testing leads to their identification as persistent and bioaccumulative toxic chemicals.

Mercury, one of the PBTs of most concern, is included as one of the president's three priority pollutants for utilities under the Clear Skies Initiative. Coal-fired power plants are the largest source of mercury air emissions in the United States, accounting for more than 30 percent of national emissions. The U.S. Mercury Report to Congress estimates, when other energy sources are included, more than 50 percent of national mercury emissions are energy-related. However, mercury is not the only problem. Energy-related sources also account for more than five percent of dioxin/furan air emissions, resulting from the combustion of wood, coal, oil and vehicle fuel. The percentage of national dioxin emissions related to energy-sources will continue to rise as reductions are made in other sources, particularly municipal and medical waste incinerators. Approximately 16 percent of emissions of the seven known carcinogenic polycyclic aromatic hydrocarbons (PAH) result from energy sources, such as residential wood combustion. Releases of other PBTs, such as cadmium, are also of concern.

International Concerns

The concern worldwide over PBT emissions continues to grow. President Bush agreed to sign the Stockholm Convention (also referred to as the Persistent Organic Pollutants (POPs) Treaty) in the summer of 2001, a global treaty to protect human health and the environment from PBTs. This was hailed as an early environmental achievement for the Bush administration. Other important international efforts currently focused on PBTs include the Canada-U.S. Great Lakes Binational Toxics Strategy, the North American Free Trade Act Commission for Environmental Cooperation (NAFTA CEC) Program, the United Nations Economic Commission for Europe Convention on Long Range Transboundary Air Pollution (LRTAP), and the Intergovernmental Forum on Chemical Safety (IFCS). The fact that many PBTs can be transported over long distances and cycle through the environment (known informally as the "grasshopper" effect -- see IJC Web site reference) means the problem cannot be solved solely within a nation's boundaries. Computer simulations led the U.S. Environmental Protection Agency (EPA) to conclude in its Mercury Report to Congress that only one-third of U.S. anthropogenic mercury emissions are deposited in the contiguous 48 states, while two-thirds are transported outside the United States.

PBT chemicals are those that partition primarily to water, sediment or soil and are not removed at rates adequate to prevent their bioaccumulation in aquatic or terrestrial species.

A Complicated Exposure Scenario

The problem with PBTs is that a legacy of relatively small releases over time can lead to major environmental and health problems due to the persistence and gradual bioaccumulation of the releases. PBT emissions from energy sources are problematic not from the standpoint of direct human exposure at the source of emission, but rather from a complex process of deposition and cycling in the environment and bioaccumulation through the food chain. When PBTs are emitted into the air, they deposit onto lakes and rivers or agricultural lands and accumulate in fish tissue and animal fat. Humans are exposed to PBTs through consumption of contaminated fish and food products. The complex pathway of exposure results in particularly difficult policy decisions about where to intervene to reduce human and ecological health effects.

For most PBTs, primary human exposure occurs through the aquatic or terrestrial food chain. Mercury accumulates most efficiently in the aquatic food web undergoing biological transformation into methylmercury, a highly toxic form of mercury. Dioxin, on the other hand, accumulates efficiently terrestrially, and 95 percent of dioxin exposure for a typical person is estimated to be associated with intake of animal fats. In both cases, however, as is typical for many PBTs, the primary entry point into the food chain is through air deposition. For this reason, energy sources, with their significant and diffuse air releases, are a logical concern.

Unfortunately, determination of the contribution of different sources to human exposure is problematic. For example, in the case of mercury, efforts to estimate the contribution of specific source categories to fish concentrations must take into account environmental cycling and transport, transformations while in the environment and contributions from global sources and reservoirs. Nevertheless, in December of 2000, EPA concluded that the contribution of utility mercury emissions was significant enough to merit regulatory action for mercury and other air toxics from coal- and oil-fired power plants.

The U.S. Mercury Report to Congress estimates more than 50 percent of national mercury emissions are energy-related.

PBTs have been linked in humans and laboratory animals with cancer, immune system disruption, developmental and reproductive disorders and neurological defects, especially when a developing fetus is exposed. New data in 2001 all indicated increasing concern related to health effects associated with PBTs. The latest release of EPA's dioxin reassessment created headlines with estimates that the risks for the general U.S. population based on dioxin exposure may exceed a one in 1,000 increased chance of experiencing cancer. The Centers for Disease Control, meanwhile, released new preliminary data from the National Health and Nutrition Examination Survey (NHANES 1999) that showed higher levels of mercury in blood than earlier estimates, with approximately 10 percent of women estimated to have mercury concentrations over the blood mercury level associated with EPA's reference dose, a level considered by EPA to be without known adverse health effects. Further research on PCBs has also raised new concerns over neuro-developmental effects associated with PCB exposure.

How significant are the potential health effects of PBTs and what precedent do they set for corporate responsibility? That can perhaps be answered by a single case that occurred last year when EPA reaffirmed its determination to make General Electric (GE) pay for a half-billion dollar dredging and cleanup of the sediments in the Hudson River. These sediments were contaminated with PCBs more than 30 years ago, primarily from GE facilities, clearly demonstrating the legacy that PBTs can leave.

The problem with PBTs is that a legacy of relatively small releases over time can lead to major environmental and health problems due to the persistence and gradual bioaccumulation of the releases.

A Complicated Control Scenario

One intent of the current multi-pollutant legislation effort is to take advantage of inter-relationships between control strategies for reducing greenhouse gases, sulfur dioxide (SO2) and nitrogen oxides (NOX), and control strategies for mercury and other air toxics releases. While laudable, history indicates that controlling air toxics, such as mercury and dioxin, comes with its own particular and expensive challenges. Cap and trade programs directed at the greenhouse gases, SO2 or NOX, will have some effect on mercury releases, but will likely not be sufficient to achieve the level of reductions desired.

Another danger associated with controlling the relatively small releases of the priority PBTs is the potential for affecting release of one PBT while controlling for another. Let's shift focus away from the large centralized power generation to the most decentralized source of energy production -- residential wood combustion. Energy for space heating in the United States represents about eight percent of U.S. energy consumption, including transportation. Surprisingly, residential wood combustion accounts for approximately nine percent of that energy consumption through approximately 9.3 million wood stoves, 27 million fireplaces, and 600,000 other wood appliances. In 1992, EPA promulgated New Source Performance Standards for wood heaters, which required certification that new stoves met particulate emission thresholds. These standards were also estimated to significantly reduce toxic emissions, primarily emissions of polycyclic aromatic hydrocarbons (PAHs), including benzo(a)pyrene (B(a)P), a priority PBT.

Residential wood stoves are one of the largest sources of anthropogenic sources of B(a)P. Currently, however, only about 11 percent of the existing stock of residential wood combustion appliances in homes are the newer certified appliances. During the course of work conducted under the Canada-U.S. Great Lakes Binational Toxics Strategy to assess the potential for reductions in dioxin releases, Environment Canada conducted testing on an older non-certified wood stove and a newer EPA-certified wood stove. Test results indicated significantly lower releases of particulate matter and PAHs from the new stove compared to the older stove, but higher releases of dioxins and furans. While the results are in no way conclusive, they highlight the need for great care when implementing requirements for reductions in these problematic toxics to ensure there are no unintended consequences. The results of the EC study paint a very complicated risk picture, attempting to weigh grams of dioxins against tons of other pollutants.


The issue of PBTs associated with energy usage illustrates the pressing need to take a comprehensive focus when assessing the combustion of fossil fuel in general versus other forms of energy. Risks from toxics, such as PBTs, must be weighed, as well as the risks from climate change and air pollution. In fact, as the nation wrestles with its energy policy, risks must be measured in many ways, in grams at times, as well as in billions of pounds.


EPA PBT Program --

Canada-US Binational Toxics Strategy --

EPA's Mercury Web site --

EPA's Utility Air Toxics Determination --

EPA's Dioxin Reassessment --

Environment Canada Residential Wood Stove Study --

CDC MMWR Report on Mercury Blood Levels --

The President's Clear Skies Initiative --

TRI PBT Chemicals Rule --

United Nations Environmental Program Persistent Organic Pollutants (UNEP POPs) Program --

International Joint Commission (IJC) Report on Transboundary Air Quality Issues --

North American Commission on Environmental Cooperation --



This article originally appeared in the 06/01/2002 issue of Environmental Protection.

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