In 1998, EPA warned that during the previous year, 107 million Americans lived in counties where the air failed health standards for at least one of the six criteria pollutants. These conditions make people sick, and occasionally kill them. Research conducted in Toronto links up to 50% of respiratory hospital admissions during pollution peaks with high levels of acid particles and ozone. ⁹ In New Jersey, emergency room visits for asthma increase substantially at ozone levels well below the current U. S. standard. ¹⁰ A Los Angeles study associates a 10% increase above average ozone levels with about two additional deaths per 1,000 people, and a 50% increase (not uncommon in the summer) with 10 additional deaths per 1,000. ¹¹ Equally pertinent, air pollution, like many other environmental problems, weighs on poor communities with disproportionate severity (see Box 1).

B. OTHER AIRBORNE TOXINS

In addition to the criteria pollutants just described, power plant combustion can release a variety of other substances, characterized by regulators as hazardous air pollutants (HAPs). EPA regulates emission rates of HAPs from specific activities, although not overall tonnage. (See Table 3 for HAP emissions from several generating technologies.)

Organic HAPs include carcinogenic dioxins, furans, and polycyclic aromatic hydrocarbons. Because these compounds result from incomplete combustion, as does carbon monoxide, measures to prevent the release of CO also generally limit organic toxins.

In addition to organic HAPs, combustible fuel may contain small quantities of toxic metals and other inorganic pollutants. These substances leave power plants as airborne particles or vapor. They also concentrate in bottom ash and collect in pollution control devices. While coal and oil combustion release the greatest quantities of inorganic HAPs, wood can present a problem as well, depending largely on bark surface and growing conditions. Waste wood may also contain metal contaminants from paint, sealant, and other sources.

Measures to limit particulate emissions also generally control inorganic HAPs. Volatile metals such as mercury and selenium represent important exceptions; only about 10% of mercury emitted from power plants takes a particulate form. The remainder takes either an ionic or an elemental form. Ionic mercury usually binds after emission to airborne particles that carry it to earth, where it enters land and aquatic ecosystems. Elemental mercury, on the other hand, may travel the atmosphere for up to two years before converting to ionic form and precipitating down. Some data suggest that 5-10% of mercury comes down within 100 kilometers (km) of its source, and another 50% within 1,000 km. The rest enters the global pool. EPA models suggest that only 30% of U. S. mercury emissions come down inside this country. ¹⁵ Utility coal and oil boilers accounted for 32.8% of American mercury pollution in 1995. ¹⁶

Mercury tends to accumulate in aquatic ecosystems, where it works its way up the food chain to top predators such as tuna, sharks, and swordfish - and to the humans who eat them. The developing fetus may be particularly vulnerable to mercury. Po tential effects on human health include losses of sensory or cognitive ability, delays in development, birth defects, tremors, and death. In most circumstances, one would not expect electricityrelated emissions to cause such severe effects, although mercury poisoning represents a particular hazard to subsistence and fishing communities, such as Native Alaskans. While much uncertainty remains about the impact of mercury on humans, some experts note that present knowledge is at least as solid as that for other federally regulated pollutants. ¹⁸

BOX 1: ENVIRONMENTAL JUSTICE The environmental impacts of electricity production weigh most heavily on poor communities and communities of color - terms that in the United States often overlap. In some cases, this situation may reflect a perceived need within the community for economic development at any cost. In others, it may reflect a lack of political influence, a lack of information about environmental risks, or a lack of awareness of alternative development and energy strategies. Indian Country as a whole holds one-third of the country's uranium mining and milling waste. 12 At 20%, 0 the poverty rate of communities located within one mile of coal-fired power plants is almost double that of the general population (11.3%). Such communities are 21.5% non-white, compared with 17% in the general population. 13 Compared with families with incomes over $35,000 per year, families with annual incomes below $10,000 suffer more than twice the incidence (per thousand people) of asthma, making them much more susceptible to pollution-related illnesses. 14

HAP emission rates vary widely among power plants, depending on combustion and pollution-control technology as well as the chemical composition of fuel, which in turn depends on geographic source or (for wood) growing conditions. One study, which compares testing data from nine coal plant with different configurations, finds the following very broad ranges of hazardous air pollutants in stack emissions: ¹⁹

In all, total emissions of hazardous air pollutants at single plants examined in the study referred to ranged between 2.8 and 1,852 tons per year. ²⁰

C. ACID RAIN

In addition to making people sick, airborne sulfur and nitrogen compounds damage ecosystems and buildings when they return to Earth as acid rain - augmented by acid snow, acid mist, acid fog, and dry deposition of acid gases and particles. A combination of high emissions and acid-sensitive soils makes the Adirondacks, upper Appalachian and southeastern Canada particularly vulnerable to these phenomena. Trees such as red spruce at high elevations and lakes suffer the most. Acid accumulates in snow packs, which jar fragile streams and lakes with an acid pulse in the spring thaw. Acid rain also damages buildings. In 1997, electric utilities contributed 64% of national SO2 emissions and 26% of NOX emissions; coal-fired facilities accounted for almost the entire utility share (see Table 1).

Largely in response to reports that rain and snow in the northeastern United States had become increasingly acidic, Congress established the Acid Rain Program in Title IV of the 1990 Clean Air Act Amendments, best known for its innovative "cap and trade" provisions. Following implementation of the program, SO2 emissions from more than 400 affected electricitygenerating units (mostly coal-fueled) fell from 15.8 million to 11.9 million tons per year. Emissions crept up in the subsequent three years, reaching 13.1 million tons in 1998. ²¹

Notwithstanding earnest and successful public policy measures to reduce sulfur emissions, sulfur concentrations in many ecosystems have fallen more modestly, and acidity continues to trouble large regions of North America. Moreover, forests are recovering only slowly - in some cases, researchers suggest, failing to grow at all. Some analysts speculate that the continuing acidity problem reflects simultaneous reductions in particulate dust; these basic (i. e., in terms of Ph factor) particles may previously have buffered airborne acids, as well as furnishing reserves of basic compounds in the soil. Other analysts note that NOX , a precursor of nitric acid, received comparatively less attention in the Clean Air Act than SO2 ; there currently exists no national cap on NOX emissions. ²²

D. THE NITROGEN CYCLE

The planet's nitrogen stock cycles between soil, living organisms, water, and the atmosphere. Viewed globally, emissions from power plants represent a modest but nontrivial additional stress on the nitrogen cycle, which is already changing rapidly due to human activity. ²³

Until very recently, the scarcity of available nitrogen was a limiting factor in most planetary ecosystems. As a result, abundant nitrogen leads quickly to increased growth. (For that reason, nitrogen is the primary ingredient in fertilizer.) Living organisms depended for their nitrogen largely on the slow process of "nitrogen fixing," by which symbiotic microbes associated with plants pull molecular nitrogen (N2 ) from the air and convert it to ammonium (NH4 ), nitrate (NO3 ), and other inorganic compounds. Previously fixed nitrogen resides in soil, where microbes recycle it into usable form.

Human activity has altered this cycle. Natural, landbased nitrogen fixation amounts to up to 140 million metric tons per year, ²⁴ mostly from microbes, plus perhaps 5 million metric tons fixed by lightening strikes. Since 1900, human activity has doubled this rate. The burning of coal and oil in power plants, which frees nitrogen previously sequestered in fossilized organic matter, contributes perhaps 6 million metric tons per year - some 4% of the global increase.

Nitrogen entering the planetary ecosystem harms the environment in several ways. Other sections of this review discuss rising concentrations of the greenhouse gas nitrous oxide, and increased regional concentrations of other oxides of nitrogen, which form smog and acid rain. As nitrogen compounds precipitate out of the atmosphere, they accumulate in soils as nitrates. Ultimately, excess nitrates leach into streams and groundwater, carrying with them soil nutrients such as calcium, magnesium, and potassium. As the level of these nutrients in the soil falls, they become the factor limiting plant growth. Plants lacking them may absorb toxic aluminum salts instead.

Meanwhile, nitrogenated runoff pouring into lakes, streams, estuaries, and coastal waters feeds explosive growth of algae and other plants, a condition known as "eutrophication." This process creates several problems, including:

• falling oxygen levels, resulting in dieoff of more complex plants and animals;
• the proliferation of nuisance algal species, which may prove toxic to fish, humans, and other mammals; and
• through surface algae growth, decreased sunlight and photosynthesis for species below. Power plants deposit 11-15% of the nitrogen in the Chesapeake Bay, thought to have contributed to the rapid growth of toxic organisms in recent years. ²⁵ Worldwide, scientists believe that such changes have accelerated the loss of biodiversity, especially of plants adapted to lownitrogen soils and the animals that depend on them. ²⁶

E. POLLUTION AT LONG RANGE

While the public generally associates energyrelated air pollution primarily with the heavy traffic and clustered industry characteristic of urban areas, high atmospheric winds can carry pollution long distances. Air pollution now troubles remote locales, including many national parks. Visitors to Great Smoky Mountains National Park could once see up to 93 miles; haze now shrinks these vistas by 60%, and by as much as 80% during summer months. ²⁷ Ontario's Ministry of Energy and the Environment estimates that over half the ground-level ozone in Toronto on hot summer days originates in the United States, and Ontario contributes substantially to sulfur pollution in Vermont and New Hampshire. ²⁸ In fact, researchers believe that pollution from Asia can reach the western United States, and some evidence hints that U. S. pollution reaches Europe as well. ²⁹

Some scientists (and regulators) assert that pollution vented originally by coal-burning power plants in the Midwest exacerbates already severe local air quality problems in the Northeast, making it even more difficult for Northeastern states to meet federal air standards. ³⁰ Evidence of longdistance transport includes, for example, ozone detected at night high over Eastern cities, when the absence of sunlight suggests that it could not have been produced locally and must therefore have arrived on prevailing easterly winds.

The question of transport aside, the states closest to power plants tend to suffer as much or more from pollution than their downwind neighbors. For example, while Northeastern cities such as Portland, Maine, often experience higher shortterm ozone "peaks," Midwestern cities such as Huntington, West Virginia, and Marietta, Ohio, suffer longer, higher ozone "plateaus." That is, cities near Midwestern power plants experience more total hours of unhealthy air. As a result, their residents are more often hospitalized for ozone-related complaints. ³¹

F. THE EFFECT OF SHIFTING ENERGY MARKETS

In addition to the obvious effects of environmental regulation, changes in energy markets have also affected air quality. For example, the Natural Gas Policy Act of 1978 eliminated gas price controls, stimulating exploration and competition. As a result, supplies rose and prices dropped, ultimately making gas - substantially cleaner than coal - the fuel of choice for new power plants. The restructuring of the electric sector under way in many states and pending in most of the rest may advance this trend by opening up competitive opportunities for highly efficient combined-cycle gas plants. ³² Depending on the rules governing the new electric sector, distributed energy resources may also thrive; these include relatively clean gas-powered microturbines, fuel cells, photovoltaic (PV) and small wind systems, energy efficiency measures, and energy storage devices. Fuel cells, for example, generate power chemically, with no combustion at all. ³³

On the other hand, restructuring may increase the competitiveness of old, dirty coal plants concentrated in the Midwest. In the 1970s, legislators struggling to build a consensus in favor of the Clean Air Act chose to exempt (" grandfather") existing and planned coal-fired plants from the act's most stringent requirements. As a result, these facilities may emit up to 10 times more pollution than new plants. ³⁴ Most members of Congress presumably shared the accepted industry view that the plants' owners would retire them after their expected lifetime of 30 or so years. Unfortunately, the exemption itself provided a competitive advantage for the plants in question, and their owners have kept many in operation.

Until fairly recently, most of the grandfathered plants operated at relatively low capacity due to saturated local demand. In 1992, however, the Energy Policy Act (" EPAct") freed the national market for wholesale power, allowing power plants to serve distant customers. But EPAct did not standardize environmental requirements for all generating facilities. In the four years preceding EPAct, the nation's fleet of coal plants increased generation by about 2%; in the six years after its passage, generation grew by almost 16%, as the plants boosted operations from 60% to 67% of capacity. ³⁵ To take a specific example, the Northeast States for Coordinated Air Use Management calculates that between 1995 and 1996, a single, large Midwestern utility, American Electric Power, increased coal-fired generation by 10%, largely to meet increased sales of wholesale power. ³⁶ The resulting annual increase in NOX emissions - over 50,000 tons - exceeded the total 1996 NOX emissions from all the fossil fuel generating plants in Massachusetts and New Hampshire combined.

A further complication concerns nuclear power. Although nuclear plants present complex environmental costs and risks, they do not directly emit air pollution. One recent analysis finds one to three dozen of America's 100 or so nuclear plants at risk of shutdown by their owners should industry restructuring expose them to competitive pressure. ³⁷ In any case, licenses for the entire nuclear industry will begin to expire in this decade, and prospects for relicensing remain unclear. Replacement of either early or "naturally" retired plants by anything other than zeroemission power plants could increase air emissions (although, of course, it would stanch the mounting problem of nuclear waste).

Finally, the uncertainty created by restructuring initially damped enthusiasm for the construction of new renewable energy facilities. More recently, seven states have integrated renewable energy into their restructuring packages through policies known as renewable portfolio standards, which require that renewables supply a certain percentage of the electricity sold. Twelve states have also created clean energy investment funds from charges levied on electricity sales. ³⁸ In addition, restructuring has allowed electric suppliers to sell "green power" generated from renewables. Although only a small number of renewable energy facilities have so far been built to meet this market, it may in the future prove significant. ³⁹

G. BIOPOWER AND AIR POLLUTION

Burning plant material, like all combustion, releases gaseous and particulate pollutants, although gasifying the biomass generally produces fewer emissions than direct combustion. In several ways, however, biopower poses less environmental risk than fossil fuel combustion. Virgin wood contains essentially no sulfur, greatly reducing SO2 emissions. In addition, most wood fuel sources contain only one-tenth to one-third the nitrogen of coal, thus limiting fuelcaused NOX emissions. (See Tables 1 and 2.)

In fact, biopower may provide a NOX-control dividend. When biomass is burned in the same boiler as coal (" cofiring"), the wood's moisture cools the combustion process, reducing the formation of thermal NOX . Bestcase results reported by the Electric Power Research Institute suggests that co-firing coal with 7% wood (by heat content) reduces NOX by 15%. Other research suggests that introducing wood to a coal boiler between the primary combustion zone and the chimney (i. e., as a "reburn" fuel) can destroy some of the NO produced in a coal boiler. In addition, cofiring wood in coal boilers, whose efficiency averages 33%, offers efficiency gains over stand-alone biopower plants, which generally run at around 20%, with resultant reductions in pollution per unit of energy produced. ⁴⁰