Just under 90,000 people live in Indiana County, Penn., home of the Homer City Generating Station. They, experts say, are the ones who are most directly affected by the thousands of tons of pollutants that the coal-fired plant's three generation units release into the air each year.

But the effects of the plant, and facilities like it, are not limited to Indiana County or the Midwestern locales where many of the country's other biggest coal plants operate. They are on display in New York City, too—like on July 23, when the state Department of Environmental Conservation (DEC) warned residents they would be better off staying indoors because of high ozone levels. A similar warning was issued in the city for the day before and the day before that, and for four other days in July alone.

Ground-level ozone, a primary component of smog, is formed through a series of complex chemical reactions involving heat, sunlight and chemicals including nitrogen oxides. The latter, known as NOx, are a byproduct of combustion—like the burning of fossil fuels. Along with sulfur dioxide, a contributor to acid rain, and particulate matter, more commonly known as soot, the chemicals can travel hundreds of miles downwind—from Homer City, in other words, to the New York metro area's millions of residents just 330 miles away.

The DEC, in fact, lists out-of-state emissions sources as a primary cause of ground-level ozone and one of the Northeast's most serious air pollution problems—along with local automobile exhaust. The scale of the problem is wide; there are hundreds of coal plants in the country, and Michael Seilback, a spokesman for the American Lung Association, says local air can be influenced by facilities worldwide, as far away as China. Still, some plants are bigger than others, some more directly upwind, and some, critically, employ pollution controls that others do not.

Coal is still the most popular energy source in the United States, accounting for about 45 percent of net power production. Mayor Bloomberg, who has called for that number to shrink, this month donated $50 million from his philanthropic fund to the Sierra Club's anti-coal efforts. Still, with public skepticism over nuclear power and the technical hurdles that alternate energy sources face, coal is here to stay for a while – and the plants, sometimes decades old, that generate power from it.

Below, a look at a few of the Midwestern power plants that affect New York, using newly released EPA data on emissions of hazardous air pollutants in 2010, and state-level data on NOx and sulfur dioxide from 2009 – the most current numbers available. As an analysis, it is necessarily incomplete—it does not, for example, include emissions of mercury, which finds its way from distant smokestacks into the local waters and fish, or carbon dioxide, a major coal-plant emission that contributes to global warming. Nor does it focus on smaller, closer facilities that are made more dangerous to New York by their proximity to the city. Even so, it is a list of power plants New Yorkers can remember the next time health officials say not to go outside.

Homer City Generating Station

Homer City, an 1,884-megawatt plant with a chimney nearly as tall as the Empire State Building (it is the tallest in the country), has become something of a poster child for coal-plant emissions.

The Clean Air Act requires new plants to employ technology like flue gas desulfurization systems, also known as "scrubbers"—devices that remove particulate matter and chemicals like sulfur dioxide from plant exhaust. Older plants are exempted from the scrubber requirements, unless their owners make significant upgrades to the plants. Two of Homer City's units opened in 1969 and one in 1977. Only the third has a scrubber.

Early this year Homer City was sued by the federal government and the governments of New York, New Jersey and Pennsylvania, which allege that the plant's operators made major modifications in the 1990s without installing modern pollution controls.

According to filings with the Pennsylvania Department of Environmental Protection, in 2009 (the last year for which data is available) the plant emitted 10,511 tons of NOx and 101,336 tons of sulfur oxides—the latter, according to New York's attorney general, more than twice as much as put out by all of New York's power plants combined.

The chimney's height, too, is more than a local curiosity. A report released in May by the United States Government Accountability Office found that tall smokestacks, intended to reduce local air-pollution impacts by dispersing chemicals into the atmosphere, contribute to transport of pollution between states. More than a third of the country's tall stacks are in five states along the Ohio River Valley, including Pennsylvania.

While the original intent of tall stacks was to dilute pollution, Conrad Schneider, advocacy director of the Clean Air Task Force, a Boston-based nonprofit aimed at reducing emissions from coal plants, says "It really just propelled the pollution into the upper part of the troposphere, and the winds would just carry it wherever they went."

Muskingum River Power Plant

The Muskingum River Plant has stood a few miles outside of Beverly, Ohio, since 1953. Four of its five boilers opened in that decade, and all four still lack major emission controls. A fifth boiler, which opened in 1968, got a scrubber in 2010. In 2009, the plant emitted more than 7,500 tons of NOx and almost 98,000 tons of sulfur dioxide. According to the Environmental Protection Agency, that year the plant emitted another 3,250 tons of hazardous air pollutants—a category that does not include NOx or sulfur dioxide.

In 2007 American Electric Power (AEP), which owns the 1,500-megawatt Muskingum River plant, settled a lawsuit that had been brought by the EPA eight years earlier, citing the emissions at 16 of the company's coal-fired plants. As part of the settlement, the company agreed to operate its selective catalytic reduction equipment—which reduces NOx emissions—year-round at Muskingum River, instead of only in the summer. It also promised to finish installation of the plant's lone scrubber.

This June, though, AEP announced that new, stricter federal emissions laws will force it to close the Muskingum River plant's four oldest generating units, along with five other coal plants and parts of five others. In its announcement, the company bemoaned the affect that the closings will have on local economies.

"Because of the unrealistic compliance timelines in the EPA proposals, we will have to prematurely shut down nearly 25 percent of our current coal-fueled generating capacity, cut hundreds of good power plant jobs, and invest billions of dollars in capital to retire, retrofit and replace coal-fueled power plants," said Michael G. Morris, the company's chairman. "The sudden increase in electricity rates and impacts on state economies will be significant at a time when people and states are still struggling."

The suddenness of the new regulations' impact, though, is relative. The Muskingum River units, nearing a half-century old, will not close until the end of 2014, when the new regulations take effect. No new pollution controls are planned before then.

Walter C. Beckjord Generating Station

The oldest of the six coal-fired units at the Beckjord plant in New Richmond, Ohio, which generates about 1,100 megawatts of power, was built in 1952. That makes the plant, east of Cincinnati, even older than the Muskingum River boilers. It, too, is closing—a decision that its principle owner, Duke Energy, announced this month.

The plant's age, the company says, is precisely what makes it cost-prohibitive to install scrubbers or other environmental controls. It is, in fact, the company's only power plant that does not use scrubbers.

"The anticipated revenue from power sales would not justify the high costs associated with implementing upgraded controls at the facility—costs that it is expected would be largely borne by our customers," the company says on its web site.

In its September, 2010 report, "The Toll From Coal," the Clean Air Task Force focused on another pollutant, particulate matter—more commonly known as soot. While federal regulations on NOx and sulfur dioxide emissions have succeeded in reducing power plants' emissions of those substances, the report argues, stronger rules are necessary to regulate fine-particle pollution.

Statistically speaking, the study found, the Beckjord plant alone can be held responsible for 141 deaths a year, 102 hospital admissions and 217 heart attacks – most, but not all, in the communities surrounding the plant. [These are not actual deaths, but rather an estimate based on emissions levels and epidemiological models of how emissions might cause "adverse health outcomes, such as hospital admissions, asthma attacks, and premature deaths."]

The plant is scheduled to close by the end of 2014—although, the company notes, the closing could be delayed if implementation of the new air-quality rules is delayed.

Until then, Beckjord will continue operating as it did in 2009: emitting more than 1,850 tons of hazardous air pollutants, almost 11,000 tons of NOx and 42,000 tons of sulfur dioxide each year.

Monroe Power Plant

There are two ways to look at the Monroe Plant, operated by the Detroit Edison Company on the shores of Lake Erie. On one hand, it emits half as much NOx as it did a decade ago, and a third less sulfur dioxide. On the other, as one of the country's largest power plants, its emissions are still massive: more than 20,000 tons of NOx and almost 86,000 tons of sulfur dioxide in 2009, the last year for which data is available.

Still, the facility is improving: In 2009 its operators installed scrubbers to reduce sulfur dioxide and other emissions on two of its four generating units and selective catalytic reduction systems to control NOx on three of the units. The two units without scrubbers should get them in 2013 and 2014, respectively. The upgrades, the company said, made it the first power plant in Michigan with both scrubber and SCR technologies in place—and when data is available from 2010, the first full year with all of the upgrades in place, it should show further improvement still.

EPA data on air emissions—excluding NOx and sulfur dioxide—indicates that chemical releases totaled 2,732 tons in 2010, down 43 percent from 4,850 tons the year before.

It's an impressive decrease. However, the installation of scrubbers is not a complete solution, because while they keep chemicals from floating out into the air, they do not eradicate them completely. The pollutants are diverted to storage, where they must be disposed of in other ways. And of course, reduction is not the same as elimination.

"These coal plants with scrubbers are still magnitudes more polluting than other sources," said Seilback, of the American Lung Association. "You're seeing less pollution coming out, but it's not that the problem is fixed."

Cardinal Power Plant

When the Cardinal plant in Brilliant, Ohio got scrubbers on two of its three boilers in 2007, the results seemed to speak for themselves. The plant's total emissions dropped by two-thirds in 2008, and they remain relatively low: 1,893 tons in 2010, excluding NOx and sulfur dioxide, according to the EPA.

NOx and sulfur dioxide emissions are now relatively low, too, for a coal plant. Cardinal emitted 1,914 tons of the former in 2009, and 34,070 tons of the latter.

But there might be a problem with the scrubbers themselves, as the Columbus Dispatch reported this month. Steel in some parts of the mechanisms had been "corroded all the way through," an official from AEP, which owns the plant, told the paper. An industry researcher, meanwhile, said the corrosion problem is "fairly widespread" nationwide.

At Cardinal, where the three boilers were built in 1963, 1964 and 1971, that is a worry. Each scrubber and the associated technology costs around $285 million, and the technology allows the plant's operators to burn local coal, which saves shipping costs but would burn dirtier without pollution controls in place. All that is in addition to the selective catalytic reduction systems in place on all three units, which were installed at a cost of about $275 million.

Problems with scrubbers are ominous for many reasons. One is that, should any of them fail entirely, it is unclear how the companies that paid for them will proceed. AEP, for one, has shown itself to be cost-sensitive when it comes to clean-air technology. This month, it announced that it was putting a years-long initiative to mitigate carbon dioxide emissions on hold, citing the weak economy and regulations that the company said would keep it from recovering its costs.

Keystone Generating Station

The Keystone plant, just outside of Shelocta, Penn., is about 10 miles away from Homer City, and almost as big – generating 1,700-megawatts. Among the facility's six units are two coal-fired boilers that opened in 1967.

In recent years, the plant's emissions of chemicals into the air have been even higher than Homer City's: 7,718 tons in 2009, not counting sulfur dioxide and NOx. Like the Cardinal plant, though, Keystone has been moving toward cleaning its emissions. A scrubber project, completed in 2009, yielded air emissions of 3,388 tons in 2010 – a 56 percent reduction over the year before.

Sulfur dioxide emissions, though, are what the scrubbers are chiefly meant to curtail. In 2009, Keystone's burners emitted 113,167 tons of sulfur oxides, along with 3,760 tons of NOx. The plant's data on those chemicals for 2010 are not yet available, but the stakes are high – for people within sight of the plant's towering smokestacks, and for people breathing the air far beyond.