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.