During the spring, buried in the thickets of the debate over congestion pricing, was a great nugget in Mayor Michael Bloomberg’s PlaNYC 2030. Deep in the document was a promise: if PlaNYC 2030 was implemented, New York City would become the “cleanest big city in America.”

That got me thinking.

How clean is the air in New York City?

The short answer is that the nation’s air is cleaner than it’s been in decades, and so is New York’s. But New York City air is still polluted enough to send thousands of people to emergency rooms every summer and to send some of those people to early graves every year.

The longer answer is more complicated, and it goes like this:

Since 1970, the U.S. Environmental Protection Agencyhas set standards for the maximum amount of air pollution that is safe to breathe. In a city like New York, ozone (a principal ingredient of smog) and particulate matter (or soot) are the key pollutants—and the city has never met EPA’s standards for either pollutant. In each case, we are in the second-worst category (Los Angeles tops the "most polluted list" for both smog and soot). By this measure, the city seems to be in pretty bad pollution shape—especially because many environmental groups think that the current EPA standards are too weak.

Soot particles are the biggest pollution concern. A decade ago, the Natural Resources Defense Council (where I worked then, and now) estimated that 64,000 Americans died prematurely every year at then-current levels of particulate air pollution. More than 4,000 people of those premature deaths were in the New York metropolitan region.

In New York, diesel engines have long been at the heart of our local soot problem. When walking up Madison Avenue, most of the soot we breathe comes from a relatively small number of diesel engines—buses, trucks, and construction equipment. In more residential neighborhoods, home heating oil is diesel’s close—and even dirtier—cousin.

Sources of Particulate Soot on Madison Avenue

Diesel (and home heating oil) soot is really toxic stuff. If it’s not effectively filtered in the tailpipe, typical diesel exhaust contains dozens of cancer-causing chemicals. This toxic soup has been found to be at least a probable or likely human carcinogen by many of the world’s leading public health agencies, including the U.S. Environmental Protection Agency, the National Toxicology Program and regulators in Germany, California and elsewhere.

Luckily, diesel soot pollution is a fixable problem. As of this year, all new diesel truck and bus engines are equipped with filters that remove more than 90 percent of this soot. And, NYC Transit buses have used these filters effectively for several years now, reducing their fleet-wide emissions by more than 95 percent since 1995. But most trucks, buses, and construction equipment don't have these filters.

So, in 2005, the Boston-based Clean Air Task Force revisited the soot problem, and focused on the specific problem of diesel soot. The task force estimated that people in Manhattan inhale enough toxic soot particles to create a lifetime cancer risk that is more than 3,200 times EPA’s so-called “acceptable” cancer risk. It gets even worse: According to the task force, more than 40,000 asthma attacks and almost 1,800 premature deaths were attributable to diesel soot pollution in neighborhoods throughout the city in 1999. That’s why the American Lung Association recently gave the City an “F” for its particulate soot levels.

But is the air really so horrible? As I write, the sky outside my window is bright blue. My daily “EnviroFlash” email from EPA and the New York State Department of Environmental Conservation tells me that today’s pollution levels are “good” for both particles and ozone.

OK, I know better: EPA and environmental conservation department give a regional estimate. That estimate tells me nothing about local conditions. Recent studies show that diesel soot emissions along roadways create corridors of increased pollution, which leads to higher health impacts. Perhaps the most famous example can be found in northern Manhattan: With five Metropolitan Transit Authority bus depots and several busy north-south truck routes, Harlem and Washington Heights have higher air pollution levels—and much higher asthma rates—than their southern neighborhoods.

All of this underscores two important themes. First, it is important to acknowledge that we’ve made significant progress in reducing many sources of air pollution over the years, and our air is cleaner as a result. But, second, it is equally important to understand the job isn’t finished—we still fail to meet EPA’s health standards for ozone and particulate soot, and we need to create new ways to address the remaining sources of air pollution that seem to be pocketed in certain areas of the city.

Mayor Bloomberg’s PlaNYC 2030 strives to ensure that “every New Yorker” should breathe the cleanest air of any big city in America. We’re not there yet. Next month, I’ll dive into the plan and explore whether the plan will us meet his goal.

Rich Kassel is a senior attorney at the Natural Resources Defense Council (NRDC), where he focuses on urban air pollution and transportation issues. He also chairs the Tri-State Transportation Campaign, a regional transportation advocacy organization, and is on the boards of the New York League of Conservation Voters and Transportation Alternatives.