But history bolted in to the present day last week and forced an ironic question: Will this famous city that celebrates its industrial heritage ever really be able to escape the grimy, dark side that came with it?

Pittsburgh, rated the "most livable city" in 1985 and again in 2007 by Places Rated Almanac, is now the "sootiest city" in the United States, worse than even smog-shrouded Los Angeles.

Who says? Well, it's the usually-worth-paying-attention-to American Lung Association.

"Los Angeles moved faster to reduce levels of particle pollution," said Janice Nolen of the ALA, based in Washington.

She added that if the trends do not change, Pittsburgh will be No. 1 for year-round pollution next year, too.

To those who live and work here, or whose job is to sell the city, this new distinction is no laughing matter. Modern Pittsburgh is taking a public relations hit for the past that made it famous.

Tourism boosters Visit Pittsburgh and the Greater Pittsburgh Hotel Association both declined to comment. Others, however, sternly criticized the testing methods and said the bad score blows in from a single source - the Clairton Works coke plant, a remnant of the region's grit-and-grime past just 15 miles from Pittsburgh's city line.

Allegheny County Executive Dan Onorato was critical of the report's methodology.

"It's an irresponsible report that offers no context of where this city is going," he said. "Anybody who lives here can tell you that if you look at the air today vs. 30 years ago, we've made great strides."

Allegheny County Health Department spokesman Guillermo Cole also said the ranking was unfair.

"It's a distortion," he said. "The American Lung Association even acknowledged that if you exclude the Liberty-Clairton monitoring station, if you don't count that area, we would rank 16th of 222 metro areas. Not that that is something to crow about, but you don't make headlines when you're 16th nationally or even locally."

Nolen said one metric in determining the rankings was the number of weighted average days of high particle pollution levels. In this year's survey, Pittsburgh recorded 62 such days. In last year's survey, it had 60.7 days.

"It's a very serious threat," she said, "whether you are breathing it in the short term at a spiked level, or day in, day out at a lower level. Unfortunately, we are seeing problems with both levels of exposure among people in Pittsburgh."

The Clairton Works is a sprawling 3.3 miles long and takes up 400 acres. Its "quench towers" shoot off bursts of steam, producing large white clouds every few minutes.

On the hillside overlooking the plant is the Allegheny County Health Department monitoring station.

Bright green pipes stretch for 11 miles alongside Route 837 and the Monongahela River. They link the three U.S. Steel facilities, the only three remaining steel mills in the region - Irvin, Clairton and Edgar Thomson. Combined, the three plants employ 3,500.

Clairton produces the oven gas, transported through the green pipes, that the two other facilities use.

Nicknamed "The City of Prayer," Clairton, with a population today of about 12,000, is far different from what it was when steel was king.

At its peak, the Clairton plant produced just over eight million tons of coke a year - used in blast furnaces to convert iron ore into liquid iron.

The plant continued that level of production until the '70s. Today, it produces about 4.5 million tons a year.

Just west on Route 837, Pittsburgh's downtown skyline glistens like a crown.

The city began taking steps to develop a more service-oriented economy, one far less dependent on manufacturing, starting in the 1980s. It attracted major banks, health centers, and high-tech firms. The universities, including the University of Pittsburgh, Carnegie Mellon and Duquesne, expanded.

"The city has transformed itself," said Mike Hoops, a business manager for PPG Industries Inc., one of the city's largest employers. Hoops, 45, had just exited PPG Place, a gleaming office tower with a fountain that opened in 1984 in the heart of downtown. "Now, when you think of Pittsburgh, you think of education, high tech, and the medical field has really taken off."

There is a new luster to downtown Pittsburgh, thanks in part to a program that sandblasts buildings, including churches, to get rid of their dark coating caused by diesel exhaust from cars and buses. The Pittsburgh District Partnership dispatches an army of workers each day to pick up trash along city streets.

New shopping squares and pricey condos attract younger people, such as Amanda Mulligan.

Mulligan, 23, of Patton, Pa., near Johnstown, arrived three years ago to attend the Art Institute of Pittsburgh. She graduates in the fall and plans to stay.

"I love it," said Mulligan, as she worked behind a sales counter at a jewelry store inside Station Square, a trendy shopping and dining complex. "There's always something to do. I like the community atmosphere."

Every hotel room this week was booked downtown as the city was host to the four-day Iron & Steel Technology Conference and Exposition at the new David L. Lawrence Convention Center. The annual event attracts about 8,000.

At Station Square, a giant replica of the Clinton Furnace, Pittsburgh's first blast furnace for making pig iron, decorates the courtyard.

In December, U.S. Steel announced it was spending $1 billion to replace two coke-producing batteries with state-of-the-art coke and emissions-control technology at the Clairton plant. The new technology would eliminate 235 of the plant's 816 coke ovens - many of which date to 1955 - reducing by 30 percent the number of openings through which emissions can escape.

"There's a lot of things out of our control, and there's a lot of things in our control," said Ray Terza of U.S. Steel. "Stuff that's in our control we're addressing with a lot of money and as expeditiously as we possibly can."

A draft permit for the first battery was issued by the Allegheny County Health Department on Tuesday and will face a 30-day public-review period.

Some locals in Clairton, such as Craig Diamond, 32, a maintenance worker at the coke plant, refused to feel Pittsburgh's "pain" over being the sootiest.

"This is all politics," said Diamond, after he got off work earlier this week. He wore soiled blue jeans and a greased T-shirt, and he had dirt under his fingernails.

Bob Moisey, 27, whose uncle, great-aunt and grandfather worked at the steel mills, said there were worse things to get worked up about than a sootiest-city label.

"Since I've been here, it's been like this," said Moisey, who was born and raised in Clairton. "My grandpap lived till he was 85 years old, and he worked at the mill," he said. "My great-aunt worked for the mill and she's still living.

"Nobody is promised tomorrow," he said. "You have to live for today."

The Ranking

The American Lung Association's "Sootiest" Ranking Method for short-term particle pollution: The association:

Downloaded data for every county that has an air-pollution monitor sent to the federal Environmental Protection Agency.

Determined the highest reading each day and compared that with the Air Quality Index.

Calculated a weighted average over a 3-year period in that county.

Compared that data to 700-plus other counties throughout the country that have air-monitoring stations.

Ranked the metropolitan areas based on the county with the highest weighted average in that metropolitan area.