Frank Smith dips a long-handled paint roller into a bucket of paint and gets to work, laying a slick coat of gray over a corrugated gate. He watches with satisfaction as the black spray-painted name of a graffiti writer vanishes, but there are hundreds more to go here, along the 900 block of Westmoreland Street.

The 28-year-old head of one of the city's dozen graffiti-abatement crews has heard the rationale.

Underprivileged kids with no creative outlet find self-expression by whooshing their signatures onto warehouse walls. Angry teenagers with poor self-esteem earn the admiration of their peers by scrawling on stop signs.

Smith has no doubt that the wall writers who slipped into this isolated industrial street in North Philadelphia to leave their mark would argue that graffiti is art. But to Smith, whose Sisyphean job for the last seven years has been to clean up after them, every "tag" is an ugly slap in the neighborhood's face.

"Without this department," Smith says as he paints, "the city would be looking real bad."

With the city reporting a significant rise in graffiti vandalism, a study published in Science magazine last week offers encouraging news for Smith and his coworkers. However frustrating their work may be, the findings indicate, the effort is not in vain.

Graffiti, as well as other forms of "disorder," such as littering, are contagious urban diseases, the study found. In order to stop the spread of blight, cities have to continuously, vigilantly, keep cleaning up.

The study's lead author, Kees Keizer, spent nearly two years creating controlled experiments in the northern Dutch city of Groningen, observing how graffiti and litter affected behavior. A 33-year-old graduate student in psychology and sociology at the University of Groningen, Keizer distributed fliers on parked bicycles in a lot with a sign forbidding graffiti. When the area was clean and graffiti-free, 33 percent of people threw the fliers on the ground or attached them to neighboring bikes. When Keizer painted graffiti on the wall, the number of litterers jumped to 69 percent.

"What we discovered is that how much people are influenced by rules depends on the context," Keizer said in a telephone interview last week. "They're willing to behave appropriately, but this has to be enforced - or underlined by an environment."

Even when a neighborhood is pristine, there is no guarantee that graffiti writers will leave it alone.

"You can explain to them that they're trashing their community," said Jonny Buss, 30, a former graffiti writer who now works with the Mural Arts Program. "They kind of realize that if they write on a wall, that someone's going to have to clean it off. But they really don't care, because they want their friends to see it on their way to school or have someone take a picture of it and put it on the Internet."

Although North Philadelphia is one of the most popular tagging venues, Smith says he's had to clean up graffiti in almost every neighborhood.

"There's not a spot in the city where I've never been," he says as he aims a thin hose at a utility pole, pulls the trigger, blasts hot water under 3,000 pounds of pressure at a tagger's mark and watches the letters wash away. "I've been under bridges, over bridges . . . . In Roxborough, they tag on poles and traffic-control boxes. They love to tag on Amtrak, where people can take a good look at their 'art'."

Graffiti is an intractable problem, said Jane Golden, director of Philadelphia's Mural Arts Program. "There's not a city in the world where it will ever be eliminated completely." Golden, who worked for the city's anti-graffiti network in the 1980s, when the problem here seemed overwhelming, said, "Philadelphia has done an extraordinary job in last 20 years reducing graffiti . . . . But there is an attraction among certain people to write on walls."

In her travels here and studying trends in other cities, Golden said, "It does seem to ebb and flow."

The city is reporting an increase in graffiti vandalism. The number of properties cleaned has risen from 93,000 in fiscal 2006 to 110,000 in fiscal 2007 and now - only halfway through the current year - to a running total of 112,000.

On Westmoreland Street, Smith, trying to stay warm despite several layers of clothes under his paint-spattered coveralls, steps back to assess his progress. A soggy pile of trash lies at his feet.

"I love what I do," he says. "You got to have it in your heart to want the city clean."

Contact staff writer Melissa Dribben at 215-854-2590 or