At 2:33 p.m. in New York City’s East Harlem, four short yellow school buses pull to a stop in front of Reece School, a private elementary school for special needs children. The bus drivers pop their doors open and idle, engines running, while they await their young passengers.

As the students trickle out of the school, a clipboard-wielding teacher checks their names off her list and guides them to their buses. On the older buses, the engines rumble as the children climb aboard. At 2:40 p.m., the first bus is full and off it goes, down the hill, merging with traffic on Madison Avenue.

The second bus inches forward and stops. Six more round the corner onto 104th Street, all with engines running.

By 2:45 p.m., all the students are heading home.

Between dismissal and departure, the diesel engines idled for 12 minutes, spewing exhaust into the air in front of the school.

Idling longer than one minute in a school zone is illegal in New York City for all vehicles, but the laws are rarely enforced. Before dismissal, around the corner on Madison Avenue, a produce delivery truck idled for several minutes, double-parked—all while a traffic enforcement cop stood two cars down.

Idling buses, cars and trucks may not seem like a big deal, but in New York City they spew out as much pollution as nine million diesel trucks driving from the Bronx to Staten Island, according to the Environmental Defense Fund. That’s roughly 130,000 tons of carbon dioxide, 940 tons of nitrogen oxide, 24 tons of soot particles, and 6,400 tons of carbon monoxide each year.

Vehicles running on diesel fuel release fine particulate matter and elemental carbon—also known as black carbon. In studies around the world, particulates have been linked to deaths from respiratory disease and heart attacks. Diesel exhaust also contains several carcinogens and other toxic substances.

For school children, health experts say diesel exhaust presents a serious health concern—especially to asthmatics—because it can trigger asthma attacks.

In New York City, the asthma hospitalization rate is almost twice the national average, and neighborhoods in Northern Manhattan, including East Harlem, have among the highest rates of pediatric asthma hospitalization and mortality in the city. Asthma is the most common cause of hospitalization for children under 14 years old, and in New York City’s worst neighborhoods, an estimated 1 in 4 school children have asthma, according to the Asthma Free School Zone, a New York based non-profit group.

In May, scientists in New York reported that school zones’ concentrations of black carbon—a predictor of diesel pollution—are related to idling of buses and trucks at the end of the school day.

“If you find ways to either curb bus emissions or curtail bus idling, and cut traffic in general, children’s exposure to black carbon should be reduced,” said Jennifer Richmond-Bryant, lead author of the study. She was an assistant professor at Hunter College in New York at the time of the study, but is now with the U.S. Environmental Protection Agency. The Asthma Free School Zone was also involved with the study.

Because of the health threat to children, California in 2003 banned idling of school buses. Under the state law, California school bus drivers must turn off their engines when they arrive and restart them no earlier than 30 seconds before departing.

At least 17 other states also have state or local rules that limit idling, some for all vehicles and others for just buses.

In most cities, kids’ exposure is greatest while they are actually inside the bus. According to one study in Los Angeles, people are exposed to 40 times more exhaust while riding in a bus than they are while waiting at the bus stop.

But in New York City, pollution at bus stops is unusually high because of urban street canyons--tall buildings and narrow streets that trap pollution at the street level. Particulates at ground level can be up to 175 percent higher than on the rooftops, depending on which direction the wind is blowing.

In the new study, air quality was monitored over 16 days at the end of the school day. Monitoring devices were positioned near the Reece School, which is also near two other East Harlem schools. Minute-by-minute readings were taken of particulate matter and black carbon and the results were compared to background pollution, according to the study, which was published in the journal Science of the Total Environment.

The researchers found that fine particulates were higher than black carbon, but changes in concentration were not related to traffic, suggesting that the street canyon was trapping the particles. Black carbon concentrations, however, were significantly related to idling and passing diesel vehicles. Because background data on black carbon have never been recorded, researchers are unable to say whether it is significantly higher during the dismissal period.

The findings have prompted several environmental and community groups to start developing policies and educational programs to manage traffic, improve air quality, and reduce absenteeism related to asthma attacks.

When the Asthma Free School Zone was founded, director Rebecca Kalin took on the idling problem by targeting the worst offenders.

“The most egregious example was yellow school buses, and nobody seemed to be paying attention at all,” said Kalin.

After a phone call in 2004 to then Attorney General Eliot Spitzer, a representative from Mayor Michael R. Bloomberg’s office met with Kalin the next day, and within eight months, the city sued the largest school bus companies, forcing the school bus operators to train drivers to limit idling.

In April, the maximum idling time in a school zone was reduced from three minutes (the maximum time elsewhere in the city) to one minute. The law, however, is difficult to monitor and rarely enforced.

A spokesperson for the New York City Department of Education said there is little that schools can do to make sure bus drivers comply because buses are privately operated and enforcement is the police department’s job.

“I’m really concerned about the city situation, but it’s improving all the time,” Kalin said. There’s a loop-hole in the law that exempts private schools and doesn’t require them to post no idling signs, she said.

The New York Department of Environmental Conservation granted traffic enforcement officers the authority to ticket school buses that violate idling laws in an effort to encourage bus companies to retrofit their buses. The fines begin at $220, and reach $2,000 for repeat offenders.

Isabelle Silverman, an attorney with the Environmental Defense Fund, said it makes sense to allow these officers to ticket idlers since “they are out there walking the streets, breathing in the pollution.”

“We want them to fix the problem, not to just have to pay,” said Anhthu Hoang, General Counsel for West Harlem Environmental Action, Inc., a community environmental justice group. “We want them to lower their emissions.”

California’s Air Resources Board also has no information on whether its ban on idling school buses is being enforced.

Federal regulations, however, are starting to clean up diesel vehicles, including school buses. In 2007, the EPA required better emission controls on new diesel truck and bus engines, such as soot filters and cleaner fuel.

Under the new EPA standards, a school bus made in 2008 will produce 90 percent less soot than a bus made in 2006. The retirement age for buses was also reduced from 19 to 16 years, so as the bus fleet turns over, the buses will become cleaner. Some buses will even be fitted with systems that are capable of monitoring idling times.

Bloomberg launched a “Turn It Off!” campaign with ads on radio, bus stops and billboards, encouraging drivers to stop idling in the city.

Advocacy groups are encouraged by the attention in recent years, and are hopeful that the momentum from new laws will lead to greater awareness. But the problem is far from solved, as seen by the continued idling of buses and cars as schools leave school each day.

Cars are not a significant source of pollution near the schools, probably because gasoline contributes less black carbon and particulate matter than diesel fuel, the New York City study found. However, the Environmental Defense Fund and the Asthma Free School Zone are targeting all vehicles with their anti-idling campaign.

“It all adds up. It’s all unhealthy,” Silverman said. “It’s right at the curbside, at stroller level, and it’s totally unnecessary. Turning it off is a win-win for everybody.”