FUMING: School buses idling as they pick up children emit significant amounts of pollutants.
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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.