The entire New Haven school bus fleet has converted to Ultra Low Sulfur Diesel to reduce the threat of diesel emissions to New Haven’s children, Mayor John DeStefano Jr. announced at a press conference Wednesday.

The New Haven Public Schools entered into a one-year agreement with Sprague Energy to supply more than 500,000 gallons of the fuel known as ULSD — a diesel blend that reduces harmful exhaust by 25 percent — to the city’s fleet of 251 buses. The $75,000 conversion was made possible through funding from the Environmental Protection Agency.

“We made the switch to Ultra Low Sulfur Diesel fuel for the sake of the children and the environment,” DeStefano said. “We wanted to be one of the first in the state to start providing a healthier means of transportation.”

A recent study by Environment and Human Health, Inc., reported that diesel exhaust from school buses poses a significant threat to children’s health, increasing their susceptibility to a variety of illnesses including chronic asthma. The Centers for Disease Control and Prevention estimated that 44,500 school-aged children in Connecticut have been diagnosed with the disease. New Haven has the highest rate of childhood asthma in the state with 25 percent of the city’s 20,000 school-aged children diagnosed with the disease.

“We are extremely pleased that the New Haven school buses are using ULSD, making them compliant with EPA mandates nearly four years ahead of the rest of the nation,” said Nancy Alderman ’94, president of Environment and Human Health, Inc.

Purchasing ULSD is the first step in New Haven’s comprehensive school bus retrofit program, which includes fitting school buses with diesel particulate filters. These two measures together will eventually reduce diesel particulate emissions from school buses by 90 percent, according to a press release from DeStefano’s office.

Wednesday’s press conference was held at Strong School, one of New Haven’s magnet schools and a location with particularly poor air quality, Alderman said. In Strong alone, there are at least seven severe cases of chronic asthma where children have been hospitalized, principal Miriam Camacho said. She said the asthma detracts from the students’ education and their attendance is severely affected.

Rose Zanders, a counselor assistant at Strong, leaves work every day a few minutes before the children are dismissed. When she gets into her car, she can smell the smog emissions from the buses lined up outside even before she rolls down her windows.

“If it bothers me, I can only imagine what it’s like for the kids,” Zanders said.

This should not be a problem anymore. In an effort to limit fuel emissions, the Connecticut passed Public Act No. 02-56 this year, which states that school buses cannot stay parked with their engines on for more than three consecutive minutes.

But Wednesday afternoon two school buses parked outside Strong had their engines running. Alderman became visibly upset, told the bus drivers they were breaking the law, and threatened to call the authorities.

ULSD fuel is widely used in Europe and is expanding across the United States, a press statement from Sprague Energy said. New Haven has already started using ULSD for the entire fleet of heavy-duty city vehicles.

“This is is an important first step,” said Yuland Daley, asthma project director for the New Haven Health Department. “There’s a lot more that needs to be done. Ultimately, we would like to see a statewide solution.”