The New York City Council has passed a bill to reduce emissions from school buses. The legislation mandates that starting next school year, 600 older buses be retired and more than 6,000 be retro-fitted with pollution control equipment to reduce students' exposure to fumes. Isabel Silverman is a lawyer with the Environmental Defense Fund who helped draft the bill. She says the newer the bus the lower the emissions.

SILVERMAN: The Federal emissions standards have gotten progressively much much tighter. So, for example, if you buy a bus today that bus is 10 times cleaner than a bus purchased in 2006.

The retro-fit program will cost the city $20 million. Mayor Bloomberg says he'll sign the bill into law.

Health officials say exposure to diesel exhaust can cause and exacerbate asthma in children.