The first of the "green" buses hit the streets of Los Angeles in 1993, nearly 20 years ago. Since the time these buses have rolled out, they've logged more than a billion miles on L.A. streets.
MTA officials say that's a billion clean-air miles.
"They are 97 percent cleaner than diesel buses and they are replacing these diesel buses. As a result, we are reducing cancer-causing particulate matter by 98 percent, carbon monoxide by 80 percent, green house gas by over 20 percent," said Los Angeles County Supervisor Mike Antonovich.
While most of the buses have compressed natural gas engines, there is one electric and six gasoline electric hybrid buses in the fleet. Officials say they've slashed the fleet's fuel cost in half with the CNG buses.
Some Metro riders believe CNG buses and other clean-air efforts have gone a long way in getting rid of dirty air.
"As they say, it's the nation's largest clean air fleet and we're proud of that," said Los Angeles resident Jim Anderson.
"Twenty years ago you could not run without breathing hard. It's like you had asthma or something. Now you can run like the wind and nothing happens. It's great," said resident Johnny Zolinierczyki.
MTA officials say while CNG buses cost about 10 to 15 percent more to operate than diesel engine buses, mainly because of increased maintenance costs, the health benefits make it worth every penny.