BOSTON — Nine school districts and bus companies throughout the state have joined the growing list of pupil transportation operations that have taken advantage of free diesel retrofit installations under the "MASSCLEANDIESEL: Clean Air for Kids" program.

This program encourages the Commonwealth's bus owners to install pollution-control devices on eligible diesel-powered school buses. There are now 18 operations statewide that have retrofitted 136 school buses to provide cleaner air benefits to more than 22,000 students.

"These investments will bring cleaner air to communities across the Commonwealth and serve as an example of how small changes can make a big difference to our environment," Gov. Deval Patrick said.

Participating in this round of MASSCLEANDIESEL school bus retrofits are: Joseph Ingle Bus Service Inc.; John B. Ormsbee Inc.; Acton Public Schools; Acton-Boxborough Regional Schools; Monson Public Schools; Southeastern Regional School District; Seekonk School Department; Upper Cape Cod Regional Technical High School; and Mohawk Trail Regional School District.

Collectively, these nine private and public bus owners have retrofitted 72 school buses that serve nearly 9,000 students across 32 municipalities in the state with diesel oxidation catalysts and/or crankcase ventilation systems. Firms based in New Bedford, Milford and Tyngsboro installed the equipment.

"The MASSCLEANDIESEL program is a great example of partnerships between federal, state and local officials working together to improve lives for our residents," Education Secretary Paul Reville said. "Our goal is to provide an excellent and equitable education for all our students and that starts with helping them arrive at school healthy and prepared to learn."

"I applaud the Patrick-Murray Administration for their continued commitment to clean air everywhere, but especially in our school environments," State Senator Benjamin B. Downing added. "These investments build off the 2008 School Campus Air Quality Act, which required all motorists to cease idling cars in school parking lots. Together, these seemingly small steps will produce great strides for the environmental health of our students, teachers and other employees."

The Massachusetts Department of Transportation provided $16.5 million in state and federal funding under an agreement associated with the Central Artery/Tunnel Project for the MASSCLEANDIESEL: Clean Air for Kids program.