According to a study by the Harvard School of Public health, heavy exposure to black carbon diesel exhaust from a bus, can cause a 3.5 percent drop in IQ.

Dr. Howard Frumkin, with Environmental and Occupational Health says, "Short term, the air in the buses is cleaner when the windows are open. So parents and school districts and bus drivers ought to know that weather permitting, it's better to keep the windows open."

He also says that bus drivers and parents should encourage kids to sit toward the front of the bus, because exhaust fumes tend to collect in the back of the bus.