An exhaustive survey shows the state’s big yellow school buses will be in good shape when they go back to work in about a month. The emissions of nearly all of the state’s 45-hundred diesel school buses were tested under voluntary plan between the schools and the state. Erin Heiden is a spokesperson for the Mirenco company which stuck a probe in the tailpipe of the buses to see if they’re letting off dangerous smoke.The pass a light beam through the smoke to measure the “opacity” or how much light passes through. If no light passes through, that shows the bus is emitting dangerous particles that could get into kid’s lungs.She says a high reading is an indication the bus needs some type of maintenance on its engine. She says the majority of Iowa’s buses passed the test.Heiden says 58 percent of the buses passed the test without the need for a tune-up. The remaining buses were fixed up. The tests are paid for through private donations and will be repeated twice a year for the next five years.