They took readings while idling with the motor running, while traveling up and down hills or driving slowly, and while making frequent stops. And most of the buses tested did not emit a significant amount of visible black smoke. The results were alarming. Our study found that levels of diesel exhaust inside a school bus can be four times higher than those found in passenger cars driving just ahead of the bus. And the diesel levels inside the buses were more than eight times the average diesel-exhaust content of California air.
Diesel exhaust has serious health effects that have been extensively documented.
For instance, numerous studies have shown that diesel fumes cause cancer, particularly lung cancer. In fact, government regulators estimate, based on lifetime risks, that diesel exhaust is responsible for 125,000 cancers nationwide. Other health effects are also troubling, though harder to quantify. In addition, exposures early in life can return to haunt them as they age, in the form of chronic health problems. Not all, but most.