The fumes come from the tailpipe and from an engine vent on older diesels that may still have many serviceable miles still left on them. They can be retro-fitted with filters that make a dramatic difference. Dr. Bruce Hill is chief scientist with the Clean Air Task Force. He's checking emissions under the hood on a bus equipped with an engine filter.
"Here we are at the engine compartment of the clean bus. With the retro-fit device here there are no extra emissions coming from the engine. In other words, the air quality around the engine is the same as the outdoor air."
The scientists are running tests to determine how much particulate matter is in the air, and how much gets inside the buses. Dr. Ramon Alvarez is with Environmental Defense and was part of the team conducting tests along a route through a neighborhood in the Woodlands.
"We have equipment set up in the middle of the bus; it's going to be sampling air inside the bus. We also have a monitor right at the front of the bus. And what we're going to do is go on a route, a bus route, we make stops along the way, we get out and measure wind conditions and temperature."
The bus door is kept open for one minute at each of the points along the route where the bus stops and instruments measure particulate matter outside and inside the bus.
Tom Smith is with the Texas Office of Public Citizen. He says buses with no filters, one filter, or both tailpipe and engine vent filters are tested along the same courses and stop at the same locations.
"And what we've found was that the retro-fits were really working well. That as you add on the filters on the emissions controls, that the amount of pollution coming in the front doors drops dramatically. Today we're running on 24 on an emissions level here at the front door. Yesterday we spikes at ten times that level on the buses that weren't retro-fit."
In addition to making school districts aware of the problem Environmental Defense also helps them get grant money to help pay the costs of making older diesel powered buses as clean or cleaner then new ones.