Over the years, school districts have responded to the growing alarm over diesel exhaust emissions that can be inhaled inside and outside school buses by retrofitting buses to cut tailpipe emissions. But a new study shows that while these measures may help make the buses cleaner on the outside, they have virtually no impact on the air quality inside bus cabins — and that's the air that kids are breathing during their ride to and from school.


Diesel engines like those on buses emit ultrafine particles that research has shown can easily penetrate the lungs and enter the bloodstream, potentially causing cancers, asthma and heart disease. A 2007 Yale University study found that children who ride a school bus are exposed to up to 15 times more particulate pollution than average because these diesel engine emissions are making their way into the bus cabin. As part of the Environmental Protection Agency's Clean School bus program, schools have been encouraged to retrofit buses to reduce these emissions.


But a new study has found that while these retrofits are making the air cleaner outside buses, they aren't doing anything to make the air inside the buses cleaner.  The study published in journal Environmental Science & Technology measured pollutant levels inside and outside the bus simultaneously with two sets of identical instruments. Researchers collected data while the buses ran actual routes and as they idled, measuring pollutant levels in the outdoor air near the bus, in emissions from the tailpipe, and inside the cabins. The results? The retrofitted buses cut tailpipe particle emissions by 20 to 94 percent, but researchers did not find any reductions in ultrafine particle concentrations inside the bus cabin after retrofitting.

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