Earth Protect Blog

Air Pollution: Tailpipe controls on buses don't clear the air inside bus cabins

As iconic yellow buses transport children to and from school, their diesel engines deliver big doses of air pollutants. In response, school districts have started retrofitting their buses to cut tailpipe emissions. But a new study shows that while these measures may help clean outdoor air, they have virtually no impact on the air inside bus cabins.

Bus diesel engines emit ultrafine particles that can easily penetrate lung tissues and enter the bloodstream, potentially causing cancers, asthma, and heart disease. Experts thought that these emissions worked their way into the bus cabin, resulting in elevated cabin pollution levels. The average concentrations of air pollutants inside a school bus can be three to 15 times greater than in average urban air, according to previous studies.

Other research has shown that retrofitting old buses with catalytic converters and other devices to reduce and capture emissions from the vehicles' exhaust systems can cut tailpipe emissions. But the effects of these modifications on air quality inside the bus have not been clear. Some studies have found reduced pollution levels inside buses, while others detected no significant changes. The confusion may stem from previous studies not controlling for confounding factors such as variations in weather, outdoor air pollution, or the make and age of buses, say Qunfang Zhang and Yifang Zhu, environmental scientists at the University of California, Los Angeles.

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