In a study published in Environmental Science & Technology, Qunfang Zhang and Yifang Zhu found that the concentration of outside air pollutants played a significant role in determining the in-cabin air quality of school buses. When idling, the in-cabin concentrations of particulates were not significantly lower after retrofitting. This makes sense, since the results show that tailpipe emissions are not the likely source of in-cabin air pollution. Retrofitted or not, it seems that outdated diesel school buses are still inefficient, yellow, tot-hauling machines.
[Source: Green Car Congress]