School bus pollution studies rap diesel, push changes

Diesel Fuel News, Feb 18, 2002 by Jack Peckham

Two studies this month claim to find potential threats to children's health from diesel particulate matter (PM) pollution on school buses. One study recommends mandatory PM filter retrofits and ultra-low sulfur diesel (ULSD), while the other pushes compressed natural gas (CNG).

The study by advocacy group, Environment & Human Health Inc. (EHHI), directed by EHHI board member and Yale University professor John Wargo, found that fine-PM ([PM.sub.2.5]) levels measured on school buses "were often 510 times higher than average levels measured at the 13 fixed-site [PM.sub.2.5] monitoring stations in Connecticut."

Levels of fine PM were higher when diesel buses were idling with doors/windows open, when moving through heavy traffic, when buses followed other diesel vehicles and when idling or queuing-up to load/unload students, the study found.

Given potential or possible links suspected between aggravation of asthma or allergic responses to pollutants such as diesel exhaust, it makes sense to try to reduce school bus diesel PM emissions, the EHHI study authors concluded.

Study methodology isn't all that clear, however, as the model years, number of diesel buses, and their tailpipe emissions rates aren't reported by EHHI. But based on the rural routes of the buses tested, EHHI claims that the PM exposure levels are "probably underestimated."

However, it's uncertain if these buses were especially high-emitters, or typical diesel buses, or even new buses. One natural gas bus was also studied for comparison; it showed much lower PM emissions than the diesel buses, none of which had PM filters.

Researchers measured bus passenger compartment emissions levels on empty buses driving and stopping along typical routes, the study says. Separately, 15 students were also fitted with portable PM monitors and carried these around for an average seven hours/day.

Most of the report and most of the data cited are compilations of many other studies, other databases, and estimates about state-wide or nation-wide exposure of children to diesel emissions on or around school buses.

However, the EHHI study doesn't include contradictory findings from a recent Fairfax County (Va.) public school diesel versus-CNG bus comparison, nor a separate study on in-use school bus emissions levels by Yale University professor Jonathan Borak.

The Borak study, partially sponsored by school bus maker, International, finds problems with the use of Aethalometer measuring devices as employed in the EHHI study as well as an earlier California study sponsored by Natural Resources Defense Council (see Diesel Fuel News 2/19/2001, p7). Borak also compares the NRDC findings with the Fairfax County study (see Diesel Fuel News 4/30/2001, p9).

The Fairfax study employed a different diesel PM measurement technology called the NIOSH 5040 method.

Finally, Borak measured actual emissions taken from two school buses operating in an Ohio school district, as well as emissions from the new International "green diesel" school bus equipped with a PM filter.

These buses for the Borak study were run on a standard vehicle test track for comparisons, using both the Aethalometer technology and the NIOSH 5040 method.

This study showed that the Aethalometers "did not perform well for higher-level DPM [diesel particulate matter] exposures," Borak found in his peer-reviewed science study, soon to be published in American Industrial Hygiene Association Journal.

A key problem with the Aethelometer is that it doesn't tolerate shaking or movement very well, as it's designed for stationary-source PM readings, not mobile sources such as buses, Borak's study discovered. Still, it's possible that confounder effects (other nearby vehicles) could explain differences between some NRDC and Fairfax study findings, he said.

* EHHI: Take Safest Path

Aside from whatever methodology issues that might raise questions about EHHI's study findings, there's a "prudence factor" to its recommendations.

That is, some old diesel buses are likely to cause potentially hazardous increases in diesel PM emissions around schools and kids. So, it's prudent to require restrictions on bus idling, require retrofits with PM filters and ULSD, mandate periodic tailpipe testing, and push for federal/state government financial aid to retrofit or replace old buses with low-emissions or alternative-fuels buses, EHHI concluded.

But rather than wait for EPA's ultra-clean diesel regulations in 2006-2007, or push gradual turnover of the high-emitter, old buses, EHHI instead recommends that the federal government should mandate PM filters/catalyst retrofits of the entire existing bus fleet "by 2003," thus implying some immediate crisis that requires drastic, across-the-board action on all 600,000 U.S. school buses.

However, the data EHHI generated for its study aren't conclusive about all buses. So it's unknown whether all the diesel buses in Connecticut (or for that matter, anywhere else) are truly dangerous, or whether just some old, high-emitter buses are a possible danger, or whether the measurements themselves might overstate a threat, based on flawed Aethalometer methodology.