NRDC: No Breathing in the Aisles: Diesel Exhaust Inside School ...

• Driving uphill increased the level of diesel exhaust inside the bus;
• Driving downhill sometimes resulted in higher levels of diesel exhaust than did driving on
the flats—this may be because the drivers shifted into a lower gear and used the engine to
slow the bus on steep hills or because smoke lingered in the bus from a recent uphill ascent.
• Levels of diesel exhaust in the bus did not rise when the bus stopped and idled—in fact, the
levels often decreased on idling;
• There was significant variability among individual buses sampled. Even two buses from the
same model year had different levels of diesel exhaust.
• A child sitting in the back of the bus with the windows closed would receive an average
exposure to diesel exhaust that is up to 4 times greater than a child riding in a passenger car
immediately ahead of the same bus. In addition, the extra exposure to a child inside the bus
was between one and 14 micrograms per cubic meter (mcg/m 3 ) higher than the exposure that
same child would get from walking or riding in a car on those same streets. Indeed, the
California Air Resources Board estimated that the average outdoor concentration of diesel
exhaust in California in 1995 was 2.2 mcg/m 3 . Thus, for the time a child is in a school bus,
his or her exposure to diesel exhaust may be up to 8-1/2 times the average statewide air
levels. Figure 1 illustrates, as an example, the average readings we obtained inside one
1986 school bus compared with simultaneous measurements inside a passenger car driving
directly ahead of the bus.
FIGURE 1
Diesel Particulate (mcg/m 3 )
Driving Conditions
Figure 2 shows an example of minute-by-minute measurements of black carbon simultaneously
inside the back of a 1986 bus and inside a passenger car driving immediately ahead
of the bus. Some of the sources of variability are marked. The only times when the levels of
diesel exhaust particles in the car briefly exceeded the levels in the bus were when the car was
behind the bus for a few minutes idling at a school, and when four windows in the bus were
opened to ventilate the interior of the bus.
8
30
25
20
15
10
5
0
Simultaneous Bus and Car Diesel Levels: 1986 Bus
Idling Uphill Dow nhill Overall
BUS
CAR