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

with children. We were unable to account for this possible source of underestimation in our
analysis. This study also did not evaluate the additional diesel exhaust children would be
exposed to at bus stops or in front of the school while waiting to load the bus. Therefore, this
study could underestimate a child’s exposure from riding to school each day. We also did not
perform this sampling protocol on any very old or very new buses, so variability related to the
age of the bus is not directly addressed in this study.
Two types of sampling equipment allowed continuous measurements of fine particles in
the air (PM2.5) and black carbon. Because it is not possible to directly quantify diesel exhaust,
scientists test for indicators that are characteristic of diesel exhaust. Diesel exhaust contains
gases, vapors, and tiny sooty particles. The particles are considered to be among the most
toxic components of the exhaust, and are so small that they penetrate deep into the lungs. The
DataRAM Real-Time Aerosol Monitor (Monitoring Instruments for the Environment, Inc.,
Bedford, Mass.) provides minute-by-minute measurements of fine particles in air (PM2.5). We
used DataRAM instruments in both the car and the bus to compare tiny particles in each vehicle.
More particles were measured in the bus than in the car; these additional particles were
most likely from diesel exhaust. Details about the DataRAM are provided in Appendix D.
The second instrument measures an even more specific indicator of diesel exhaust. The
Aethalometer Real-Time Aerosol Analyzer (Andersen Instruments, Inc., Atlanta, GA)
provides minute-to-minute measurements of black carbon particles in the air. Black carbon
particles are a telltale emission from diesel engines; diesel, in turn, is the major source of
black carbon particles in air. The Aethalometer therefore confirms that the tiny airborne
particles of smoke actually come from a diesel engine. Details about the Aethalometer are
provided in Appendix C. Comparison of the Aethalometer readings in the car and the bus
allowed a very accurate continuous measurement of the difference in black carbon particles
between the two vehicles and a very good estimate of the diesel smoke attributable to the
school bus. When run side by side under a variety of conditions, the aethalometers generally
provided measurements that were within one percent of each other.
Because black carbon is only one portion of the entire mixture that comes out of a diesel
tailpipe, it is necessary to multiply the black carbon measurements with a correction factor to
derive an estimate of the total diesel particulate. The results below are measurements of black
carbon from the Aethalometer, adjusted to represent estimates of total diesel exhaust
particulate. 26
RESULTS
There were significantly higher levels of black carbon particles inside the school buses
compared to outside the buses on the same streets and compared to inside passenger cars
driving along the same streets immediately ahead of the buses. This means that there were
higher levels of diesel exhaust inside the school buses where children ride. The levels were
variable, but some patterns emerged:
• Diesel exhaust is higher in the back of the bus compared to the front of the bus;
• When some windows on the bus are open, the levels of diesel exhaust go down, and when
all of the windows are closed, the levels rise;
7
There were higher
levels of diesel
exhaust inside the
school buses where
children ride.