Summary of Findings
- Diesel Buses: Each day, nearly 600,000 school buses
transport 24 million students to schools in the U.S. Within Connecticut,
nearly 387,000 children ride 6,100 school buses, and most are powered
by diesel fuel.
- Children’s Time on Buses: The time spent on
buses by individual students varies between 20 minutes and several
hours per day. For one child, a half-hour ride to school, and a half-hour
ride home each day amounts to 180 hours per school year—90 full
24- hour-days over 12 years of school. Annually, U.S. children spend
3 billion hours on school buses. Connecticut children spend 50 million
hours on buses each year.
- Background Particulates: Connecticut background
fine particulate matter levels (PM2 . 5) a re near or above national
standards, when averaged over 24 hours. Childre n’s exposure
to diesel exhaust from school buses constitutes an additional exposure
beyond backgro u n d l e vels of particulates re p o rted from current
- Background Ozone: Connecticut is not in compliance
with current federal ozone standards. In 2001, portions of the state
exceeded the 8-hour limit on 26 days, and the 1-hour limit was e xceeded
on 9 days. Ozone is known to exacerbate asthma, and is normally highest
in the afternoon, when childre n’s exposure to diesel particulates
fro m school bus rides is also likely to be high. NOx p recursors to
ozone have increased over the past 10 years. In 2001, nearly 109 million
people lived in 272 counties where federal ozo n e limits we re exc
e e d e d.2
- Carcinogenicity of Diesel Exhaust: Diesel exhaust
is classified as a probable human carcinogen by many governmental authorities,
including the International Agency for Research on Cancer (WHO), the
U.S. National Toxicology Program, the U.S. Environmental Protection
Agency, and as a known carcinogen by the State of California. The California
South Coast Air Quality Management District recently estimated that
nearly 71% of the cancer risk from air pollutants in the area is associated
with diesel emissions. Diesel exhaust includes benzene, 1,3-butadiene,
and soot, all classified as known human carcinogens. Nearly 33 studies
have explored the association between diesel exhaust exposure and bladder
cancer. A recent meta analysis of this literature found increased risk
between 18-76%. These findings are based primarily upon studies of
truck drivers, railroad workers, bus drivers and shipyard workers.
- Diesel Exhaust Contains 40 Hazardous Air Pollutants: In
addition, diesel exhaust contains both carbon particulates and 40 chemicals
that are classified as “hazardous air pollutants” under
the Clean Air Act.
- Particulates and Respiratory Diseases: Exposure
to particulates has been associated with: increased mortality among
those with cardiopulmonary diseases; exacerbation of symptoms for asthma,
bronchitis, and pneumonia; decreased lung function; and retarded lung
development. It has also been correlated with increased hospital admissions
and emergency room visits for respiratory illnesses.
- Children’s Susceptibility: Children may be
especially susceptible to adverse respiratory effects following exposure
to fine-diameter particulate matter (PM2.5) emitted from diesel engines.
Nearly 94% of diesel particulates have diameters less than 2.5 micrometers
(um).4 The average diameter of diesel particulates is 0.2 m i c ro
m e t e r s. Smaller particles are able to penetrate children’s
narrower airways reaching deeply within the lung, where they are more
likely to be retained. Higher rates of respiration among child ren
may lead to their higher exposure, when measured per unit of their
b o d y weight.
- No Known Safe Exposure to Diesel Exhaust: There
is no known safe exposure to diesel exhaust for children, especially
those with asthma or other chronic re s p i r a t o ry disease. T h
e re is no single standard for acceptable cancer risk from diesel exhaust
in the U.S.
- Asthma Prevalence: Nationally, 4.8 million children
have asthma. More than 44,500 Connecticut school children have the
- Asthma Costs: Asthma costs an average of $500 per
child per year for medications, physician care, and hospital treatment.
Annual direct medical costs are estimated to be nearly $22 million
for Connecticut school students alone. This estimate does not account
for other costs that often include school absenteeism, lost parental
work while caring for ill children, psychological effects, and abnormal
- Children’s Exposure to Particulates on Buses : Children
were exposed to airborne p a rticulate concentrations in tested buses
that we re sometimes 5-15 times higher than b a c k g round levels
of PM2 . 5.
- Variability Within Buses: Particulate and black
carbon levels vary within individual buses over time. The most important
influences on variability include: bus idling behavior, queuing practices,
bus ventilation via windows, and outdoor concentrations on bus routes.
Particulate and carbon concentrations did not vary by sampling location
within diesel buses, e.g., front vs. rear. Engine model, age of engine,
number of miles since last overhaul, maintenance cycles, location of
bus engine (front, next to driver, or rear), elevation change, passenger
load, and climate may all influence levels of interior pollutants and
- Exhaust From Other Traffic: The intensity and type
of traffic along bus routes significantly affects air quality on buses.
Buses following diesel-powered vehicles, including other buses, h a
ve increased levels of carbon and particulate concentrations within
passenger compartments. Pa rticulate levels rose rapidly within the
passenger cabin when buses pulled behind other diesel vehicles in traffic.
No buses tested had air filtration equipment capable of re m oving
the fine particles detected in the buses.
- Idling Buses: Idling buses tested had higher concentrations
of particulates and carbon than moving buses. Higher concentrations
occurred when idling buses had open windows when compared with buses
with closed windows. There is a current Connecticut Department of Environmental
Protection (DEP) regulation, DEP 22a-174-18 (a)(5), that limits idling
time to 3 minutes, yet it is neither monitored nor enforced.
- Queued Idling Buses: Queued idling buses had the
highest levels of particulates and black carbon measured. Idling buses
tend to accumulate diesel exhaust which may be retained during the
ride, depending upon bus ventilation rates. Particulate and carbon
concentrations rise rapidly once idling begins.
- Length of Bus Route: The length of bus routes affects
the magnitude of childre n’s e x p o s u re to air pollutants
in the interior compartment. Time in transit between home and school
spent by Connecticut students varied between 20-180 minutes per day
in the towns sampled. The longest routes may occur in the rural parts
of the state, especially in large regional school districts.
- Lower Emissions From Natural Gas Buses: Natural
gas buses studied emitted 60- 98% less carbon than diesel-powered buses.
- Findings Are Likely to Underestimate Exposure: Exposures
to carbon and particulates found in this study were measured in environments
with exceptionally low traffic and few other sources of pollution.
Most children are exposed to additional pollution from traffic and
other residential, commercial and industrial activities. These findings
therefore are likely to underestimate levels of fine particulates and
carbon found in more urban areas and routes with higher traffic intensity.
- Additional Sources of Particulate Exposure Threaten Children: Residential
use of tobacco products, wood stoves, candles, kerosene heaters, and
poorly ventilated cooking stoves are for many children additional sources
of exposure to carbon-based particulates and organic gases that result
from combustion. Federal and state monitoring efforts fail to account
for these exposures despite the fact that most people spend more than
80% of their time indoors. Most epidemiological studies that associate
PM10 levels with adverse respiratory health effects consider particles
measured by outdoor stationary monitoring facilities, neglecting indoor
- School Buses Are Exempt From Emissions Testing: School
buses are currently exempt from routine emissions testing in Connecticut.5
There is no federal requirement that all state governments monitor
school bus emissions, although some states require testing.
- Federal Particulate Standards Exceeded: EPA estimates
that in 2000, 11 million U.S. children lived in areas that exceeded
one or more federal air quality standard. Nearly 3.5 million children
lived in areas where the particulate standards were exceeded in 1998.
Within Connecticut, bus exposures when combined with background outdoor
particulate levels may elevate children’s average daily exposure
beyond the current federal 24-hour PM2.5 standard.
- Absence of Passenger Cabin Air Quality Standards: Current
law does not regulate air quality within buses.
- Federal Monitoring vs. Personal Monitoring: Federal
law and regulation permit the testing of air quality by means of fixed
monitors. In Connecticut, 13 fixed monitors measure PM2.5. This sampling
design fails to capture the local variability and severity of air pollution
in the state. National standards permit averaging particulates over
24- hour periods. These practices ensure that shorter episodes of intense
pollution—such as those experienced in bus rides—are neither
recognized nor regulated by the state or federal government.
- Tougher Federal Diesel Standards Delayed Until 2006: Tougher
new diesel emissions standards will not be phased in until 2006. This
delay poses respiratory health threats to Connecticut citizens, who
now experience air pollution at levels above acceptable federal standards
for ozone. Compliance with current standards does not ensure health
protection. EPA estimated that the new standards would result in 8,300
fewer premature deaths, 17,600 fewer cases of childhood acute bronchitis,
and 360,000 fewer asthma attacks. These estimates demonstrate the scale
of respiratory health threat EPA believes exist under current conditions.
- Federal Particulate Standards: The exposures identified
in this study will not be affected by the tougher federal PM standards
adopted in 1997 (which are different from the diesel standards described
in 26 above), since monitoring to determine compliance with the PM
standards is done outdoors.
- Bus Parking Yards: Bus parking and maintenance facilities
have the potential to create localized particulate air pollution that
far exceeds ambient outdoor levels reported from State monitoring efforts.
Pollution may routinely migrate to adjacent properties, as buses are
left idling, or during periods of peak use—early mornings and
afternoons. If vehicles are parked near schools, both outdoor and indoor
school air quality may be diminished.
- Bus Drivers: Bus drivers’ exposure to motor
vehicle and diesel exhaust is significantly higher than children’s,
due to longer periods of time spent on buses.