U.S. Environmental Protection Agency
Office of Transportation and Air Quality
(formerly: Office of Mobile Sources)

January 7, 2000


     Several documents and technical reports on assessment of
highway vehicle toxic emissions have been posted here. These
documents were used in the Final Regulatory Impact Analysis for
Tier 2 Motor Vehicle Emissions Standards and Gasoline Sulfur Control
Requirements. 

The documents are:

1.  EPA. 1999.  Final Technical Report on the Analysis of Carbon 
Monoxide Exposure for Fourteen Cities Using HAPEM-MS3. Report No. 
EPA-600/R-99/096 (NERL-RTP-0-661). Prepared by G. Glen and D. Shadwick, 
Mantech Environmental Technology, Inc. for the U.S. Environmental 
Protection Agency, March 1998.

     This contractor report contains a description of the HAPEM-MS3b
exposure model, presents model results and discussion, and
provides an uncertainty analysis.  EPA used the CO exposure
results as a tracer for toxics exposure.  

     A comprehensive database of all CO exposure modeling results
has been placed in the docket for the Tier 2 Final Rulemaking and 
is also available in electronic form from EPA upon
request. Individuals or organizations interested in obtaining
this information should contact Pamela Brodowicz at
(734)214-4364.

2.  E. H. Pechan and Associates, 1997.  Determination of Annual
Average CO inventories and The Mobile Source Contribution in
Selected Urban Areas Using the 1990 OAQPS Trends Data Base.
Prepared for U. S. EPA, Office of Mobile Sources, Assessment and
Modeling Division, Ann Arbor, MI, Pechan Report No.
97.09.013/1817, September, 1997.

     The Pechan report provides OTAQ with the CO emission inventory
data needed for the HAPEM-MS3 modeling in order to allocate the
CO to only onroad mobile sources.  This data includes the 1990
annual average CO emission inventories for the 13 urban areas
with a breakdown of the amount for CO contributed by onroad and
nonroad mobile sources.  This report also provided the seasonal
daily average CO inventories from the mobile sources in these
urban areas, broken down by onroad and nonroad contributions.
The data source used for this analysis was the draft 1997
National Emission Trends Inventory.  

3.  EPA, 1999. Analysis of the Impacts of Control 
Programs on Motor Vehicle Toxics Emissions and Exposure in Urban 
Areas and Nationwide.  Prepared for U. S. EPA, Office of 
Transportation and Air Quality, by Sierra Research, Inc., 
and Radian International Corporation/Eastern Research Group.  
Report No. EPA 420-R-99-029/030.

     A draft of this contractor report was subjected to
technical peer review.  This final report presents emissions and 
exposure estimates for highway vehicles.  Nonroad engines are 
not included in this assessment.  The following compounds are 
included --benzene, acetaldehyde, formaldehyde, 1,3-butadiene, 
MTBE, and diesel particulate matter. Base scenarios for 1990, 1996, 
2007, and 2020 were included in the assessment, as well as several
control scenarios in 2007 and 2020.  Toxic emissions and exposure
were modeled for the following urban areas: Chicago, Denver,
Houston, Minneapolis, New York, Philadelphia, Phoenix, Spokane,
and St. Louis.  Sensitivity analyses were also performed.    

     The gaseous toxics emissions model used in these analyses,
MOBTOX5b, is unofficial. It includes revisions to emission rates 
and other elements in MOBILE5b intended to reflect proposals for 
MOBILE6.  Diesel particulate matter emissions were modeled using 
PART5. It should be noted that PART5 assumes in-use vehicles meet 
PM standards and calculates total emissions based on this assumption. 
 
     It should be noted that analyses in this report focus on
average exposures for several demographic groups.  We recognize
that distributions of exposures and of most exposed individuals
within demographic groups is an important aspect of risk
characterization.  EPA is developing such estimates.  The 
HAPEM-MS3 report described above does contain information on 
distribution of 1990 CO exposures, which should be indicative of 
motor vehicle toxics exposures.  

    It should also be noted that HAPEM-MS3 was used to assess 
exposure to ambient air toxics, and does not account for exposures 
originating within microenvironments.  For instance, the model would 
not account for exposure to evaporative benzene emissions indoors 
from vehicles parked in attached garages, or to vehicles during 
refueling.  Furthermore, the modeling done in this assessment assumes 
that the highway fleet (gasoline plus diesel) emissions ratio of CO 
to diesel PM can be used as an adjustment factor to convert estimated 
CO personal exposures to diesel PM exposures.  This assumption does 
not take into account that diesel-fueled vehicle temporal and spatial 
patterns are different than gasoline-fueled vehicles. 

     Finally, the analyses do not account for exposures to aldehydes
formed secondarily in the atmosphere from chemical transformation
of other compounds.

     OTAQ is currently conducting a formal risk assessment on health 
risk of toxic emissions from mobile sources.

     MOBTOX5b, as well as model code, input files, and output
files are also available from EPA in electronic form upon request.  
Individuals or organizationsinterested in obtaining this information 
should contact Rich Cook at (734)214-4827.

4.  Peer Review Comments on the April 1998 draft contractor report.

     EPA conducted a formal peer review of the above draft report.
Peer reviewers were asked to comment on methodologies and 
assumptions used in EPA's assessment.  Comments were provided by the 
Northeast States for Coordinated Air Use Management, Mark A. DeLucchi 
of the University of California, Davis, and Ted Johnson of TRJ
Environmental.