10/24/94

     Please find in the attached zipped files final peer reviews
of the Motor Vehicle-Related Air Toxics Study.  The five peer
reviewers were Tadeusz E. Kliendienst, Andrew Sivak, Thomas H.
Stock, Jaroslav J. Vostal, and Clifford P. Weisel.  Also included
is a summary of the most significant comments received from the
peer reviewers.  These reviews will be included in the docket for
the Motor Vehicle-Related Air Toxics Study (docket no. A-91-19). 
At this time, we do not plan to revise the study based on these
comments.  The reviews are being provided to the Regulation
Development and Support Division (RDSD) as input for the air
toxics rulemaking.

     Please contact Rich Cook (313-741-7827) if you have any
questions or comments. 

===================================================================

      Summary of Peer Reviews of Motor Vehicle-Related Air
                          Toxics Study

1) Tadeusz E. Kliendienst

a) Recent research indicates that oxidation of hydrocarbons leads
to formation of products (e.g., peroxyacetyl nitrate, PAN) that
are substantially more mutagenic than precursors.  Also, NOx
limited systems tend to be less mutagenic.  The significance of
these observations with respect to human health impacts is
unknown.

b) In EPA's urban airshed modeling of benzene in St. Louis, there
is a large discrepancy between modeled and measured values of
benzene.  Possible explanations for this discrepancy should be
considered.

c) In EPA's urban airshed modeling of 1,3-butadiene in St. Louis,
there is a rapid rise in 1,3-butadiene after 20:00 hours LST when
NOx levels are highest right after sunset, and 1,3-butadiene
reacts relatively rapidly with NO3.  This apparent inconsistency
should be addressed.  

d) It is important to note that at the present time, ambient
concentrations of toxic compounds cannot be accurately predicted
based solely on emission rates, atmospheric dispersion, chemical
removal and formation, etc.  At the present time, experimental
measurements of toxic compounds are the most reliable means of
obtaining ambient concentrations and they must certainly be used
to validate air quality models for toxic compounds.

2) Andrew Sivak 

a) EPA should include three recent human studies relating benzene
exposure and leukemia in its benzene risk assessment.

b) Since there are non-linear dose-response data for benzene, it
is unclear how the Agency can state that they support a
theoretical linear low dose extrapolation.

c) EPA should have used the 1991 draft unit risk for
formaldehyde, based on monkey and rat DPX data, in its
calculations, rather than the 1987 unit risk.

d) EPA should have used newer and more rigorous NTP
carcinogenicity data for 1,3-butadiene to calculate unit risk
estimates used in the study.

e) EPA should have included more discussion of the mutagenicity
of polycyclic aromatic hydrocarbons and nitrated polycyclics in
the gaseous phase of diesel exhaust.  

f) The use of a linearized model for the calculation of risk from
particle exposure is simply wrong scientifically.

g) There appears to be no cause for a public health concern for
cancer from exposure to motor vehicle exhaust, and it would seem
that the agency could use its resources more productively on
matters of higher concern.

h) The Agency should consider recent publications by Adam Finkel
and others on communicating uncertainties in risk assessment
calculations.

3) Thomas H. Stock 

a) Although use of HAPEM-MS is a laudable first effort in
assessing the health impact of exposure to air toxics from motor
vehicles, it has many severe limitations which may render it
inadequate for its intended use.

b) Although HAPEM-MS is a national exposure model, the
microenvironmental exposure factors it uses to convert fixed-site
concentrations to microenvironmental concentrations are based on
data from only one city, Denver.  Thus, there are many questions
on how much these data can be used to generalize to other cities.

c) EPA developed an integrated exposure adjustment factor based
on activity data from a California study.  It is questionable how
representative this is of the nation as a whole.

d) Given the limitations of the HAPEM-MS model, it is reasonable
to "correct" modelled exposures to agree with adjusted ambient
data, until a more realistic model is developed.

e) At this point in time, there are insufficient data on any air
toxic to explore the relationships among ambient,
microenvironmental, and personal exposure.

4) Jaroslav J. Vostal

a) Any new assessment of the potential public health risk of
diesel particles should include recent data which indicate
carcinogenicity of diesel exhaust is not associated with
particle-bound organics.

b) Tumor responses to diesel particles are not linear at low
ambient levels; thus, applicability of the linear dose-response
model is restricted.

c) Since existing epidemiological data for diesel exhaust
exposure does not include documented exposure, the use of
epidemiological data in the risk assessment remains questionable.

d) EPA's default model of linear multistage extrapolation has no
universal validity and needs to be replaced with more realistic
approaches once mechanisms are better understood.

f) Reanalysis of study results should take into account EPA's new
risk assessment guidelines.

g) The application of threshold-displaying models may reduce risk
estimates for diesel particulate matter, formaldehyde,
acetaldehyde or gasoline vapors to levels not statistically
different from zero.  The same mechanisms may potentially apply
also for benzene.  This leaves 1,3-butadiene as the only
measurable risk of the health effects of air toxics from mobile
sources.

h) The magnitude of uncertainties and numerical results should be
presented in the form of frequency distributions or iterative
computer runs rather than by point values.  Also, sensitivity
analyses should be conducted on the variability and distribution
of different ambient concentrations or exposure estimates and the
final results should be presented as ranges starting from zero up
to the upper bounds.

5) Clifford P. Weisel

a) EPA's HAPEM-MS model is severely limited by extrapolating
ambient CO measurements to microenvironmental CO measurements
nationwide from a single study done in Denver during four months
of the winter.  These extrapolations need to be validated in
other regions of the country in other seasons.

b) Time activity pattern data for areas other than just
Cincinnati need to be included in the HAPEM-MS model.

c) The HAPEM-MS model included five microenvironments, but
others, such as public and private parking garages, need to be
included.

d) It is problematic to extrapolate air monitoring data collected
for CO to air toxics because: (1) CO is purely an exhaust
emission, while some air toxics such as benzene have
contributions from both exhaust and evaporative emissions; 
(2) atmospheric reactivity of CO and some air toxics is very
different.

e) Exposures in microenvironments should be considered in
calculating cancer risks, since these exposures can contribute a
measurable percent of the total daily exposure.

f) The inclusion of more accurate estimates of microenvironmental
exposure data are expected to produce higher exposure estimates
than presented in the study and to reduce the uncertainty of the
estimates.