United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S4-85/036 Apr. 1987
Project Summary
A Sensitivity Analysis of the
Enhanced Simulation of Human
Air Pollution Exposure (SHAPE)
Model
Jacob Thomas, David Mage, Lance Wallace, and Wayne Ott
A sensitivity analysis was undertaken
of the Simulation of Human Air Pollution
Exposure (SHAPE) model, which in-
corporates an enhanced version of the
Coburn-Forster-Kane (CFK) physiologi-
cal model for predicting a person's blood
carboxyhemoglobin (COHb) as a func-
tion of time in response to carbon
monoxide (CO) exposure. The SHAPE
model simulates the physical activities
of a sample of people in an urban area,
exposing them to pollutant concentra-
tions from appropriate microenviron-
ments as they move through time and
space in a 24-hour period. The CFK
model dynamically calculates their blood
COHb from their CO exposures, and
their ventilation rate which is altered in
response to their level of activity. To
conduct this sensitivity analysis, the
SHAPE program was run many times
using different combinations of values
for its parameters, thus allowing the
contribution to COHb of each of many
variables to be examined. The following
phenomena were found to have signifi-
cant effect, over and above the influence
of the ambient CO on the predicted
frequency distribution of the maximum
COHb levels of the population: (1) CO
exposure contributed by the highway
microenvironment; (2) altitude of the
city; (3) the CFK physiological param-
eters (e.g., Haldane constant and en-
dogenous CO production). In contrast,
if it is assumed that a person using an
indoor parking garage spends less than
10 minutes there, then use of a parking
garage has very little effect on the COHb
frequency distribution of the population.
For low-level CO exposure, use of either
the linear or nonlinear form of the CFK
model yields essentially the same
results.
This Prelect Summary was developed
by ERA'S Environmental Monitoring
Systems Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that Is fully docu-
mented In a separate report of the same
title (see Project Report ordering In-
formation at back).
Introduction
The Simulation of Human Air Pollution
Exposure (SHAPE) model estimates
human exposure to carbon monoxide (CO).
It develops an "activity pattern profile"
for an individual by allocating each minute
of the day into any of 14 microenviron-
ments (home, car, office, parking garage,
etc.). By knowing where the person is and
approximately what he or she is doing,
another part of the program "exposes"
the person to CO concentrations appro-
priate for that microenvironment. Random
number generators provide minute-by-
minute concentration levels roughly
equivalent to those found in the actual
microenvironment, based on empirical
field studies. For each person, these
sequential microenvironmental exposures
are brought together to form an "exposure
profile" providing CO concentration and
estimates of the carboxyhemoglobin
(COHb) of the blood as functions of time
over the 24-hour period.
The previous version of the SHAPE
program used a simplified Coburn CO-
COHb model, a first-order linear differ-
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ential equation with three constant coef-
ficients. This formulation did not permit.
physiological and other subcomponent
parameters (for example, blood volume,
ventilation rate, altitude) to be varied by
themselves, because the latter param-
eters were lumped together into the three
constants.
The research summarized in the full
report covers enhancements to the
SHAPE model and a sensitivity analysis
of the enhanced model.
The goal of this analysis is to determine
how changes in a variety of fundamental
parameters (highway CO concentrations,
CO concentrations inside a parking ga-
rage, time patterns of ambient CO con-
centrations, and various Coburn physio-
logical parameters) affect the following
three dependent variables for the
population:
• Maximum 1 -hour average exposure
(M1AE)
• Maximum moving 8-hour average
exposure (MM8AE)
• Maximum 1 -hour blood carboxy-
hemoglobin(MICOHb)
In each case, we are interested in the
population's frequency distributions of
these variables and the impact of a change
in each parameter on these frequency
distributions. In general, we examined
changes in frequency distributions that
result while one parameter is allowed to
vary and all the others are held constant.
In some cases, two parameters were
varied at the same time.
Our simulation sample consisted of
400 non-smoking people employed in a
large urban area. Their simulated com-
mute habits are based on data from the
U.S. Census Bureau's "Nationwide Per-
sonal Transportation Study", (1973).
Modifications to SHAPE
The following enhancements have been
made to the SHAPE computer program:
• All individual Coburn parameters
appear explicitly.
• Both linear and the more accurate
nonlinear versions of the Coburn
model can be used to compute blood
COHb.
• Once the age and sex of the person
have been established, height is
determined by random sampling from
distributions based on data from the
National Health and Nutrition Ex-
amination Survey (HANES) con-
ducted by the National Center for
Health Statistics.
• Weight is determined from empirical
height-weight equations, also based
on sex and age.
• Blood volume is determined from
height and weight, and hemoglobin
content is randomly sampled from
the HANES data, based on sex.
• Some of the person's lung functions
(alveolar ventilation and diffusion
rates, for example) are allowed to
vary throughout the day and are
determined from the activities he or
she performs, based on metabolic
rate assumptions for each physical
activity level.
• For each person, the program runs
through an extra 24-hour exposure
sequence on the previous day in
order to set the initial conditions for
the day of interest.
Once these enhancements were made,
the program was run many times with
different combinations of values. Ambient
data for three different dates were used,
each with relatively high CO levels se-
lected from an earlier sensitivity analysis.
In this study, only subsamples of an
urban population were simulated for their
exposure experience to CO. The sub-
sample consisted of nonsmoking em-
ployed commuters during a week day. As
such, the distributions generated of maxi-
mum 1 hour CO exposure, maximum 8
hour exposure and maximum 1 hour
COHb are seen to have small standard
deviations and less than 10% coefficients
of variation.
Conclusions
The findings of the study are among
the following:
• Changes in CO levels inside auto-
mobiles exert a statistically signifi-
cant effect on computed blood COHb
of the population.
• Altitude exerts a statistically signifi-
cant effect on computed blood COHb.
• Changes in the levels of CO in indoor
parking garages have little effect on
total CO exposure and blood COHb
of the population due to the brief
time (10 minutes) assumed spent
there.
• For low-level CO exposure, the linear
approximation of the solution to the
Coburn equation does not differ
significantly from the nonlinear
solution.
This sensitivity analysis has identified
the relative significance of a variety of
parameters in generating estimates of
blood COHb and has provided insight into
the manner in which the parameters
affect person-to-person variability of blood
COHb. It also has identified the likely
contribution of various human activities
— driving a car, parking in a garage — to
population exposure distributions and
human dose distributions.
Jacob Thomas is with General Software Corporation. Landover, MD 20785;
the EPA authors,David Mage (also the EPA Project Officer, see below) and
Lanco Wallace, are with Environmental Monitoring Systems Laboratory,
Research Triangle Park, NC 27711; and the EPA author, Wayne Ott is with
Office of Acid Deposition, Environmental Monitoring and Quality Assurance.
Washington, DC 20460.
Tho complete report, entitled "A Sensitivity Analysis of the Enhanced Simulation
Lf Human Air Pollution Exposure (SHAPE) Model ." (Order No. PB 85-201
101 /AS; Cost: $18.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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