United States
Environmental Protection
Agency
National Risk Management Research
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-96/037 May 1996
EPA Project Summary
IAQ Model for Windows, RISK
Version 1.0: User Manual
Leslie E. Sparks
A computer model, called RISK, for
calculating individual exposure to in-
door air pollutants from sources is pre-
sented. The model is designed to cal-
culate exposure due to individual, as
opposed to population, activity patterns
and source use. The model also pro-
vides the capability to calculate risk
due to the calculated exposure. RISK
is the third in a series of indoor air
quality (IAQ) models developed by the
Indoor Air/Radon Mitigation Branches
of U. S. EPA's National Risk Manage-
ment Research Laboratory. The model
uses data on source emissions, room-
to-room air flows, air exchange with
the outdoors, and indoor sinks to pre-
dict concentration-time profiles for all
rooms. The concentration-time profiles
are then combined with individual ac-
tivity patterns to estimate exposure.
Risk is calculated using a risk calcula-
tion framework. The model allows
analysis of the effects of air cleaners
located in either/or both the central air
circulating system and individual rooms
on IAQ and exposure. The model al-
lows simulation of a wide range of
sources including long-term steady
state sources, on/off sources, and de-
caying sources. Several sources are
allowed in each room. The model al-
lows the analysis of the effects of sinks
and sink re-emissions on IAQ. The re-
sults of test house experiments are
compared with model predictions. The
agreement between predicted concen-
tration-time profiles and the test house
data is good. The model is designed to
run in the Windows operating environ-
ment.
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory's Pollution Pre-
vention and Control Division, Research
Triangle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
RISK is the third in a series of indoor
air quality (IAQ) models developed by the
Indoor Air/Radon Mitigation Branches of
U. S. EPA's National Risk Management Re-
search Laboratory. The first model, IN-
DOOR, was designed to calculate the in-
door pollutant concentrations from indoor
sources. The second model, EXPOSURE,
extended INDOOR to allow calculation of
individual exposure. RISK extends EX-
POSURE to allow analysis of individual
risk to indoor pollutant sources. Note that
risk estimates based on currently avail-
able data are projections containing a great
deal of uncertainty. This is particularly true
when using a model such as this one to
calculate risk estimates for individuals, be-
cause such numbers as carcinogenic po-
tency, upon which the model depends for
calculating individual and population can-
cer risk, are projections of population risks
based upon a variety of extrapolations
and assumptions. Risk estimates gener-
ated by models such as this one are use-
ful mainly for the purpose of comparing
scenarios rather than for determining ab-
solute risks to individuals or populations.
The three models were all developed as
tools to carry out the mission of the engi-
neering portion of the EPA's indoor air
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research program "To provide tools nec-
essary to reduce individual exposure and
risk to indoor air pollutants."
The three models reflect the status of
EPA source and sink characterization re-
search at the time the models were writ-
ten. RISK includes new empirical source
models and mass-transfer-based source
models in addition to the common first
order decay source models used in previ-
ous models. The mass-transfer-based
source models are particularly useful for
gas-phase-limited mass-transfer situations.
RISK is the first version of the IAQ
model designed for the Windows operat-
ing environment. Most of the material in
the full manual is contained in the on-line
help file provided with RISK.
Purpose of IAQ Engineering
Modeling Research Program
The modeling component of the IAQ
engineering research program is designed
to support the overall mission of IAQ engi-
neering research. In meeting this mission
the IAQ modeling program:
• Provides tools to integrate the results
of IAQ research.
• Provides tools for analysis of the ef-
fects of IAQ control options on indi-
vidual exposure.
• Provides tools for improving under-
standing of interactions of sources,
sinks, ventilation, building parameters,
air cleaners, and individual activity
patterns on individual exposure to in-
door air pollutants.
Relationship Between Modeling
and Source Characterization
The role of the model relative to source
characterization can be seen from Figure 1.
Data related to source characterization
are developed as part of EPA's indoor air
source characterization program. These
data are used to develop source emission
models that are used in this IAQ model.
The source models are updated when-
ever new information is developed by the
source characterization research program.
Model Overview
RISK is designed using the concepts of
buildings and scenarios. The fixed infor-
mation about a building (the number of
rooms, the room dimensions, and the ar-
rangement of the rooms) is contained in a
building file. The changing information
(sources, sinks, air exchange, room-to-
room flows, etc.) is contained in scenario
files. The steps in using the model are:
define the building, save the building in-
formation to a file, define the scenario,
save the scenario to disk, and run the
model. Repeat the scenario definition
model execution process for each sce-
nario of interest.
RISK provides a wide range of graphi-
cal and tabular outputs of the results of
the calculations. Summary outputs of risk
and exposure are provided in tabular form.
Full risk, exposure, and concentration out-
put are also provided in tabular form. The
tabular outputs are supplemented by
graphs of concentration and exposure ver-
sus time. The calculated results can also
be saved to disk for later analysis.
Building
In model terms a building is a collection
of rooms and their interconnections. Room
dimensions and room-to-room interconnec-
tions, but not room-to-room air flows, are
fixed for a given building. The actual val-
ues of room-to-room air flow and air flow
between the building and the outdoors
can be changed for the various scenarios.
A building may have an unlimited number
of scenarios associated with it.
Scenario
In model terms a scenario is a collec-
tion of data for a given building that can
change from model run to model run. All
information on sources, sinks, air flow
rates, individual activity patterns, type of
Source
Testing
Source Si-
Size
Source
Modeling.
->• Emissions
Ventilation
Building factors (e.g., sinks).
Source usage
IAQ
Modeling
Air
Exposure
Modeling
Concentration
Occupancy..
Risk
Modeling
- Exposure
Occupant sensitivity..
Dose-response
Figure 1. Risk analysis process.
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pollutant, and risk factors is included in
the scenario.
Data input
Data are entered into the model using a
"fill in the form" interface. The data in the
form are not available for use by the pro-
gram until they are "transferred" from the
data entry form to the model. Data are
transferred from the data entry form to the
model by selecting the transfer command
from the file sub-menu, or by clicking on
the transfer button provided with the vari-
ous data entry screens.
Model output
Graphs
The model can display the results of
the various calculations in several graphs:
• Concentration versus time.
• Instantaneous exposure versus time.
• Cumulative exposure versus time.
• Total exposure for each activity pat-
tern.
• Time exposed to irritant concentra-
tion for each activity pattern.
Most of the graphs can be displayed in
the following formats:
• Linear x linear y.
• Log x linear y.
• Linear x log y.
• Log x log y.
All graphs can be printed.
Tables
In addition to the graphs, the model
provides numerous tables displaying the
results of the calculations, including:
• Summary, showing time exposed to
irritant concentration, total exposure,
and risk.
• Concentration versus time.
• Exposure versus time.
• Concentration and exposure versus
time.
• Summary risk.
• Risk analysis.
Data in the output tables may be copied
to the Windows clipboard and pasted into
other Windows applications. This is a con-
venient way to enter data into a graphics
package for plotting.
Files
The results of the calculations can be
saved to a disk file. This file is a Tab
delimited file that can be read by many
spreadsheet and graphics packages.
Model Assumptions
The model is based on two assump-
tions:
1. All rooms in a building are well mixed.
2. Mass is conserved.
Assumption 1 means that the concen-
tration leaving the room through all exits
is the same as the concentration in the
room. Numerous experiments in EPA's IAQ
test house show that this assumption is
valid.
The assumption that the rooms in a
building are well mixed does not mean
that the building is well mixed. Individual
rooms in a building may have different
concentrations.
A room in the model does not always
equal a room in the physical building. For
example, a single large room may need to
be divided into two or more model rooms
to meet the well mixed assumption, and
several physical rooms may be treated as
a single room in the model.
Assumption 2 means that the amount
of air entering a room must equal the
amount of air leaving the room. This as-
sumption also means that the amount of
outdoor air entering the building as a whole
must equal the amount of air leaving the
building for the outdoors. The model will
warn the user if the mass balances are
incorrect. However, the model will not bal-
ance the flows for the user. The user is
responsible for ensuring that the flows bal-
ance.
Modeling decisions
Several decisions were made in de-
signing the model. Some of the major
decisions were:
• The emphasis of the effort was on
model ease of use.
• The data requirements were mini-
mized as much as possible.
• Data defaults would be provided as
much as possible.
• Results of ongoing source and sink
research would be incorporated into
the model as soon as possible.
• User would be responsible for bal-
ancing flows.
• Room-to-room flows and ventilation
rates were model inputs and would
not be calculated from pressure/tem-
perature data.
Most of these decisions were made to
make the model easier to use. Having the
user balance the flows will help the user
understand the input data. The computer
will determine if the flows balance, but it
will not actually balance them. The pur-
pose of this decision is to reinforce the
idea that mass must be conserved. The
user must determine where air is coming
from and where it goes.
Model limitations
There are two types of limitations on
the model:
• Data limitations.
• User limitations.
Data limitations
Data limitations are limitations about our
understanding of the processes occurring
and imperfect scientific knowledge. The
major data limitations are:
• Limited data on source emission rates.
• Limited understanding of sinks and
inadequate models for sinks.
• Limited risk information.
• Limited library of default data for room-
to-room flows, ventilation rates, etc.
Research is underway to correct each
of these data limitations. The results of
the research will be incorporated into the
model as soon as they are ready.
User limitations
User limitations fall into four broad cat-
egories:
• User does not understand what the
model can do.
• User does not understand or know
what question the model should ad-
dress.
• The user does not fully understand
the input data requirements.
• The user does not understand the
model output.
Often these limitations interact with each
other and result in failure of the model to
meet the user's needs.
These limitations often reinforce limita-
tions caused by data gaps.
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Author Leslie E. Sparks is also the EPA Project Officer (see below).
The complete report, entitled "IAQ Model for Windows, RISK Version 1.0: User
Manual," (Order No. PB96-501 929; Cost: $38.00, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air Pollution Prevention and Control Division
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
National Risk Management Research Laboratory (G-72)
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/SR-96/037
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