Working draft paper - do not cite or quote

This is a preliminary document prepared by a subgroup of members of the
Base Programs Analyses & Policies Work Group as part of the Federal
Advisory Committee Act (FACA) Subcommittee process. It is not an EPA
document. It is subject to further work group discussion. Interested
people should forward their comments to a member of the Base Programs
Analyses & Policies Work Group.

July 23, 1996

[Contact Person]{.underline}: Steve Pezda/David Chock

Phone: 313 322-9213 / Fax: 313 594-4271

E-Mail: \<usfmc3gq@ibmmail.com\>

[I. ISSUE TITLE]{.underline}

Utilization of an Exposure-Based Monitor System

[II. DESCRIPTION]{.underline}

A. [Background and Problem]{.underline}

The NAAQS have been promulgated to protect the risks to public health.
Therefore, in implementing the NAAQS, some have recommended that ambient
monitors should be located, and measured monitor concentrations should
be utilized, in a manner that is reflective of actual exposure and
health risks, i.e., where people actually spend their outdoor time.
Others, however, perceive that this approach could sacrifice the health
of individuals in less populated areas. In either case, it should be
noted that people, on average, spend approximately 90% of their time
indoors. (Robinson, J.; Nelson, W. C. \[1995\] National human activity
pattern survey data base. Research Triangle Park, NC: U.S. Environmental
Protection Agency.)

Monitors provide important information but are expensive to purchase,
utilize, and maintain. All monitoring data should be used to the fullest
extent possible. Important uses of monitoring data are to: 1) provide
the temporal and spatial distribution of measured species, 2) determine
the attainment status of an area, 3) determine possible unique areas of
concern where special attention may be required, 4) help design State
Implementation Plans (SIPs) and specific control measures to reduce
pollution levels, 5) prioritize control measures for cost effective
reductions in overall personal exposures, and 6) determine the progress
toward achieving NAAQS. A methodology to consider for accomplishing
these tasks is one that considers human exposures throughout the MSA.

Presently, EPA air pollution monitoring data is obtained through the use
of some 4469 monitoring sites located throughout the U.S. (EPA\'s
Aerometric Information Retrieval System). Monitoring objectives exist
for choosing general locations for new monitoring stations. For State
and Local Air Monitoring Networks (SLAMS), the objectives are to
determine: 1) the highest concentrations in an area, 2) representative
concentrations in areas of high population density, 3) the impact
significant sources have on ambient pollution levels, and 4) general
background concentration levels. For National Air Monitoring Stations
(NAMS, which are a subset of SLAMS), two categories of stations
exist: 1) urban scale sites located in areas of expected maximum
concentrations, and 2) neighborhood sites located in areas that combine
poor air quality and high population density. Thus monitors are often
not located so as to obtain the best estimate of pollution levels to
which the overall public is actually exposed. Instead, emphasis is
placed on locating monitors where the pollution concentrations are
expected to be the highest. In addition, the worst (highest reading)
monitor (design monitor) in each MSA is used as the basis of
attainment/nonattainment status. As a result, the present monitoring
system (monitor location and monitor data) is not being used to its
fullest extent because data from other monitors is essentially being
ignored. By using the data from all monitors it is possible to obtain
better estimates of overall exposures/health risks throughout the MSA.
Such information can then be used to develop, improve, and prioritize
cost effective mitigation strategies for maximum risk reductions
throughout the MSA.

The current approach is viewed by some as an appropriately conservative
approach that maximizes protection of the public health and provides an
adequate margin of safety. The reasoning is that if the highest reading
within the MSA can be reduced to the standard, then the whole area
should be safe. This may not necessarily be the case, especially with
respect to PM, since PM emissions (primary) are much more local in
origin. Acid aerosols (secondary particles), on the other hand, can be
transported over long distances. Also, for ozone, reducing the areawide
peak ozone downwind of a major urban area may require a decrease in
NO~x~ emissions in the upwind urban area, which would lead to an
increase in ozone exposure there.

The scientific community is now coming to the conclusion that there is
no \"threshold\" for health effects for some pollutants, especially
ozone. Additionally, analyses have shown that some NAAQS may never be
attained due to natural background levels of these pollutants. This
being the case, it is more and more being recognized that no \"zero
risk\" solutions are available. Full protection is impractical if not
impossible. Therefore, the cost effectiveness of mitigation strategies
becomes important, as do the development, improvement, and use of
important tools such as risk assessment. Since some risk must remain, it
is logical to prioritize utilization of resources in such a fashion as
to maximize beneficial results. One way to do this is through
utilization of some form of exposure weighting or averaging of monitors.

B. [Advantages & Disadvantages of Present Monitoring System]{.underline}

The advantage of the present monitoring system is that it provides
maximum health protection to individuals in an area in the vicinity of
the design monitor, where peak pollution concentrations are measured.
(Note: The size of the \"area\" where health protection is achieved
depends on the pollutant, meteorology, sources of emissions, transport,
etc. For ozone and secondary particles, the concentrations are likely to
be more uniform than for primary particles. Compared to ozone, the total
PM concentrations may have many local peaks. Overall then, PM pollution
will not be as uniformly distributed as ozone.) The disadvantages
associated with the present monitoring system are: 1) The system does
not require an estimate of overall health risks associated with exposure
to ambient pollution, 2) Human and financial resources may not be
efficiently spent in that they are used to reduce peak pollution levels
in the vicinity of the highest-reading monitor rather than overall
pollution risks throughout the nonattainment area, 3) The present system
provides disincentive for states to install additional monitors (because
of the fear that any additional monitors will cause or reinforce
nonattainment status), and 4) The present system cannot be used to
design and implement the most effective SIPs, and prioritize control
measures, that lower overall ozone exposure and PM exposure.

C. [EPA Consideration of an Exposure-Based Monitor System]{.underline}

In their review of options associated with the ozone NAAQS, EPA is
considering an approach that averages across multiple monitors (OAQPS
Staff Paper, \"Review of the National Ambient Air Quality Standards for
Particulate Matter: Policy Assessment of Scientific and Technical
Information\", U.S. Environmental Protection Agency, April 1996, page
VII-21). Such an approach is also being considered for the PM NAAQS
monitors. According to EPA, such an approach \"would better focus risk
management activities on reductions in area or regionwide fine particle
concentrations. Because the health effects information is keyed to
fluctuations in areawide fine particle concentrations, such a form would
also be more directly related to reduction in population risk.\" EPA
also understands that, in such an approach, consideration should be
given to how peak localized exposures might result in unacceptable
individual risks.

It is worth noting that at the May 16 - 17, 1996 Clean Air Scientific
Advisory Committee (CASAC) meetings related to the PM Staff Paper,
support was expressed for an approach that utilizes some form of
averaging of monitor readings. Support was tied to the fact that such an
approach would better represent population health risks, and would
further provide incentive for states to add monitors to the existing
system.

[III. OPTIONS]{.underline}

The main purposes of the monitoring system are to: 1) provide
information on pollution concentrations to which people are actually
exposed, 2) enable the use of this information to design and implement
the best pollution control programs for reducing overall health risks,
3) enable the use of this information to determine and prioritize
control options, and to measure progress toward attainment of the NAAQS,
and 4) to verify modeling results (e.g., transportation phenomena).
Other factors to consider in deciding on the best option(s) related to
the placement and use of monitors include costs, and ease of
implementation and use.

Four options, along with a discussion of their advantages and
disadvantages, are detailed below, followed by a description of other
important related issues.

A. [Option #1 - No Change from Present Methodology]{.underline}

This option would represent the status quo, i.e., attainment status
would continue to be based on the worst single monitor in the MSA.
Criteria for the location of new PM~2.5~ monitors would be identical to
those established for existing monitors, i.e., generally placed where
highest pollution levels are expected. This option would not require any
added expense for ozone, but added costs would be required if a new PM
fine NAAQS is established. This option would continue to be easy to use.
It also provides for maximum health protection for individuals in the
vicinity of the design monitor. This option precludes the use of a risk-
and exposure-based approach to human health protection, and does not
necessarily reduce health risks throughout the MSA. In fact, it discards
most of the monitoring data and therefore may actually worsen ozone air
quality of populated urban areas if NO~x~ emissions have to be
significantly curtailed. It could not be used to help design best SIPs
and pollution control measures for the entire MSA, nor could it be used
to prioritize the use of limited resources. It would not represent the
most cost effective approach to overall risk reduction. It could not be
used to measure overall progress toward NAAQS attainment. It also does
not provide incentive for states to establish an expanded monitor
system.

B. [Option #2 - Total Exposure Weighting of Existing Monitor
Data]{.underline}

For a situation where the monitor system is already in place (e.g.,
ozone and PM~10~), a viable option would be to use monitor data to
estimate total human exposures to air pollution. Control strategies and
pollution reduction techniques (SIP design) could then be prioritized
most efficiently (e.g., health risk reductions and cost effectiveness)
in terms of reducing exposure to the greatest number of people. In this
regard, a viable option would be population weighting of monitors. This
option would avoid the increased costs associated with movement of
present monitors and/or addition of new monitors (necessary in some
areas to more accurately reflect actual exposures). In addition,
population data already exist and can be acquired from the U.S. Census
Bureau. This option represents a sensible and practical alternative to
characterize the human exposure to ozone and PM more accurately. This
option assumes that outdoor exposure is correlated to where people live.
To implement this option, the simple approach described below could be
used:

1.  Group the monitors in each MSA (if it has more than one monitor and
    contains several counties) into sub-areas (counties). The number of
    people residing in each county is readily available from the U.S.
    Census Bureau.

2.  Use the annual mth-highest monitor reading (according to the form of
    the standard) for each monitor. If a county has only one monitor,
    use the annual mth-highest reading at that monitor for the county
    pollution concentration. If a county has more than one monitor, take
    the average of the annual mth-highest readings of all the monitors
    within that county to obtain the pollution concentration for that
    county.

3.  Calculate the total exposure-weighted pollutant concentration for
    the MSA by taking the sum of the products of the county population
    and county pollution concentrations, and dividing this sum by the
    total MSA population.

4.  Use this exposure-weighted concentration to determine the MSA
    attainment/nonattainment status.

5.  Adjust the weighting factor periodically to reflect any shifts in
    population over time.

This option can be used immediately, is easy to use, will entail little
or no added cost (some added costs would be required if a new PM fine
NAAQS is implemented), and will provide a reasonable estimate of total
exposure to pollution levels that can be used as the basis of attainment
status and to design SIPs for reducing overall population health risks.
It can be used to prioritize most cost effective mitigation strategies.
It can also be used to measure overall progress toward NAAQS attainment.
This option would also provide incentive for states to add additional
monitors. However, special attention would be needed for those areas
where peak pollution levels may occur (see section below on \"Other
Issues\").

C. [Option #3 - Move Monitors and then Conduct Total Exposure
Weighting]{.underline}

In this option, existing monitors (ozone and PM~10~) are first moved to
more accurately reflect total exposures to air pollution. (For new NAAQS
that require new monitors, such as for PM~2.5~, monitors should be
originally placed to best reflect total exposures.) Once moved, the
exposure-weighted pollution level is determined as in option #2. Option
#3 will require added time and expense to move the monitors. Once this
is done, however, the methodology will be easy to use and will produce a
weighted pollution level that can be used as the basis of reducing
overall exposure risks, e.g., prioritizing control strategies and
pollution reduction techniques most efficiently (e.g., health risk
reductions and cost effectiveness) in terms of reducing pollution
exposure to the greatest number of people. The advantage of this option
over option #2 is that the new monitor locations, if properly cited,
should better represent total exposures and health risks. This option,
like option #2, will allow for the use of exposure weighting of monitor
data to design effective SIPs, and to measure overall progress toward
attainment of the NAAQS. This option will also provide incentive for the
states to add additional monitors. This option does not focus on those
areas where peak pollution levels may occur (see section below on
\"Other Issues\").

D. [Option #4 - Averaging of Monitor Data]{.underline}

This option would provide for the averaging of data gathered by multiple
monitors that represent the same temporal and spatial air mass. One
advantage of this option would be designation based upon monitoring data
that would be more reflective of the total air mass versus designation
based on a single isolated monitoring site. This option also provides
incentive for the citing and installation of additional monitors. This
option would allow the use of monitor data to help design an effective
SIP that reduces overall health risks. Averaged monitor data could also
be used to measure progress toward overall MSA attainment. This option
would likely require the addition of more monitors so that sufficient
monitor data (for averaging) is available within the air mass, wherever
that air mass may occur. Under this option there would continue to be a
requirement to investigate and address local issues relating to the
single monitor data, but those data would not cause the designation of
an entire area to be classified as nonattainment (see section below on
\"Other Issues\").

> [1. Option #4a - Total Exposure Weighting in Conjunction With Option
> #4]{.underline}
>
> This option is appropriate when the air mass described in option #4
> does not encompass the entire MSA. In this option, the averaged
> monitor data for the temporal/spatial air mass detailed in option #4
> is exposure weighted with the monitor data outside the air mass but
> still within the MSA. There are several ways to conduct the weighting.
> One way is by averaging the average concentration within the air mass
> with the average concentration outside the air mass. Another way is to
> weight (by number of people) the average concentration within the air
> mass and the average concentration outside the air mass. The exact
> averaging scheme would depend on the precise location of the air mass
> within the MSA versus the segmentation of the remaining areas within
> the MSA. This option has the advantage of incorporating data from all
> the monitors in an MSA under the condition that the temporal/spatial
> air mass does not encompass the entire MSA. In this way, consideration
> is given to the exposure of all individuals within the MSA.

[IV. Other Issues]{.underline}

The following issues should be considered when utilizing the options
described above:

-   The need for judgment in determining which monitors to include in
    the averaging (some special purpose monitors should likely not be
    included because they were established for unique purposes, such as
    measuring pollution levels from upwind sources).

-   The need to investigate and address \"hot spot\" mitigation
    strategies (e.g., ozone alert days) for areas and affected
    populations around a monitor which shows high levels of the measured
    pollutant (judgment is needed with respect to the amount of time
    that the population would be exposed to the pollution levels
    recorded at this monitor). Such a situation would not, by itself,
    cause an entire area to be classified as nonattainment

-   The need to design averaging programs that prevent \"gaming\", that
    is, prevent location and use of monitors in such a fashion that
    attainment is \"achieved\" through inappropriate averaging schemes.

-   The adequacy of the monitoring system in terms of being \"robust\"
    enough (number and placement of monitors) to enable use of an
    averaging methodology.

-   The impact that transport may have on the averaging methodology. For
    example, an area may be designated attainment via an averaging
    methodology yet still impact a downwind nonattainment area.
    Consideration still needs to be given to control requirements for
    the contributing area to help bring the downwind nonattainment area
    into attainment.

[V. Recommendations]{.underline}

The NAAQS review process for PM and ozone has raised the possibility of
utilizing exposure based air monitoring data for determining compliance
with the NAAQS. This represents a departure from current practice and
raises questions regarding the current approach to air quality
monitoring network design. The workgroup examined potential approaches
to the development of exposure weighted monitoring data. This included
the use of existing monitoring networks as well as the development of
revised networks. This also included the weighting of air monitoring
data by population and the spatial averaging of air quality data. Based
on discussion of these options, the workgroup makes the following
recommendations.

1\. The establishment of acceptable levels of risk and exposure are most
appropriately conducted as a part of the NAAQS setting process.
Therefore, any use of exposure based monitoring to determine compliance
with the NAAQS must be allowed by the NAAQS.

2\. Regardless of the outcome of the NAAQS review process, there is a
potential role for the utilization of exposure based air monitoring data
in the implementation of emission control programs. The workgroup
identified two specific possibilities. The first involves the
prioritization of the implementation of control actions. Consideration
of exposure based monitoring data could be used as a way to maximize the
reduction in population exposure and risk. The second is in the area of
measuring progress. The use of exposure weighted monitoring data could
be used to measure and judge the progress of emission control programs.
In either case, the use of exposure based monitoring could provide an
incentive to increase monitoring to better define the actual air quality
situation. The workgroup intends to more fully develop these approaches
as a part of Phase II.

3\. The current monitoring network design and siting criteria should be
reviewed. This issue paper raises a number of concerns regarding the
current design of monitoring networks and the utilization of air quality
data. Specifically, the workgroup recommends that this review focus on
insuring, at a minimum, that the monitoring system design is consistent
with the form of the NAAQS including any applicable secondary standards.
This is consistent with the recommendation of the workgroup in the
development of the issue paper on designations.