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OCR for page 53
Sampling and Physical
Chem~cal Measurements
INTRODUCTION
This chapter considers sampling and physical-chemical measurement meth-
ods available for assessing human exposures to airborne pollutants and em-
phasizes recent advances. These advances could be used to improve e~osure-
assessment methods.
In assessing human exposures to airborne pollutants, numerous factors
besides the contaminant must be measured especially if the assessment is
based on fixed-site sampling or modeling. Accurate estimates in these instan-
ces depend not just on concentration measurements from f~xed-site monitors
in various locations, but also ore knowledge of numerous factors that influence
the environments where the exposures occur (see Chapter 2~. Outdoors, these
factors include temperature, humidity, precipitation, barometric pressure, wind
speed and direction, turbulence, and mixing height. Insolation, as well as light
scattering and absorbance, might also be important. Some of these factors
also must be measured to mode! indoor environments. However, other fac-
tors are unique to indoor environments such as: ventilation rates, pressure
differentials across building shells and between building compartments, re-
moval efficiencies of building filters, and contaminant deposition rates on
indoor surfaces. Furthermore, modeling frequently requires measurements of
source strengths. Outdoors, source-strength measurements include emission
rates from a major point source (e.g., power plants). Indoors, source-emission
rates could include volatile organic compound (VOC) emission rates derived
from chamber studies of building materials, consumer products, and home
furnishings (Tichenor, 1987~. These areas are too broad to be discussed com-
prehensively in this chapter, whose focus is the measurement of airborne con-
taminants. Nonetheless, measurement methods that produce information
about environmental factors or emission rates should be accounted for in the
53
OCR for page 54
54 ASSESSING HUMAN EXPOSURE
development of useful indirect methods to identify and control the factors
most significant to human exposure.
As outlined in Chapter 1, the choice of sampling and physical and chemical
measurement methods to be used in an exposure assessment is driven by a
studys specific aims. The analytical procedures should be chosen with atten-
tion to the specific needs of the study. The "why," "what, ~when," and ~where" all
influence the selection of the ~how,' discussed in this chapter. It is important
for the analyst to ask, "What are we ultimately trying to accomplish?"
Sampling frequency and duration are important elements of a sampling
strategy. Certain analytical procedures provide real-time or instantaneous
measurements of contaminant concentrations (e.g., long-path-length Fourier
transform infrared spectrophotometers), while others provide an average value
for the interval during which sampling occurs (e.g., collection of VOCs on the
sorbent Tenax). Real-time measurements can be made consecutively to yield
a continuous record of a contaminant concentration, or they can be made
intermittently to yield a series of concentration "snapshots.- Integrated meas-
urements can be made consecutively or intermittently, or they can be over-
lapped, if more than one set of sampling apparatus is available.
If monitoring is done for compliance purposes, the sampling frequency and
duration likely are specified by regulation. However, rigid specification usually
is not necessary for most types of exposure-assessment monitoring. If peak
concentrations are important in assessing a potential health effect, then a
sampling procedure should be integrated over a time scale no longer than that
at which contaminant concentrations fluctuate. Furthermore, the sampling
should be frequent enough to measure major fluctuations. Real-time continu-
ous monitoring for a contaminant that causes a chronic health effect would be
unnecessary, because the total contact is of concern. The time scale of the
relevant biological effect for a contaminant must be considered in choosing
the time scale of the sampling and measurement process (Lioy and Daisey,
1987~.
Emissions of various airborne contaminants can be time dependent. For
example, at a manufacturing site, time of day and day of week can influence
emission rates and, consequently, the airborne concentrations of various spe-
cies. Diurnal, weekly, and seasonal variations in emission rates can affect
outdoor airborne concentrations. Such factors must also be considered when
planning sampling frequency and duration.
Spatial considerations are important in f~xed-site monitoring. As stated in
the National Research Council (NRC) report, Complex Mixtures (NRC, 1988),
The primary consideration should be the relevance of the sample site to
potential human exposure." Selection of a sampling site can be purposive or
probabilistic. Purposive sampling normally is conducted to answer questions
OCR for page 55
SAMPLING AND PlIYSICAL-CHEMICAL MEASUREMENTS 55
about a specific location (e.g., sampling near known emission sources, such as
a power plant or a waste dump). Probability sampling seeks to provide an
overall picture of an area. The choice of sampling sites should be influenced
strongly by the nature of the potential human exposure. Table 3.1 is a sum-
mary of designs that can be applied to the selection of sampling sites. It also
includes a brief evaluation of when the different strategies are most useful.
(For a further discussion of sampling sites, see NRC, 1988~.
QUALITY ASSURANCE
Using advanced techniques in exposure studies does not ensure the acquisi-
tion of better qualitative data, but allows the potential of obtaining better data.
Whether that potential is realized depends on the quality-assurance (QA)
program that is designed into the study.
The terms precision and accuracy often are used in quantitive studies.
Precision is a measure of the agreement among individual measurements
made of the same property of the sample. Accuracy refers to the degree of
agreement of a measurement (or an average of measurements of the same
property) with an accepted reference or true value. QA and its complementa-
ry concept, quality control (QC), have many definitions. QA often is used to
also include QC, and this report uses this convention. For environmental
measurements, QC comprises operational activities that are carried out before
and during the measurement process that are intended to ensure that data are
of sufficient quality~ata whose precision and accuracy are known and are
sufficient to meet the needs of ~ study. Examples of QC are calibration pro-
cedures, maintenance of constant line voltage and temperature, use of blank
and spiked samples, and use of traceable standard reference materials. QA
also includes activities carried out to ensure that the collected data achieve the
precision and accuracy required, such as interlaboratory comparisons, meas-
urement system audits, and statistical procedures to highlight bad data or
extreme values. These activities should be carried out by persons not involved
in routine data-gathering operations. EPA has developed a comprehensive
QA handbook that gives principles and recommended procedures for achiev-
ing quality data in air-pollution measurement systems (EPA, 1976a,b).
ERRORS
In designing a QA program to meet the needs of a specific exposure study,
it is useful to consider the four activities involved in any environmental meas-
urement that can cause errors in the data obtained:
OCR for page 56
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OCR for page 57
SAMPLING AND PHYSICAL-CHEMICAL MEASUREMENTS 57
· Selection of representative sampling sites.
· Collection of the environmental sample.
· Sample analyses.
· Data handling.
Site-Selection Errors
The representativeness of the sampling site refers to selection for appropri-
ate spatial and temporal definition. For example, a study to determine com-
pliance with ~ ambient air~qu;ality steward for ~ given pollutant would re-
quire air samplers to be placed at community sites that represent typical out-
door air and to have the same measurement time as the standard. On the
other hand, a study to determine total air exposure of a population to the
same pollutant could require a sampling strategy that involves personal sam-
plers or microenvironmental measurements combined with activity diaries. In
many studies, site-to-site variability is the largest component of the total meas-
urement error. EPA (1988b) guidelines for exposure studies provide general
information on proper sitting of outdoor air-monitoring stations.
Collection Errors
The study of most air contaminants requires that the air sample be moved
from the microenvironment into a collection device or analytical instrument.
Errors can result from physical and chemical changes in the sample during
and after sample collection. Air-collection procedures usually concentrate
molecules that normally are diffuse and isolated, thus enhancing the possibility
of concentrated molecules interacting with each other or with the collection
medium or sampler components. These interactions can render some collect-
ed molecules immeasurable by the chosen analytical procedure. Errors can
also occur during handling, shipping, and storage of the samples.
Pumps can be significant sources of artifacts in collected samples. For
example, particle artifacts may arise because of mechanical wear or oil-droplet
emissions. Gas-phase artifacts may arise as a result of emissions from hot
pump oil or other pump lubricants. Also, the magnitude of the pump flow
rate is an important consideration in microenvironmental sampling. The flow
rate can be set so high that the sampling system decreases the contaminant
concentration in the microenvironment being measured, and, near the end of
the sampling period, the contaminant concentration in the microenvironment
could be lowered artificially.
OCR for page 58
58 ASSESSING HUMAN EXPOSURE
Another common source of error when sampling with a pump is a poorly
defined flow rate. It is extremely important to calibrate and periodically check
flow rates in any device that uses flow rate to quantify the volume of air sam-
pled.
Sample collection artifacts can arise from components other than the pump.
These include tubing, improper sealants, and incompatible plastics. Collection
artifacts can also be a problem: (a) during sample collection set up (potential
sources include idling motor vehicles, smoking, and the use of insect repel-
lents); (b) when using a sorbent, if the collection efficiency is poor (sorbent
breakthrough); and (c) during vapor- and particle-phase sampling when the
collection procedure itself may alter the distribution between the phases.
Errors also can occur with samplers when study subjects do not wear or
carry a personal sampler when they are expected to. Sometimes subjects are
embarrassed by the noise or size of the samplers; subjects might change their
activity patterns when wearing samplers to avoid embarrassing situations. A
subject might wear the sampling device, but forget to turn it on. Other unin-
tentional errors include accidentally sitting on the sampling line for a pump,
effectively stopping the flow to the sampler. Although passive samplers might
seem to alleviate many of these problems, an outer garment over the device
seriously reduces sampling capability. Therefore, the design of personal sam-
plers should include provisions to minimize their misuse and to ensure that
they have been used properly.
Analytical Errors
Analytical errors are associated with the identification and quantitation of
the chemical of interest in the sample collected. Qualitative errors can be
minimized by increasing the selectivity of the analytical method that is used
and by confirming compound identity by a second technique. This selectivity
should minimize potential interferences, that is, the ability of chemicals other
than the one of interest to interfere with the measurement process so as to
give results either higher or lower than the true value.
The metrics used to describe analytical quantitation errors are precision
and accuracy. Methods that have good precision and accuracy can be used.
However, methods that have poor accuracy but good precision often can be
useful in studies that require understanding only of the relative differences
among properties of environmental samples (Watson et al., 1983~. Critical
operational procedures that can lead to analytical quantitative errors are poor-
ly conducted calibration procedures and use of inadequate reference materials
when carrying out calibrations. Even if these procedures are carried out care
OCR for page 59
SAMPLING AND PHYSICAL-CHEMICAL MEASUREMENTS 59
fully, some errors are inherent in the analytical method-every method has a
limit for precision and accuracy. Another factor that can lead to quantitative
errors is the ~rugge~ess" of the method: its sensitivity to variations in the
factors that affect the measurement (such as temperature, relative humidity,
=d line voltage). Furthermore, many measurement methods cited in the
literature glare reasonable precision and acewac,T when used by highly framed
research staff, but lose their precision and accuracy when used by less well-
trained personnel who might not maintain stable operating procedures. The
routine use of field blanks and field spikes can reduce the occurrence and
magnitude of analytic errors
Data-Handling Errors
Data-handling errors are among the errors that can occur during data ma-
nipulation. These errors include mistakes in reading instrumentation, In trans-
pos~ng data from one system to another (e.g., data-entry errors), and in cal-
culating results in appropriate units. The EPA QA handbook (EPA, 1976a,b)
gives guidelines for minimizing these errors. Many of these errors have been
reduced through the extensive use of microprocessors that often collect and,
~ some cases, even analyze the data (Barrett, 1988; de Monchy et al., 1988),
thus reducing human error in transcribing data and bias in data evaluation.
Periodic human inspection of all steps of data collection, reduction, and re-
porting is essential as one further QC measure (Taylor, 1987~. Inspection
ensures that the automation of the data-analysis process does not obscure
significant information not considered when the initial microprocessor
programs were established. This is particularly true when programs are set
to accept or reject data automatically.
One common error in data reporting is the error of omission an omission
of precision and accuracy information when reporting data. The literature is
replete with misinterpretation and overinterpretation of data thought to be
more certain than they really were. Expenmer~tal data always should be ac-
companied by precision and accuracy information. Precision and accuracy are
integral parts of the measurement.
Designing a measurement strategy for a field study seems straightforward,
but designing one that minimizes the errors almost always is difficult. Fur-
ther, designing a good QA program for environmental field measurements
might be more difficult than one designed for laboratory experiments. More
people of varying skills usually are involved in environmental field studies than
In laboratory studies: different members of a staff will design the study, col-
lect the samples, analyze the samples, report the data, statistically analyze the
OCR for page 60
60 ASSESSING HUMAN EXPOSURE
data, and interpret the data. Without an organized QA program, such sharing
of responsibility for data quality can result in information of poor quality, or
perhaps worse, data of unknown quality. Furthermore, environmental field
measurements cannot be replicated, i.e., a measurement made today cannot
be repeated tomorrow
Obviously, QA is a critical part of exposure studies, and a QA program
must be established as part of the initial study design. The plan should fit the
specific aims of the study. The QA program also must be considered when
establishing the budget, because an effective QA program costs about 15-25%
of the measurement budget. From a practical point of view, this translates
into significantly less data but data of a higher quality~han if QA were neg-
lected. Those designing the study must take this reality into account when
determining the statistical power of the study. Painful though it is to reduce
the amount of exposure data, obtaining less data that are all good is much
better than obtaining more data that are bad or unverifiable.
AIRBORNE ANALYIES
The nature of a given airborne pollutant its physical, chemical, and in
some cases, biological characteristics~etermines the procedures appropriate
to its sampling and measurement.
A contaminant can exist in a vapor phase or particle-associated condensed
phase, or it can be partitioned between these phases. The partitioning can
result from the adsorption of a vapor-phase compound onto the surfaces of
airborne particles, in which case the contaminant distribution is a function of
the compound's liquid-phase (or subcooled liquid phase if the compound is a
solid at ambient temperature) vapor pressure and also the surface area of
airborne particles per unit volume of air (Pankow, 1987; Bidleman, 1988;
Junge, 1977; Ligocki and Pankow, 1989~.p,p'-Dichlorodiphenyltrichloroethane
is an example of an ambient contaminant commonly partitioned in this man-
ner. Partitioning also can be due to dissolution of a vapor-phase compound
in a liquid associated with airborne particles. An example is the dissolution
of sulfur dioxide (SO2) in water associated with hydroscopic particles. Organic
vapors also can dissolve in liquids associated with airborne particles. Still
another partitioning process involves attached and unattached radon daugh-
ters. Because a pollutant's dose to the lung can be very different in the vapor
phase from that of the condensed phase, care must be taken that a sampling
procedure does not alter the distribution between phases and present a false
picture of the pollutant's physical state (Van Vaeck et al., 1984; Bidleman,
1988; Coutant et al., 1988; Ligocki and Pankow, 1989~.
OCR for page 61
SAMPLING AND PHYSICAL-CHEMICAL MEASUREMENTS 61
If a pollutant is present in the condensed state, its distribution as a function
of particle size is important information for assessing human exposure. Air-
borne particles range in size from a few nanometers to hundreds of microm-
eters (Finlayson-Pitts and Pitts, 1986~. Particles frequently are classified as
After (~2~ am diem.) and "coarse" (>2.5 am diam.~. Fine particles are some-
t~mes subclassified as nuclei mode (~-.~-1 '`m diem.) and accumulation mode
(0.1-2.5 am diam.~. Fine and coarse particles tend to have different sources
and, consequently, different chemical compositions. They also have different
transport characteristics, such as settling velocities and diffusion coefficients,
which lead to different atmospheric lifetimes. For these reasons, collection of
s~ze-fractionated particles (at lead ~ fine and ~ coarse Faction) is usefo1 when
sampling with the intention of future chemical analyses. The American Con-
ference of Governmental Industrial Hygienists has established cut sizes ap-
propriate for fractionation in relation to inhalation hazard (Phalen et al.,
1986~.
In addition to size, other physical properties of airborne particles, such as
morphology and water content, can strongly influence their effects on living
organisms and should be considered in any sampling methodology. Particle
shape and surface texture are important morphological features and are char-
acteristic of the nature of the particle; research is in progress to represent this
information as a few characteristic numbers (FIopke et al., 1988~. Such re-
search is concerned with the use of optical and electron microscopes as the
major tools for determining these features with a primary focus on electron
microscopes.
Chemical species are not always distributed uniformly throughout a particle.
For some species, the surface concentration is significantly larger than the
bulk concentration. Examples include semivolatile organic compounds that
have been adsorbed on particle surfaces and trace metals with low boiling
points, such as lead, zinc, and cadmium, that are surface enriched by high-
temperature processes. In such cases, bulk analyses would yield much lower
concentrations than those actually in contact with environmental surfaces.
Hence, surface analyses using techniques that provide elemental or chemical
information such as SAM (scanning auger microprobe), SIMS (secondary ion
mass spectrometry), XPS (x-ray photoelectron spectroscopy), total reflectance
IR (infrared), LAM MA (laser microprobe mass analysis), and FTIR (Fourier
transform IR) are integral to a thorough evaluation of the health effects of
certain pollutants.
The water content of airborne particles can affect partitioning of an inor-
ganic gas between the vapor and condensed phases. The water content de-
pends on the relative humidity (RH) of the air that contains the suspended
particles, water-soluble salts associated with the particles, and different RHs
OCR for page 62
62 ASSESSING HU~4N EXPOSURE
at which various salts deliquesce. When measuring the water content of parti-
cles, it is important to remember that this value can depend strongly on RH.
It is also important to ensure that the measurement procedure does not alter
the water content. However, in some situations, protocols might require that
the measurement be made at a specified RH.
In considering the chemical nature of the analyte, some type of general
classification scheme is useful in choosing sampling and analysis procedures.
Such schemes can be as detailed as the outlines for inorganic and organic
textbooks, or they can be fairly general. At the very least, the contaminant
should be classified as organic or inorganic. If the contaminant is organic,
subclassification into polar and nonpolar and as volatile, semivolatile, or non-
volatile is useful. If it is inorganic, subclassification by periodic group, solu-
bility, acidity, hardness, and radioactivity might be helpful.
For biological analyses, an obvious distinction is that between viable and
nonviable contaminants. The former include bacteria, viruses, spores, molds,
and fungi. The latter include allergenic materials, such as arthropod frag-
ments and insect excrement. Sampling for viable biological pollutants is com-
plex, costly, and time consuming. A protocol for such sampling recently was
developed by Morey and coworkers (1987~.
CRITERIA FOR METHOD SELECTION
This section evaluates the requirements under which a method must oper-
ate, including sampling and analysis. The conditions for an ideal analysis are
summarized in Table 3.2. However, optimal conditions for an analysis might
require compromises.
Sensitivity
A method with adequate sensitivity is one in which an analyte can be de-
tected at or below the level at which an adverse human-health problem is
anticipated or observed. Ideally, a detection limit of at least an order of mag-
nitude below the health-effect level is desirable. It also is desirable to have
a broad linear range of O.lX-lOX the level of interest (i.e., a linear range of
two orders of magnitude from the detection limit). Reproducibility of +2%
for replicate analyses and stability of +5% during an 8-hour period also are
desirable.
Achieving high sensitivity during a continuous analysis is a very difficult
task. Therefore, one of the first compromises made ir1 achieving high sensitivi
OCR for page 63
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104 ASSESSING HUMAN EXPOSURE
rations. If the sample can be packaged in a container that permits irradiation
in the reactor and subsequent gamma-ray counting, it can be analyzed by
INAA. Although there is some low-level residual radioactivity associated with
the samples, the technique is relatively nondestructive of the sample. Neutron
activation analysis can provide information on as many as 40 elements with
sensitivities ranging from picograms to micrograms of analyte element. There
are elements, such as lead, that do not produce gamma-ray emitting products.
These elements cannot be analyzed by INAA. Neutron activation is a well-
established method that is now widely used to characterize environmental
trace elements (De Soete et al., 1972~.
X-ray fluorescence (XRF) is another common multielement method for
aerosol samples. A good discussion of XRF applied to the analyses of air-
borne particles is presented in Malissa and Robinson (1978~. The resolution
of XRF is such that particles <100 am diameter are effectively analyzed as if
they were homogeneous.
Radon and Radon Progeny Measurements
Radon is a naturally occurring gaseous radioactive element that is found
ubiquitously throughout the environment and is typically found in higher con-
centrations in indoor atmospheres than in outdoor air. It decays to a series
of four short-lived decay products that can be deposited in the human respira-
tory tract either directly or after attaching to pre-existing ambient particles.
EPA reported in a September 1988 press conference that the decay of these
radioactive progeny can induce lung cancers and might be responsible for
20,000 lung cancer deaths per year in the United States. Measurements can
be made of radon and decay products, and methods for each of these meas-
urements are presented.
Radon
Radon can be measured directly at environmental levels using the ability
of the emitted alpha particle to excite a ZnS(Ag) scintillator to produce meas-
urable emitted light. Hemispherical (Lucas, 1957) and simple right-circular
cylindrical (George et al., 1976) detector cells with optically clear, flat win-
dows, and interior walls coated with ZnS(Ag) have been used. For simple
grab measurements, the cell is evacuated. A valve into the chamber is
opened, and the chamber is filled with ambient air. The cell is then placed
on a photomultiplier tube, and the count rate of light pulses is measured.
OCR for page 105
SAMPLING AND PHYSICAL-CHEMICAL MEASUREMENTS 105
With proper calibration, this count rate can be related to the ambient radon
concentration. However, the concentration of radon In a building is highly
variable. Thus, grab samples of radon rarely reflect the long-term, average
indoor concentration.
An alternative approach is an active system to pull air through the scintilla
ration chamber using inlet and outlet connectors. The counts in a given tune
(e.g., 15 minutes) can be converted to an approximate radon concentration.
Because of the accumulation of decay products in the chamber, the actual
radon concentration is related to the activity counted through complex cal-
culational methods. As in all active systems, pump failure is possible and
careful flow control is needed. Therefore, this system is not convenient for
long-term monitoring.
Two methods are available to obtain an integrated measure of radon con-
centration. For short intervals (2-7 days), canisters containing activated car-
bon can be set in a room. The radon is adsorbed on the carbon and ac-
c~nulates over time. After the sampling period is over, the canister is sealed,
and the decay products build up in the container. Because equal activities of
the shorter-lived decay products will develop after about 4 hours, a measure-
ment of the emitted gamma radiations from the progeny can be used to deter-
mine the radon concentrations. However, the carbon canisters, dependent on
diffusion for eventual contact between the radon and the sorbent, are useful
only for a limited period and can have difficulties because of water-vapor
adsorption reducing the amount of adsorbed radon.
A better, long-term, integrated measurement can be made using a track-
etch detector. In these detectors, a small piece of a special type of plastic is
placed In a small plastic cup and sealed with a moisture-proof plastic film.
The radon can diffuse through the plastic film. Its decay and those of its
decay products will cause the plastic detector to be bombarded with alpha
particles. The alpha particles produce a track of radiation damage in the
plastic that can be more easily dissolved than the bulk material. Thus, when
the plastic detector is etched in a mild alkaline solution, microscopic holes can
be seen where the number of holes per unit area can be related to the radon
concentration. These detectors can be used for several weeks to as long as
one year and thereby provide more accurate annual average values for the
radon exposure.
A recent development is the use of an electret for passive radon measure-
ments (Kotrappa et al., 1988~. The concept is similar to the track-etch detec-
tor in that the detector is placed in a cup so that the radon can enter it.
However, the measure of radon exposure is the decrease in surface electric
charge on an electret made of Teflon FOP. Such a system has a simple read-
out device that only has to measure the residual surface charge on the electret
OCR for page 106
106 ASSESSING HUMAN EXPOSURE
and provides a useful alternative measurement method for periods of several
weeks to several months.
Radon-Decay Products
The measurement of the radon-decay products is difficult, because the
concentration and size distribution must be determined for the decay products.
The decay-product behavior is quite complex because of the ability to attach
to airborne particles, as well as to indoor surfaces such as walls, ceilings, and
furnishings. The particle size determines the ability of the radioactivity to be
deposited in the room and In the respiratory system. The respiratory deposi-
tion provides the dose to the critical tissues; the sizes of most concern are
those less than 10 nm. These highly diffusive particles can most easily deposit
in the body, whereas only about 20% of the particles with diameters around
100 nm are retained in the respiratory tract (James, 1988~.
Measurement of the total concentration of alpha-emitting particulate matter
is relatively easily determined. Air is drawn though a membrane filter, and
the collected alpha activity can be measured using either a ZnS(Ag) scintil-
lator for total alpha counting or a solid-state detector that provides alpha-
spectroscopic determination of the specific decay products. From multiple
sequential counts and the known decay kinetics of the radon progeny, the
concentrations of each of the decay products (hippo, 2~4Pb, rabbi) can be cal-
culated. The proper choice of filter can provide quantitative collection of
activity and an adequate radioactive source for accurate measurement of the
collected activity.
Size measurement of the decay products is a difficult problem. George
(1972) proposed a method for measuring the "unattached" fraction ~ which air
is drawn through a filter that is covered with a 60-mesh screen. The highly
diffusive activity attaches to the screen. The activity counted on the screen is
the unattached activity, and that which passes through the screen to the filter
is the attached activity. Activity-size distributions have been measured with
diffusion batteries down to sizes of 10 to 15 rim (Knutson, 1988), because
conventional diffusion batteries have relatively little resolving power for ultra-
fine particle sizes. Ramamurthi and Hopke (1988) reviewed several measure-
ment systems for unattached fractions in the context of the improved under-
standing of penetration of particles through single screen.
It is now understood that the unattached activity is not a single species with
one fixed diffusion coefficient, but rather an ultrafine mode in the particle size
spectrum between 0.5 and 5 nm. Improvements in measurement methods
(Reineking and Porstendorfer, 1986; Holub and Knutson, 1987) have made
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SAMPLING AND PHYSICAL-CHEMICAL MEASUREMENTS 107
it possible to determine the full range of sizes for grab samples. There re-
mains a need to develop a system that will provide a series of size and con-
centration measurements.
Radon concentrations are determined as a measure of exposure rather than
decay-product concentrations' because integrated radon measurements are
relatively simple, inexpensive, and accurate, and dosimetric models suggest
that radon is a reasonable surrogate for the decay products. However, the
development of new methods to measure decay product size and con-
centrations would permit direct long-term measurement of the species that are
Me proumate cause of the health effects.
Chemometrics
Chemometrics is a new field of chemistry that deals with the extraction of
maximal information from chemical data that have been produced using the
techniques described in the previous sections. Chemometric methods have
been extensively described by Sharaf et al. (1986) and by Massart et al. (1988~.
It has applications to analytical chemistry in that often information content of
a detector system is used only partially by simple data-reduction techniques.
In addition, many of the newer analytical methods are truly multivariate tech-
niques. To optimize the sensitivity and selectivity of such methods, it is neces-
sary to use a proper multivariate design for calibration (Deming and Morgan,
1987~. It is impossible to fully optimize many modern analytical instruments
with a one-variable-at-a-time approach. Similarly, interferences also must be
treated in a multivariate manner Lath a proper statistical design if the value
of multiple-species-measurement instruments are to be used fully.
One immediate application is in the analysis of data from several sensors,
where qualitative and quantitative analysis can be performed on the air sample
(Stetter, 1986~. These methods can be used to resolve and quantitate the
components of mixtures that the separation methods do not separate fully.
Often the patterns obtained from the chemometric analysis of the analytical
data can reveal the origin of the contaminants or information on their reac-
tivity in the air.
In addition, time-dependent response of a detector such as a drifting cali-
bration can contain information that can be extracted easily using techniques
such as Kalman filtering (Brown, 1986~. Thijssen et al. (1984a) developed a
model for random drift, and a criterion was developed for deciding when to
recalibrate or to measure the next unknown. Such methods have been ex-
tended to include optimization of the calibration scheme (Thijssen et al.,
1984b) and to incorporate a generalized standard additions approach to cali
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108 ASSESSING HUMAN EXPOSURE
bration (Vandeginste et al., 1983~. In many instances in the past, drifting
calibration data would not have been recognized as such or would have been
discarded because of the observed drift.
Considerable work has been completed on developing structure-activity
relationships so that the behavior of some of the easily measured compounds
might be useful in predicting the atmospheric or physiological behavior of
other structurally related compounds. Although much of the prior work has
been on compounds of possible pharmacological activity, structure-activity
studies also could help predict other biological responses, such as toxicity,
bioconcentration potential, and other aspects of ecosystem behavior.
Chemometric methods have been used to identify the sources of mutagen-
icity observed in collected airborne particulate matter (Daisey et al., 1986;
Wallace, 1987; Lewis et al., 1988) and are likely to be more widely used in the
future to help relate observed atmospheric composition to various toxicological
responses. When applied to resolving sources of airborne particulate matter
or applications to interpreting airborne mutagenicity, chemometric methods
commonly are called receptor models and are discussed more fully in Chap-
ter 6.
SUMMARY
In assessing human exposures to airborne pollutants, numerous factors
besides the contaminant must be measured, especially if the assessment is
based on fixed-site sampling or modeling. Accurate estimates in these in-
stances depend not only on concentration measurements from fixed-site moni-
tors In various locations, but also on knowledge of numerous factors that ~nflu-
ence the environments where exposures occur. The measurement of an air-
borne contaminant can be visualized as a three-step process. First, the pollu-
tant is sampled; then it is separated from other species also collected in the
sample; finally, it is detected The choice of the measurement methods to be
used in an exposure assessment is driven by a study specific aims and by the
nature of a given airborne pollutant.
Quality Control/Quality Assurance
Quality assurance is a critical part of exposure studies and must be es-
tablished as part of the initial study design, at which point it should be decided
what precision and accuracy are needed to test the study hypothesis. An ef
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SAMPLING AND PHYSICAL-CHEMICAL MEASUREMENTS 109
fective quality assurance program is costly (approximately 15-25% of total
expenses) and should be considered when establishing a project's budget.
The use of field and lab spikes and blanks should be a routine practice.
Simple techniques for generating or supplying analytical standards in the field
need to be developed. Particular attention should be given to standards for
highly reactive compounds, which often are of the greatest concern from
human health perspective and for which stable standards often are difficult to
prepare. Techniques could include the use of permeation devices; novel on-
site generation of standards (e.g., reactors that quantitatively produce an ana-
lyte); and compressed-gas standards, where containers are made of highly
deactivated materials. High passivated, sta~nless-stee] canisters might be
useful for this purpose.
Validation studies of samplers and analytical instruments used in exposure
assessments are required. Validations should be performed in settings similar
to those used in exposure studies.
Sampling Techniques and Strategy
The choice of a sampling strategy and a measurement method hinges on
the study specific aims and hypotheses. Physical, chemical, and biological
characteristics of the pollutant dictate the method chosen to sample and meas-
ure airborne concentrations.
A contaminant can have very different health effects in the vapor phase and
in the condensed phase. Care must be exercised that a sampling procedure
collects all of the appropriate phases and does not present a false picture of
a contaminant's physical state.
Personal monitoring (active or passive) is the most direct approach for
assessing human exposure to airborne pollutants. However, the portability
requirement of this technique typically decreases method sensitivity compared
with stationary microenvironmental monitoring. Personal monitors need to
be developed for many contaminants, including certain metals, PAHs, other
semivolatile organic compounds, polar VOCs, and radon decay products. In
some cases, personal monitors already exist, but need to be refined, reduced
in weight and size, and validated (e.g., airborne particles, certain pesticides).
More general instrument needs include personal sampling pumps with im-
proved reliability, stability, and wide ranges of accurate flow rates and low
noise levels. Lighter batteries with longer lives will contribute to improved
personal monitoring.
For pollutants whose effects might be related to peak exposures, personal
samplers are needed that will monitor continuously for only short-term peak
OCR for page 110
110 ASSESSING HUMAN EXPOSURE
exposures. Samplers based on electrochemical principles potentially could
meet this need. In such an application, Bath the use of an electronic discrimi-
nator, the signal characteristic of the analyte could be recorded only when it
exceeds a preset threshold value. For monitoring devices using other chemical
or physical principles, the measurement of short-term peak exposures is a
more difficult problem and requires additional research.
Quiet and unobtrusive microenvironmental samplers (active or passive) are
needed if they are to be used more widely; such samplers should be available
for the majority of atmospheric contaminants.
Passive samplers are well suited to the collection of long-term integrated
samples collected over days or weeks and can be extremely useful in a per-
sonal or microenvironmental study. However, long-term sampling with a
passive monitor places great constraints on the sorbent; it must retain the
analyses without promoting unintended reactions among adsorbed analyses
during the sampling period. Improved sorbents are needed that would permit
long-term sampling for a wide variety of analyses. New sorbents are also
required for polar organics, highly volatile compounds, and very reactive spe-
cies. Ideally, passive samplers should be easily desorbed with the proper sol-
vent or thermal techniques. New Resorption processes need to be developed.
Procedures that permit desorption with a minimum of dilution, such as super-
critical fluid extraction, would be especially useful. Such improvements would
avoid the compromise in analytic sensitivity that often results from the large
volumes used in liquid Resorption techniques.
Instrumental Techniques
Numerous advances have been made in instrument design, operation, and
experimental deployment during the past 10-15 yearse LC techniques are
being used to analyze for compounds not amenable to GC. In particular, IC
(ion chromatography is being used increasingly to analyze for highly polar air
contaminants. In addition, LC-MS is developing into a productive technique
to complement GC-MS. The development of new MS instrumentation (e.g.,
ion trap mass spectrometers) has made MS a valuable air-analysis device.
Microsampling and microanaIytical devices have been developed but are not
yet widely applied. A summary of attributes of different measurement tech-
niques is presented in Table 3.5. (Table 3.5 does not summarize all the
techniques discussed in this chapter; it does include some established tech-
niques that were not discussed in this chapter.)
Some contaminants are not distributed uniformly over the surfaces of air-
borne particles. To evaluate health effects properly, particulate analyses
OCR for page 111
111
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OCR for page 113
SAMPLING AND PHYSICAL-CHEMICAL MEASUREMENTS 113
should include surface techniques that provide elemental or chemical infor-
mation.
Field-Study Instruments
Research and development should focus on better instrumentation for field
studies. These include more portable, reliable, and rugged gas chromato-
graphs, gas chromatograph/mass spectrometers, and ion trap mass spectrum
eters. Sampling methods, instruments, and software that interfaces with such
instruments are also needed to permit unattended sample collection and
analyses in field settings.
A sensitive, highly specific detector applicable to numerous compounds is
needed for the liquid chromato~aph; continued improvements in LC-MS are
beginning to fill this gap.
OCR for page 114
Representative terms from entire chapter:
assessing human
