INDUSTRIAL HYGIENE
AIR SAMPLING
UNIVERSITY OF HOUSTON DOWNTOWN
PURPOSE
Introduce the techniques available for
Industrial Hygienists to evaluate
EXPOSURES to particulates, gases, and
vapors arising in or from the workplace.
Also:
be aware of technology available for
assessment of environments
indoor and ambient air, and
capabilities and limitations of
methods.
SAMPLING STRATEGY

When developing a particular sampling
strategy, review sampling and analytical
methods available for the contaminants of
interest. Select most suitable for the
specific application.
e.g. OSHA, NIOSH
i.e. published and validated methods
EPA methods used for lower level indoor air
pollutants and toxic compounds in ambient
air media.
SAMPLING METHOD
Select a method that meets the sampling
and analytical ACCURACY and PRECISION
requirements of the standard in its unique
field
conditions.
Usually
stipulate
measurement at the PEL within a +/- 25%
of the “true” value at a 95% confidence
level.
EPA – indoor air pollutants and toxic
compounds associated with ambient air.
ANALYTICAL LABORATORY
Select and consult with a qualified
analytical laboratory, e.g. AIHA that
participates in Laboratory Accreditation
Programs. Labs can assist in choosing
methods that meet the sensitivity and
specificity criteria for the environment
being evaluated.
Choose sampling
media and strategy compatible with
method selected and advise on special
handling.
SAMPLING PLAN

Designing
a
sampling
plan
involves
consideration of the following: location of
samples, the number of workers to be
sampled, and the duration of sampling.
Also consider other factors – noise,
equipment, size, flow rate, and security.
• Personal vs. Area Sampling
• Grab vs. Integrated Sampling
• Active vs. Passive Sampling
GRAB SAMPLING
This technique involves the direct
collection of an air-contaminant mixture
into a device (i.e. sampling bag,
syringe, or evacuated flask) over a
short interval of a few seconds or
minutes. Represents the atmospheric
concentrations at the sampling site at a
given point in time.
GRAB SAMPLING
This type of collected sample measures gas
and vapor concentrations AT A POINT IN
TIME and are used to evaluate “PEAK”
exposures for comparison to “Ceiling” limits.
Can
be
used
to
identify
unknown
contaminants, to evaluate contaminant
sources, or to measure contaminant levels
from intermittent processes or other
sources.
Collected
using
syringes,
canisters, or sampling bags.
Instantaneous (as well as integrated)
measurements of gases/vapors also may be
performed using detector tubes or directreading instruments.
GRAB SAMPLING – ADVANTAGES
-
After collection, can frequently be
analyzed immediately by GC or directreading instruments.
-
Therefore, quick decisions can be made
in field or at the site about source of
leaks, Confined Space Entry (CSE),
PPE, etc.
GRAB SAMPLING –
DISADVANTAGES
- For most applications, contaminants
are
collected but not integrated over
time. Only
some devices will allow
use of a metering
device to collect
sample(s) at or near constant
flow
over period of time for TWA.
- For low contaminant concentrations,
analytical instrument may not be
sensitive for detection.
INTEGRATED SAMPLING
For gases and vapors, involves
passage of a known volume of air
through an absorbing or adsorbing
medium to remove the desired
contaminants from the air during a
specified
period
of
time.
Contaminants of interest are collected
and concentrated over a period of time
to obtain the average exposure levels
during the entire sampling period.
INTEGRATED SAMPLING
This type of sampling to cover the
entire period of exposure is required
because
airborne
contaminant
concentrations during a typical work
shift vary with time and activity. Grab
samples do not reflect average
exposures. Most integrated sampling
is done to determine the 8-hour TWA
and/or STELs to compare with OSHA
PELs, ACGIH TLVs and NIOSH RELs.
INTEGRATED SAMPLING
CONSIDERATIONS
Appropriate sample duration and flow rate
need to be chosen relative to the purpose of
sampling, the sensitivity of the analytical
method, and the expected concentration of
the contaminant of interest.
It is also
essential that the flow rate and time be
accurately measured. The accuracy depends
on the precise determination of the mass of
contaminant collected as well as the volume
of air sampled.
ACTIVE SAMPLING
Means of collecting an airborne substance
that employs a mechanical device such as an
air
sampling
pump
to
draw
the
air/contaminant mixture into or through the
sampling device.
Examples: sorbent tube, treated filter, or
impinger containing a liquid media.
A key element is calibration that reliably
measures the pump flow rate, thus allowing
for an accurate determination of air volume.
ACTIVE SAMPLING –
ADVANTAGES
-
-
-
Select method to be used by compliance
personnel during OSHA inspection.
Offers calibrated, measured airflow for
assurance in accuracy of sample volume.
Sorbent tube samples have a secondary
layer for back-up indicating
breakthrough.
Multiple phases can be assessed by a
series of samplers.
ACTIVE SAMPLING –
DISADVANTAGES
-
-
Cumbersome equipment and may
interfere with job of workers throughout
shift.
Pump calibration is time consuming and
requires technical training on tasks.
Pump may become somewhat less
reliable at maintaining constant flow over
the entire sampling period, and more
frequent calibration may be necessary.
PASSIVE SAMPLERS
Commercially available for a variety of
airborne contaminants. Some samplers are
designed to collect a broad range of
compounds, whereas others because of their
collection media preferentially collect a single
chemical or family of chemicals.
Examples:
activated charcoal sorbent –
organic vapors and GC analysis; chemical
treated sorbents or filter paper for preferential
collection for HPLC analysis.
Direct-reading passive samplers
based on
colorimetric techniques.
May not be as
accurate as lab analytical methods. Discuss
examples.
DIFFUSIVE SAMPLERS
Diffusive
samplers
rely
on
the
movement of contaminant molecules
across a concentration gradient, which
for steady-state conditions can be
defined by Fick’s first law of diffusion.
Consist of diffusion gap between
external air and a sorbing medium
which serves to collect the chemicals of
interest, but also to maintain the
concentration as close to zero as
OSHA ISSUES
Research report in 1998 that attempted to
determine sampling rate variation of specific
passive sampler designs.
Concept of
passive
sampling
equated
to
active
sampling with pump error of +/- 5%.
Significant in that use of the sampling rate
variation for a passive sampler along with
the analytical error component allowed the
calculation of the overall Sampling and
Analytical Error (SAE). SAE must be used
by OSHA inspectors along with sample
results to determine if PEL exceeded.
Therefore, passive sampling methods can
be used by OSHA.
PASSIVE SAMPLING –
ADVANTAGES
-
Easy to use, allowing samples to be
collected by personnel with less technical
training.
- Less expensive.
- Less obtrusive to wearer for monitoring.
- For most applications, the mass of
contaminant collected by passive
samplers is not significantly affected by
temperature or pressure.
PASSIVE SAMPLING –
DISADVANTAGES
May not be OSHA/NIOSH
methods to reference
in order to
insure reliability of data.
- Sampling rate, if theoretically
calculated, may
not prove to be
valid under field conditions.
- Reverse diffusion may be a factor.
- Environmental parameters may
influence the
collection efficiency of
passive samplers.
Examples: stagnant air; high face
velocities.
AIR SAMPLING
INSTRUMENTS

Five basic components of air sample
collection devices :
• Air inlet orifice
• Collection device
• Airflow meter
• Flow rate control valve
• Suction pump
COLLECTION DEVICES FOR
PARTICULATES







Filters
Impactors
Impingers
Elutriators
Electrostatic precipitation
Thermal precipitation
Cyclones
Fundamentals of IH reference book (5th Edition): Table 16-C, pg 528
AIR SAMPLING PUMPS



Integrated methods require a relatively
constant source of suction that can be
calibrated to the recommended flow
rate (within +/- 5% with collection
media in-line).
Personal sampling within the worker’s
breathing zone or can be used as area
samplers.
Features – constant flow
capabilities/back pressure; intrinsically
safe; electromagnetic susceptibility,
FLOW RATE METERS


Pressure compensating devices
Critical flow orifice
SAMPLE COLLECTION
MEDIA
Consult published air sampling methods to
determine the appropriate collection media
for a specific chemical contaminant.
Review methods to determine applicability
relative to field conditions. Such as: vp,
bp, reactivity; interferences as well as also
humidity/temperature
effects,
proper
measuring range; physical state of the
contaminant
being
sampled;
multiple
phases (i.e. particulate and vapor phase).
SOLID SORBENT MEDIA
Adsorb onto surface; Effectiveness
determined by:
 Trap and retain nearly all contaminant
from air
 Amenable to desorption from sorbent
 Sufficient capacity to retain quantity of
contaminant to facilitate analysis
without creating large pressure drop
across sample media
 Not cause chemical change of
contaminant except by analytical
COLLECTION EFFICIENCY OF
SOLID SORBENTS
Various Factors:
 Temperature
 Humidity
 Sampling Rate
 Other Contaminants
 Sample Breakthrough
- 25%
- Migration
TYPES OF SORBENT MATERIALS



Inorganic Sorbents – silica gel (polar;%RH);
less reactive than charcoal
Elemental Carbon – charcoal types;
organics; high adsorptive capacity; stable
compounds; high humidity parameters
Carbonized or Graphitized Sorbents – low to
moderate surface area; intermediate to high
volatility; stable compounds for thermal
desorption
Fundamentals of IH reference book (5th Edition): Table 16-B, pg 527
OTHER MEDIA



Chemically treated filters –
derivatize/desorb
Liquid absorbers
• Gas washing bottles – e.g. impingers
• Fritted glass bubblers
Sampling bags/partially evacuated rigid
containers (canisters)
• Situations: use of direct-reading
instruments; leaks/emergencies; peak
concentrations; highly volatile compounds
- Precautions – storage time; reaction;
diffusion
CALIBRATION
Pump flow must be calibrated with the
entire sampling train assembled as it
will be used in the field. Good IH
practice requires both pre- and postpump calibration on the same day
under pressure and temperature
conditions similar to those at site.
Should not be done with built-in
rotameters (not precision devices and
will not give a quantitative measure of
the rate of airflow).
CALIBRATION STANDARDS
Two terms:
 Primary – direct and measurable linear
dimensions (length and diameter of
cylinder)
• Examples: spirometers and bubble
meters
 Secondary – flowmeters that trace
calibration to primary standards and
maintain accuracy with reasonable care and
handling in operation.
• Examples: precision rotameters, wet test
meters, and dry gas meters.
Refer to instructions from manufacturers.
OPERATIONAL LIMITS OF
SAMPLING AND ANALYSIS
Inherent limitations of method:
- Sampler capacity
- Limit of Detection (LOD)
- Limit of Quantification (LOQ)
- Upper measurement limits which define the
useful range of the method.
These factors determine the minimum,
maximum, or optimum volume of air to be
sampled and may determine the confidence
that can be placed in the results.
Discuss with lab before sampling!
SAMPLER CAPACITY
Predetermined conservative estimate
of the total mass of contaminant that
can be collected on the sampling
medium without loss of overloading.
NIOSH definition of 2/3 of the
experimental breakthrough capacity of
the solid sorbent, that is 67% of the
mass of contaminant on the sorbent at
the breakthrough volume.
Breakthrough volume is defined as
that volume of an atmosphere
containing two times the PEL for the
CALCULATIONS







Total Mass of Contaminant
Airborne Concentration by sample
volume (mass over volume)
Air Volume (flow rate x sample time)
Unit Conversions – mg/M3 to/from
ppm
Temperature/Pressure Corrections
Time-Weighted Averages
Potential Work Shift Adjustments
REMEMBER
CALCULATIONS
A range of temperature and pressure
changes can be tolerated before
corrections are applied to the volume
or air sampled during an exposure
assessment.
All OELs and environmental exposure
standards and limits are expressed at
25 degrees C and 1 atmosphere (760
mm Hg), defined as normal
temperature and pressure (NTP).