Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
suitability for personal, indoor or ambient monitoring is also
included.
7.2.1 Gases and vapours
7.2.1.1 Passive samplers
Commercial passive samplers are available for a variety of air
pollutants, including inorganic gases such as carbon monoxide,
nitrogen dioxide, sulfur dioxide and ozone, and VOCs (e.g., benzene,
toluene, xylene, etc.). Passive air samplers are probably the most
convenient tool for conducting large-scale personal exposure
assessments because they are small, inexpensive and easy to use.
However, sampling rates are of the order of 10-50 ml/min and absorbing
capacity is limited. Passive samplers operate on the principle of
molecular diffusion. The rate of diffusion is related to the diffusion
coefficient of the compound, the cross-sectional area of the absorbing
surface and the length of diffusion path. Specific information on the
calculation of sampling rates can be obtained from the manufacturers.
The collection mechanism relies either on physico-chemical absorption
or adsorption or chemical reactions. The samplers for inorganic gases
rely on reaction of the contaminant with a chemical coating on the
collection surface. The samplers for VOCs typically rely on absorption
by a liquid or adsorption by a solid collection medium. Selection and
use of passive samplers should take into consideration potential
sources of error such as wind effects, temperature, humidity and
interfering gases.
In practical applications, personal monitoring is performed by
mounting the passive sampler on a participant's collar to estimate air
pollution concentrations in the breathing zone. After collection, the
adsorbent is removed from the sampler and extracted with the
recommended solvent. The extract is then analysed by a suitable method
(e.g., spectrophotometry, gas chromatography with specific or
unspecific detectors, HPLC, etc.). As with any monitoring procedure,
measures should be taken to evaluate sample preservation and
integrity. These procedures should be described as part of the quality
assurance (QA) protocol and the standard operation procedures (SOPs)
(see Chapter 11).
7.2.1.2 Active samplers
There are many commercially available liquid-media samplers for
reactive and soluble gases, such as liquid-containing bottles, and
solid-sorbent tubes for insoluble and non-reactive gases and vapours,
such as activated charcoal, silica gel, porous polymers or other
materials. Pollutants are transported with the carrier gas (air), and
are captured by collecting media. The most frequently applied
mechanisms in the collection of air pollutants in these media are
chemical reactions (e.g., acid-base and colour-forming), and
absorption/adsorption of the pollutant molecules on collecting media.
Solid sorbent collection efficiency depends on contacting surface
area, air flow rate, temperature, humidity and presence of interfering
compounds.
The sampling rate, breakthrough volume and method limit of
detection are important parameters which need to be considered for an
accurate exposure assessment by active samplers. The identification
and quantification of collected air pollutants are usually performed
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