Handbook of Solvents - George Wypych - ChemTech - Ventech!

1084 Myrto Petreas
lection at the alveolar plateau constitutes
end-exhaled air. When the
total expiration is collected (combination
of all three phases) the
sample is considered mixed-exhaled
air. Simultaneous monitoring
of CO2, 6,19-21 or oxygen and
nitrogen 22 has been suggested to
normalize the measured solvent
vapor concentrations in breath
samples in order to facilitate data
interpretation and comparisons.
The various sampling techniques
must be evaluated and the
one most applicable to the purpose
of sampling chosen. The most
commonly used sampling techniques
based on physiological
principles include mixed-exhaled
air and end-exhaled air:
Mixed-exhaled air. This
technique involves the collection
of the entire volume of exhaled air. It corresponds to a mixture of the alveolar air with air
from the dead space. The collection apparatus may also contribute to the dead space. Total
dead space should be considered and the concentration adjusted, either by subtraction, or by
regression against some other technique unaffected by the dead space. Timing of the breath
collection is important here since the concentration of the air in the dead space may equal
that of the air in the workroom if the sample is taken during exposure, or it may equal zero if
taken after the end of exposure.
End-exhaled air. This technique excludes air from the dead space and collects only the
last part of the breath, in order to estimate the concentration in the alveolar air which is in
equilibrium with the arterial blood. Use of a Haldane-Priestley tube, 23 or simultaneous monitoring
of the peaking of the temperature of the breath 19 or its CO2 concentration, 6,19,24 will
assure a valid alveolar air sample. 6,25,26
Two less frequently used techniques include breath holding or rebreathing to homogenize
the breath sample:
Breath holding. For compounds that achieve equilibrium slowly, some researchers
have used the breath holding technique, which involves holding the breath for 5-30 seconds
prior to exhaling into the collection device. This results in a more constant concentration in
the exhaled breath. The extent to which the lungs are filled with air (as with a deep inhalation)
will affect the results. Kelman 14 showed, however, that breath holding is not necessary
for solvents with blood/gas partition coefficients greater than 10.
Rebreathing. With this technique the subject rebreathes his/her exhaled air to provide
a more homogenized breath sample. This technique is applicable to solvents that reach
steady state very slowly. For prolonged rebreathing, removal of CO2 to avoid pH changes
and/or supply of oxygen may be necessary. 27
Figure 15.2.2. Solvent concentration in breath during exhalation.
15.2.1.5 Breath sampling methodology
The theoretical principles discussed above lead to two choices for collecting breath samples:
Should a mixed-exhaled air or an end-exhaled air sample be collected?