Types of exhaled air samples

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 brea 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 die alveolar air which is in equilibrium with the arterial blood. Use of a Haldane-Priestley tube, or simultaneous monitoring of the peaking of the temperature of the breath or its COj concentration, will assure a valid alveolar air sample.  

Two less frequently used techniques include breath holding or rebreathing to homogenize the breath sample  

Breath holding. For compounds fliat achieve equilibrium slowly, some researchers have used the breath holding technique, which involves holding die 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 affeet the results. Kelman showed, however, that breadi holding is not necessary for solvents with blood/gas partition coefficients greater than 10. 

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Methodologieally, the differenee between these two types of samples is the control over which portion of the breath is collected. Many techniques make use of the Haldane-Prestley tube to collect the last portion of the expiration. Earlier work used the simultaneous monitoring of COj or temperature in the exhaled breath to identify the moment (CO2 or temperature reaching a maximum) when air from the anatomical dead space has been purged and alveolar air can be sampled. 

In addition to field studies, controlled human exposures to PERC have been reported. In this type of studies, subjects were exposed to various constant levels of PERC for different lengths of time with or without exercise. Samples of blood, exhaled air and urine were collected according to a variety of schedules both during exposure and for varying lengths... 

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