Environ. Sci. Techno/. 1995, 29, 104-107
Sampler for Measmment of KIYOUNG L E E * A N D Y U K I O YANAGISAWA Department of Environmental Health, Harvard School of Public Health, 665 Huntington Avenue, Boston, Massachusetts 02115
A simple sampler for measurement of alveolar carbon monoxide (CO) has been developed. The alveolar CO sampler consists of a mouthpiece, an alveolar air trap system using two one-way valves, and an adsorbent tube. The alveolar CO sampler can collect alveolar CO without subjective discard of dead space air in the subject respiratory system. The small size of the alveolar CO sampler could make the measurement of alveolar CO less laborious. Accuracy and precision of the alveolar CO sampler were evaluated with a pseudo-expired test air at a relative humidity of 80% and at a relative humidity of 100%. The alveolar CO sampler can measure alveolar CO ranged from 3 to 70 ppm without relative humidity effect. As one of the applications of the alveolar CO sampler, halflives of CO for smokers were determined to be 214, 243, and 252 min by taking the alveolar CO samples at 15, 30, 45, 60, 90, 120, 150, 180, and 240 min after smoking. Since the alveolar CO sampler is simple, accurate, reproducible, and not affected by subject cooperation, it would be useful for epidemiological studies.
Introduction Personal exposure is usually measured as a pollutant concentration in the breathing zone, the area within 1 ft from a mouth. However, the transfer rate of air pollutants from the breathing zone to body and sorption efficiencies can have wide variability (1).Precise measurements ofbody burden or dose are important in evaluating adverse health effects, since the personal exposures do not always show a linear relationship with the dose level (2). The recent development of a small personal exposure monitor (PEM) for carbon monoxide (CO) made direct measurements of personal CO exposures possible (3).The use of the PEM contributes understanding of causes, severity,and variability of personal exposures of population as well as individuals. As a result of personal exposure studies using the PEM, personal exposures to CO are found not to be directly correlated with ambient concentrations at fixed-site monitors (3, 4). The personal exposure data with dairies can be used to determine the effect of human activities on the personal exposure. For example, commuting activity is known to significantly influence personal exposures (5). A passive sampler was developed to measure low ambient levels of CO (6). The passive sampler can be used for measurement of integrated personal exposures to CO as well as for stationary measurements in indoor and outdoor environments. Carbon monoxide is collected by a solid adsorbent, Zn-Y-zeolite, after passing through a 5 cm long diffusion tube of 0.32 mm i.d. The collected CO is thermally desorbed and analyzed by gas chromatography with a flame ionization detector (GC/FID) with methane conversion. Passive samplers can measure CO exposures of 30-1600 ppmh. The effects of environmental factors, such as humidity, temperature, and wind velocity, are negligible in the detection of CO exposure. Carbon monoxide is one of few pollutants for which the indicator.of dose is available. The dose of CO can be assessed by carboxyhemoglobin (COHb) levels since inspired CO is rapidly transferred to blood and the most CO in the body is present as a form of COHb. Although COHb is the biological marker directly indicating the amount of CO in the body, measurements of COHb levels are not always possible due to invasive and tedious natures of collecting blood samples. Measurement of alveolar CO can overcome the disadvantage. In order to estimate CO in the body using the alveolar CO, two conditions are required: an equilibrium of CO in alveolar air within arterial blood and a removal of dead space air. There are several maneuvers to accomplish equilibrium between the alveolar CO and CO in arterial blood: rebreathing 5 L of oxygen for several minutes with the removal of COz (7); estimation of the alveolar CO from the mixed expired CO level using measured inspired ventilation volume, inspired CO concentration, and estimated dead space volume (8);and a breath-holding method (9). The breath-holding maneuver has been commonly used due to its simplicity (2, 10). Several inspirations and breathholding times were evaluated to provide simple and * Corresponding author; FAX: 617-432-3349; e-mail address:
[email protected].
104 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 1, 1995
0013-936)(/95/0929-0104$09.00/0 0 1994 American Chemical Society
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I
Mouth piece
Alveolar air trap system
FIGURE 1. Configuration of a sampler for measurement of alveolar carbon monoxide.
reproducible measurement of the alveolar CO (9). Breathholding for 20 s after inspiring full lung capacity was found to allow enough time for the equilibrium of CO in arterial blood and alveolar air (9). Then, about 300-500 mL of initial expired breath is discarded by the subject for the elimination of the air from nongas exchange regions ofthe lung, and the end-tidal portion of expired breath is collected and analyzed. After the equilibriumwas reached by the breath-holding, dead space air is discarded by subject. The subjective discard of dead space air can induce significant error depending on the subjects willingness, age, and so on. The sample volume for measuring the alveolar CO usually depends on ananalyticalmethod, for example, acontinuous analyzer, such as NDIR and ecolyzer, that requires at least several hundred milliliters of the breath sample. Therefore, a plastic bag is usuallyused for collection of the air sample (2, 10). However, the handling of many hags may be laborious when they contain several hundred milliliters of air samples. To solvethe disadvantages described above, the sampler for measurement of alveolar CO must achieve two major goals: to objectivelydiscardthe deadspaceair and to collect expired air only from the alveolar region. In addition, the small size of a collection device is desirable. A simple sampler that satisfies the requirements described above has been developed for measuring the alveolar CO.
Experimental Section ContigurationofSamplerForAlveolar CO. The alveolarC0 sampler consists of three parts: a mouthpiece, an alveolar air trap system, and an adsorbent tube (Figure 1). The alveolar air trap system is made from an acrylic tube and two one-way valves (Hanes Rudolph, miniature one-way valve). The sampler air volume collected by the air trap systemhas to bedeterminedbysensitivityofthecoanalysis and capacity of the CO adsorption. Higher sensitivity is expected by collecting more air sample volume. However, the collection volume is restricted by the CO adsorption capacity of the Zn-Y-zeolite adsorbent. The sue of the adsorbent tube was designed to be the same as the CO passive sampler (6).since it is convenient to analyze the air sample and the alveolar CO sample with one GC/FID system. TheGC/FIDforanalysisoftheCO passivesampler is described elsewhere (6).In the consideration ofthe sue of the adsorbent tube which holds 350 mg of adsorbent, the collection volume of 5 mL was used in the alveolar CO sampler. The sampling procedures for the alveolar CO are as follows: A subject, while standing, inspires room air deeply and then expires completely. The subject is then asked to inspire the room air deeply again, to bold it for 20 s, and to blow the deep breath into the alveolar air trap system.
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When the subjects blow all of the deep breath into the alveolar air trap system, the air trap system holds the final portion of the expired air in the 5 mL of the acrylic tube sealed with the two one-way valves. Since the 5 mL of air in the trap system is continuously replaced by the further expiration of the deep breath air, the expired air coming from dead space of the lung is pushed out. At the end of expiration, only the expired air from the alveolar region is retained by the 5-mL air trap system. The adsorbent tube is a glass tube filled with 350 mg of the solid adsorbent. Both ends of the adsorbent tube were blocked by screen and sealed by rubber septum. The adsorbent, an ion-exchanged Y-type zeolite with a Zn ion, is the same adsorbent used in a CO passive sampler (6). After 5 mL of the alveolar air was held in the alveolar air trap system, the adsorbent tube is connected with the air trap system (Figure 1). By pushing the adsorbent tube toward the mouthpiece, the retained alveolar air in the air trap system can be delivered to the adsorbent tube. The adsorbentadsorbsCOinthe5-mLalveolarairsamplewhile the air sample is passing through it. The adsorbent tube can be analyzed by the GC/FID system developed for the analysis of the CO passive sampler (6'). By replacing the mouthpieceandadsorbent tube, thealveolar airtrap system can be used repeatedly. Evaluation Methods. Prior to evaluation of the whole alveolar CO sampler, including an alveolar air trap system and an adsorbent tube, an appropriate mesh type of the adsorbent was determined by injecting pseudo-expiredtest air into the adsorbent tube. The test air with a known CO concentration was generated by mixing 200 ppm of CO and breathing quality air, as shown in Figure 2. The test air was humidified at 80% relative humidity by passing a humidity generation system in which distilled water was heated and bubbled to generate water vapor. The test air of 5 mL was injected to the adsorbent tube using a gastight syringe through a rubber septum sealing one end of the adsorbent tube. Two different particle sizes of the adsorbent, i.e., diameters of4255850 and 250-425pm, were compared to determine the effect of the mesh type on the collection efficiency. The adsorbents with diameters of 425-850 and 250-425 pm were prepared with 20-40 and 40-60 mesh sieves, respectively. The effectofflow rate onthe collection VOL. 29.
NO. 1,1995 I ENVIRONMENTAL SCIENCE 8 TECHNOLOGY m 105
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efficiency was evaluated by injecting the test air with different flow rates, such that 5 mL of the test air was injected to the adsorbent tube in 1 or 10 s. The 425-850 pm adsorbent was chosen for the adsorbent tube based upon experimental results described later. Accuracy and precision of the alveolar CO sampler were evaluated using the pseudo-expired test air, which was introduced to the alveolar air trap system, and then an adsorbent tube was connected to the air trap system (Figure 1). Carbon monoxide in the alveolar air trap system was collected by pushing the adsorbent tube toward the mouthpiece. The adsorbent tube was analyzed by the GC/ FID. The sampling was duplicated at each CO level. Humidity effect was determined by a test air at a relative humidity of 100%. After distilled water was injected into a plastic bag, various volumes of 200 ppm of CO and breathing quality air were injected to the bag. The plastic bag with the air sample was placed inside an incubator at a temperature of 37 "C. The 1-mL sample of the test air was collected by a syringe and analyzed by the GC/FID. The rest of the air sample was pushed to the air trap system. Carbon monoxide in the air trap system was collected by the adsorbent tube, which was analyzed by the GC/FID. As an example of applications of the alveolar CO sampler, alveolar CO samples of smokers were collected many times at short intervals after smoking to calculate the half-life of CO in the body. Alveolar CO of three male smokers was measured just before, just after, and 0.25, 0.5, 0.75, 1, 1.5, 2, 2.5, 3, and 4 h after smoking a cigarette.
Results and Discussion Collection Efficiency ofAdsorbent Tube. When collection efficiency of the adsorbent tube was sought by injecting the test airby a gas-tight syringe,the amount of CO collected by the adsorbent tube increased with the use of a small adsorbent, as shown in Figure 3. When particle diameters of the adsorbent were 250-425pm (40-60 mesh), collection efficiency of the adsorbent tube was increased up to 90% of the amount of CO injected in the range of 25-340 nL. The collection efficiencyof the adsorbent tube significantly decreased with the amount of CO more than 340 nL, which may be due to the saturation of the adsorption sites with CO and water molecules. The adsorbent tube with 425850 pm (20-40 mesh) adsorbent collected about 50% of CO injected in the range from 25 to 340 nL. The collection efficiency was gradually decreased from 50% when more than 340 nL of CO was injected. Although the use of the small adsorbent (40-60 mesh) provided high collection efficiency,the handling of the small 106
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 1, 1995
0
0
50 100 150 200 CO collected by the sampler for alveolar CO (nl)
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FIGURE 4. Relationship between amount of CO collected by alveolar CO sampler and CO level in pseudo-expired test air.
adsorbent was practically difficult because of significant pressure drop. When the adsorbent tube with the small adsorbent was pushed toward the mouthpiece, the pressure drop by the adsorbent tube was high enough to disintegrate the one-way valve of the air trap system. Therefore, the small adsorbent cannot be used in the alveolar CO sampler. The adsorbent tube with adsorbents of diameters of425850 pm can collect CO in the alveolar air trap system with a consistent collection efficiency of 50% in the range of 25-340 nL. In addition, the collection efficiency of the adsorbent tube was not affectedby different injection speed. The difference of collection efficiency was less than lo%, when 5 mL of the test air was injected within 1 and 10 s. Therefore, the adsorbent with diameters of 425-850 pm was selected for the alveolar CO sampler. EvaluationofAlveolar CO Sampler. The alveolar airtrap system was evaluated with the adsorbent tube containing the adsorbents with diameters of 425-850 pm. When the pseudo-expired test air of various CO concentrations was introduced into the air trap system and collected by the adsorbent tube, the amounts of CO collected were linearly related with the CO levels between 5 and 70 ppm, as shown in Figure 4. In the range of test air CO levels between 5 and 70 ppm at a relative humidity of EO%, the linear regression line between test air CO concentration and the amount of CO collected by the alveolar CO sampler has an intercept of 0.40 (standard error = 2.91) and a slope of 0.50 (standard error = 0.02) with a correlation coefficient of 0.9918. The differences of duplicates were less than 9% of the average of duplicates in the detection range. The minimum detection limit was determined by the variation of the amount of CO in the unexposed adsorbent tube. The adsorbent tube unavoidably contains a small amount of CO as a result of the assembly procedure. The standard deviation of the amount of CO in unexposed adsorbent tube is usually f2.0nL. If a signal to noise ratio is assumed to be 3 for the minimum detection limit, the detection limit of 6.0 nL corresponds to an alveolar CO of 3.2 ppm. The maximum detection limit was determined by collection efficiency. The adsorbent tube maintained the collection efficiency of 50% up to 70 ppm of CO. When the CO concentration was higher than 70 ppm, the collection efficiency was significantly decreased. Therefore, the maximum detection range was determined as 70 ppm alveolar CO. The maximum detection limit is closely related with collection by the adsorbent tube, as shown in Figure
CO may allow the identification of activities which significantly affect CO exposures.
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FIGURE 5. Change of alveolar CO before and after smoking.
3. The collection efficiencyof the adsorbent tube with 425850 pm adsorbent was 50% in the range of 25-340 nL. The collection efficiency of the adsorbent tube was significantly decreased when CO of the pseudo-expired test air was higher than 340 nL. The 340 nL of CO corresponds to about 70 ppm, since the volume of air in the air trap system is 5 mL. The relative humidity of the pseudo-expired test air was 80%,while the relative humidity of human end-expired air is close to 100%. Test air at a relative humidity of 100%was generated by a plastic bag with distilled water. Since CO concentration in the test air was measured by the GClFID system, the measurement could be free from water vapor interference (11). When the test air at a relative humidity of 100% was collected by the alveolar CO sampler, the amounts of CO collected by the alveolar CO sampler were not affected by the relative humidity of loo%, as shown in Figure 4. Application ofAlveolar CO Sampler. One of advantages of alveolar CO measurement is that it is possible to repeatedly measure alveolar CO with a relatively short interval due to its noninvasive nature. When alveolar CO of three male smokers was measured by the alveolar CO sampler before and after smoking with intervals of 15,30, and 60 min, alveolar CO profiles were varied with the subjects, as shown in Figure 5. Increases of alveolar CO after smoking were 8, 6, and 5 ppm. Alveolar CO of one subject decreased just after smoking, and alveolar CO of other two subjects decreased after 30 min. The half-lives of the alveolar CO of the three male smokers were 214,243, and 252 min when the subjects were quietly sitting after smoking. The half-lives of alveolar CO with the subjects sitting in the room were comparable with the values in a previous investigation (12). A relationship between alveolar CO and COHb levels was determined in many studies using the breath-holding technique (2, 13, 14). The alveolar CO was used as an indicator of the previous several hour exposure and supported the selection of 8-h average CO exposure as the National Ambient Air Quality Standard (15). The alveolar CO was used to evaluate the effects of home heating systems on CO exposure (16)and to identify the cause of indoor air problems ( 1 7). These studies measured the alveolar CO only once, usually at the end of an exposure period. Multiple measurements of alveolar CO can accurately estimate the CO exposure in environmental settings with various CO concentrations, since the half-life of CO in the body is relatively short. Multiple measurements of alveolar
The alveolar CO sampler consisting of an alveolar air trap system and an adsorbent tube has been developed and evaluated using pseudo-expired test air and real human breath. The air trap system collects the end-tidal 5 mL of expired air without subjective discard of dead space air. The CO in the 5-mL alveolar air is collected by an adsorbent tube. The detection range of the alveolar CO sampler was from 5 to 70 ppm. The amounts of CO collected by the alveolar CO sampler were linearly related to the CO levels in the test air as well as the alveolar CO levels of human breath collected by a plastic bag. With repeated measurements of alveolar CO, alveolar CO was increased by 8-14 ppm and decreased with a half-life of 214-256 min after smoking. The simple alveolar CO sampler can be used for multiple measurements of alveolar CO to predict accurately the previous CO exposure.
Acknowledgments This research is supported by Biomedical Research Support Grant. The authors thank the participants for their time and their careful adherence to our instructions. We also thank Dr. Robert Banzett for useful discussion and Joan Arnold for editing the manuscript.
Literature Cited (1) Droz, P. 0. Appl. Ind. Hyg. 1989, 4, F-21-F-24. (2) Lambert, W. E.; Colome, S. D.; Wojciechowski, S. L. Atmos. Environ. 1988, 22 (lo), 2171-2181. (3) Wallace, L. A.; Ott,W. R. J. Air Pollut. Control Assoc. 1982, 32, 601-610. (4) Cortese, A. D.; Spender, J. D. J. Air Pollut. ControZAssoc. 1976, 26, 1144-1150. (5) Akland, G. G.; Hartwell, T. D.; Johnson, T. R.; Whitmore, R. W. Enuiron. Sci. Technol. 1985, 19, 911-918. (6) Lee, K.; Yanagisawa, Y.; Hishinuma, M.; Spengler, J. D.; Billick, I. H. Environ. Sci. Technol. 1992, 26 (4), 697-702. (7) Hackney,J. D.; Kaufman, G.A.; Lashier,H.; Lynn, K.Arch. Enuiron. Health 1962, 5, 300-307. (8) Rawboned, R. G.; Coppin, C. A.; Guz, A. Clin. Sci. Mol. Med. 1976, 51,495-501. (9) Jones, J. G.; Ellicott, M. F.; Cadigan, J. B.; Gaensler, E. A. J. Lab. Clin. Med. 1958, 51, 553-564. (10) Wallace, L.; Thomas, J.; Mage, D.; Ott,W. Atmos. Enuiron. 1988, 22, 2183-2193. (11) Lee, K.; Yanagisawa,Y.; Spengler, J. D.; Billick, I. H. Proceedings of the Conference of Air & Waste Management Association; No. 92-80-04; AWMA: Kansas City, MO, 1992. (12) Peterson, J. E.; Stewart, R. D. Arch. Enuiron. Health 1970, 21, 165-171. (13) Guyatt, A. R.; Kirkham, A. J. T.; Mariner D. C.; Cumming, G. Clin. Sci. 1988, 74, 29-36. (14) Wald, N. J.; Idle, M.; Boreham, J.; Bailey, A. Thorax 1981, 36, 366-369. (15) Wallace, L. A.; Thomas, I.; Mage, D. T. U.S.Environmental Protection Agency, Research Triangle Park, 1984; EPA-6OO/D84-194. (16) Cox, B. D.; Whichelow, M. J.J. Epidemiol. Commun.Health 1985, 39,75-78.
(17) Wallace, L. A. 1.Air Pollut. Control Assoc. 1983, 33, 678-682.
Received for review M a r c h 30, 1994. Revised manuscript received September 13,1994. Accepted September 14,1994.@
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Abstract published in Advance ACS Abstracts, October 15, 1994.
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