Environ. Sci. Techno/. 1995, 29,2311-2316
Evaluation of Elemental Cadmium as a Marker for Environmental Tobacco Smoke DE W U , + SHELDON LANDSBERGER,*st AND SUSAN M. LARSONt Department of Nuclear Engineering, University of Illinois, 214 Nuclear Engineering Laboratory, 103 South Goodwin Avenue, Urbana, Illinois 61801, and Department of Civil Engineering, University of Illinois, 3230 Newmark Civil Engineering Laboratory, 205 North Mathews Avenue, Urbana, Illinois 61801
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The evaluation of cadmium as a marker of environmental tobacco smoke (ETS) has been carried out on the basis of several criteria. First, cadmium was found to be a good indicator source in ETS. For example, elevated Cd concentrations (4-38 ng/m3) were measured in smoking areas compared with less than 2 ng/m3 in ETS-free indoor air. Secondly, the cadmium concentrations in various types of unburnt cigarette tobacco are not significantly different, with the average concentration being 1.28 f 0.17 p g of Cd/g of tobacco. Furthermore, the emission fraction of cadmium is quite large: approximately 44% of the total cadmium in cigarette tobacco is released in ETS. Cadmium in ETS was primarily found in the small particles (diameter less than 1 pm), indicating Cd to be a good marker of respirable ETS. Finally, the Cd concentration in ETS was observed to decay slowly and to increase linearly with the strength of the source. On the basis of these evaluations, it is concluded that cadmium is a good marker of ETS.
Introduction Environmental tobacco smoke (ETS) contains many toxic organic and inorganic substances (1)and is thus considered a major source of indoor air pollution, causing adverse health effects not only for smokers but also for nonsmokers exposed to ETS. A unique and readily-measured marker of ETS would be valuable in monitoring ETS concentration, in predicting exposure to ETS, and in evaluating ETS health risks. Cigarette smoke consists ofmainstream smoke (MS) and sidestream smoke (SS). MS is the smoke inhaled by a smoker, and SS is the smoke released from the burning tip of a cigarette between puffs. ETS consists of SS and the smoke exhaled by the smoker, which is part of MS. It has been found that SS contains higher levels of cancer-causing substances than MS per unit of Particulate material of smoke (2). +
Department of Nuclear Engineering
* Department of Civil Engineering. 0013-936x/95/0929-2311$09.00/0
D 1995 American Chemical Society
ETS is a very complicated substance. More than 4000 components have been identified,accounting for more than 95% of ETS mass (1). The major components include particulate matter, nicotine, carbon monoxide, nitrogen oxides, formaldehyde, and volatile organic compounds. Thus,when exposure of ETS is evaluated,the concentrations of these components should be determined. It is however inconvenient to directly measure all ETS components, even just the major ones. Therefore, an ETS marker, which must meet several requirements, is helpful to serve as a tracer to quantify the concentration of ETS components using the concentration of the marker. Once ETS concentration is known, the exposure to ETS can be estimated. For a substance to be considered a marker of ETS, the National Academy of Sciences (3)suggests that the following four requirements be met: (a) The species is unique or nearly unique to ETS, that is, there is a low contribution of the species from other sources. (b) There is a determination method available for the species even for low level measurements. (c) There is a similar emission factor for the marker from various cigarette products. (d) The marker is present in constant proportion to the ETS components, which cause adverse health effects. Almost all major ETS components have been proposed as markers. For example, carbon monoxide (CO) was widely used as a marker of ETS before the mid-1980s (4). CO is easy to measure but is unfortunately not unique. It is a common substance released from many sources (such as gas stoves, heaters, and motor vehicles) and is therefore no longer considered a suitable marker of ETS (1). Respirable suspended particulate (RSP) matter has also been used as an ETS marker. It, however, is also not unique as it could come from other sources. Thus, using RSP as an ETS marker may overestimate ETS exposure. In many cases though, the elevated RSP level from ETS is significantly higher than that from other sources. Therefore, RSP is still widely used as a marker of ETS. Because of its uniqueness to ETS, nicotine is a commonly accepted marker. A problem associated with nicotine is that it is mostly found in the gas phase (go%), making it a relatively poor particulate marker. Furthermore, Eatough et al. (5) reported that the fraction of nicotine in ETS vaned with the measurement conditions. For example, 5-10% of ETS nicotine was found in the particle phase in controlled atmospheres, while 20% was found in field environments. While other compounds, such as pyridine, are not commonly reported as ETS markers, Eataough et al. (6) demonstrated that 3-ethenylpyridine, present in the gas phase, is unique to tobacco smoke and is more stable under Wconditions than is nicotine. It has been thus proposed as an ETS marker (I). Solanesol has also recently been evaluated as a marker of ETS (7, 8). It is unique to ETS and exists in air as condensed particulate matter, making it suitable for a particle phase marker. But the determination of solanesol is not simple (1). It involves an extraction from a filter to a solution with recovery about 60-90%, and then samples are analyzed by gas chromatography (GC) and supercritical fluid chromatography. Through an indoor air survey conducted in many residential homes, Lebret et al. (9) found that airborne cadmium was exclusively high in smoker’s homes. They suggested therefore that cadmium may be a good marker
VOL. 29. NO. 9, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of ETS. Although this suggestion has been made, there has been no complete evaluation of cadmium as an ETS marker reported in the literature. One of the reasons for this may be the previous lack of a sensitive analytical method, which is demanded by the criteria b. In previous work (IO),we successfully determined elevated levels of airborne cadmium in public places where smoking is present, using instrumental epithermal neutron activation analysis (NAA) in conjunction with a Compton suppression system. With this method, the detection limit of Cd analysis can be as low as 2 ng/sample, which satisfies criteria b. Therefore, we undertook a complete evaluation of cadmium as an ETS marker. As stated above, an ideal marker is one that only comes from the targeted source. Realistically, a marker can be successful if it has a low background concentration or if there are few sources other than the targeted source. This criteria (a) is fulfilled for Cd as an ETS marker. Except for buildings where kerosene heaters are used or where there are workshop facilities (111, cadmium levels are very low in most indoor environments that are ETS free. We have found cadmium concentrations in environments such as a library and an ofice to be less than 2 ng/m3,which is our detection limit of the method. Cadmium concentrations in ambient air varies with geographical region. For example, in remote areas, such as Antarctica, cadmium was found at a level of 5 pg/m3 (12). Purghart et al. (13)measured cadmium in rural Switzerland at a level of 1 ng/m3. In urban areas, this level can be as high as 3-2270 ng/m3 (14-16). The major cadmium sources in outdoor air are waste incineration and non-ferrous smelters. Kauppinen and Pakkanen ( 1 7) measured cadmium concentrations in hospital refuse incineration aerosols and found values as high as 6-28pg/m3. Our study will assume criteria a holds and that background cadmium levels are less than 1.5 ng/ m3. (Thus cadmium may not be a suitable marker for ETS in highly polluted urban areas.) In this study, the suitability of cadmium as a marker for ETS was examined focusing on criteria c and d. To investigate criteria c, the experimental approach centered on determining cadmium concentration in cigarette tobacco as well as on finding the cadmium emission factor for MS and SS. Then, measurements were made in a controlled chamber to determine the ratio of cadmium to total suspended particles (TSP)in ETS to test criteria d. (We assumed that TSP represents the particulate component responsible for ETS health effects.) Finally, field samples in public places where smoking was allowed were taken and analyzed in order to examine the fraction of particles from ETS using cadmium as an ETS marker.
Experimental Section Collection of Cigarette Component Samples. Six brands of American commercial cigarettes. (Vantage, Camel, Marlboro, Merit, Kool, and Winston) were chosen for this study. The first five brands include both 1983 and 1993 products to get an indication of the difference between recent and aged tobacco. Two brands of research cigarettes 1R1 and 1R4F references, from the University of Kentucky, were also included in the study. The cigarettes were kept in a refrigerator for long-term storage. They were placed at a temperature of 20-25 "C and a relative humidity of 50-60% for at least 24 h prior to being used. Three cigarettes from each brand were separated into tobacco, paper, and filters for the samples. 2312 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 9.1995
8
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e s s t1st i l SS t efilter r h oholder i&r
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Glass container
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Smoking machine
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? Cigarette
Pump Air in
FIGURE 1. Schematic diagram of the mainstream and sidestream smoke collection device.
Cigarette ash and butts (with used filters if any) from smoked cigarettes were also saved. The cigarette butts fromfiltered cigarettes were further separated as cigarette tobacco butts and used cigarette filters. These cigarette componentswere analyzed for their elemental concentrations. Collection of MS and SS Samples. Mainstream and sidestream smoke were sampled using facilities at the Oak Ridge National Laboratory (ORNL). MS samples were collected by a filter placed between a cigarette and the draw port of a smoking machine (Figure 1). The SS sample needs a special device for containing the smoke and for channeling it to the sidestream filter (Figure 1). Overall, the apparatus includes a smoking machine, two mainstream smoke filter holders, a sidestream smoke hood with a metal screen on the bottom for incoming fresh air, two sidestream smoke filter holders, and a small sampling pump (Gilian GWr-5). Other types of MS and SS sample collectors have been used in the literature (18). The smoking machine can simulate human smoking with an adjustable puff volume and frequency. The standard smoking conditions [I puff/min, puff volume of 35 mL, and puff duration of 2 s (19)l were used for the sample collection. As can be seen from Figure 1, two filters were placed between the cigarette end and the port of the smoking machine. (The necessity for two filters will be discussed in the next section.) The particles in MS were only collected during the puffing stage by air drawn by the smoking machine. Each cigarette usually had about 10 puffs. Therefore, the total sampling volume for MS was about 0.35 L. Sidestream smoke was captured by a glass container or hoodwith avolume of about 3.4 L. A cigarette was lit outside of the container in the first puffing, and then the cigarette was quickly moved into the container. Meanwhile, the small sampling pump was turned on to draw the smoke out of the container. The particles in the SS were collected by a series of two filters located on the top of the hood. The air flow rate of the pump was 4 L/min. The total sampling volume for SS was about 44 L. The air velocity upward in the lower part of the hood was approximately0.38 cm/s. Because the particle settling velocity is only 0.001 cm/s for 0.5-pm particles and 0.013 cm/s for 2-pm particles (ZO),particle settling loss was negligible. Sample collection of SS continued for approximately2 min after the cigarette was extinguished.This was necessitated by the high concentration of cigarette smoke in the SS hood that must be collected. There may have been some loss of SS onto the walls of the container or escape of SS from the hole used for inserting a cigarette. However, there was not much material wiped off the walls at the conclusion of each test, and therefore the fraction of these losses was assumed negligible.
The filter used in MS and SS sample collection was a membrane filter from Millipore. The filter pore size is 0.8 pm for the first filter in the series and 0.22pm for the second one. This setup was designed to measure the filter penetration of the particles in the smoke. Each filter was weighed before and after sampling to determine the captured mass of particulate material. The cadmium emission factor of a cigarette was determined as the ratio of the amount of cadmium on the SS filters to the amount of cadmium in the cigarette. Collection of ETS Sample in a Chamber. To further characterize ETS and to validate the emission factors measured in the MS/SS experiments,cigarettes were burned in a controlled atmospheric chamber at ORNL. The chamber has a room-size dimension: 3.7 m long, 3.0 m wide, and 2.7 m high. There is a door 1.0 m wide and 2.0 m high and two 0.6 m x 0.6 m windows. The interior walls, ceiling, and floor are made ofstainless steel. Electric power is supplied through wall plugs. The makeup and recirculation air comes into the chamber through uniformly distributed holes in the ceiling. The chamber is air conditioned, with a temperature ranging from 10 to 32 "C and arelative humidityrangingfrom20% to 80%. Operation mode of the chamber can be static (without air circulation), air circulation bypass, filtered air circulation, or outside makeup air circulation. The blower for air circulation can be adjusted from 0 to 100% of 1000 ft3/min (1699 m3/h). The chamber was normally operated at a temperature of 20 "C and a relative humidity of 50% for this study. Three experiments were performed in the chamber for this study. The first was to examine the airborne cadmium concentration as a function of the number of cigarettes burned. This will show whether cadmium concentration is proportional to ETS source strength. The second was to evaluate the decay of airborne cadmium concentration with time to ensure that cadmium does not decay more rapidly than the TSP in the ETS. A third experiment measured the cadmium size dependence to determine whether cadmium was a tracer for respirable particulate ETS. Environmental tobacco smoke was generated for these experiments by either freely burnt (smoldered) 1R4F cigarettes (only SS produced) or cigarettes smoked by a machine (both MS and SS injected into in the chamber). The conditions for each experiment were as follows: (1) Cd concentration as a function of the number of cigarettes burned. The chamber was operated at normal circulation mode, and the cigarettes were freely burnt in the chamber. In five separate experiments, the number of cigarettes burnt was 0,2,4,6,and 8. Three small sampling pumps, placed at different locations in the chamber, were used simultaneously in sample collection. (2) Cd concentration as a function of decay time. Two experimental conditions were chosen: air circulation and static conditions. In the normal condition (air circulation), eight cigarettes were freely and simultaneously burnt. Then three small pumps were used to collect samples at four time intervals: 0-1, 1-2, 2-3.5, and 3.5-5.5 h. In the static condition, six cigaretteswere freely burnt. Then three small sampling pumps were used at time intervals of 0-1, 1-2, 2-3, 3-4, and 4-5 h. Again, the pumps were used simultaneously in sample collection. (3) Cd concentration as afunction of particle size, In the static condition, 10 cigarettes were freely burnt. A small pump was used to collect total suspended particles (TSP), and a cascade impactor (Anderson 1ACFM)with six stages
was used to collect samples with a range of different particle sizes. The total sampling time was 110 min. In another experiment using normal air circulation,two cigarettes were smoked by two separate smoking machines at 10-min intervals (total 12 cigarettes). Both the small sampling pump and cascade impactor were used. Total sampling time was 90 min, which means there was no ETS generated in the last 30 min. The substrates and backup filter for the impactor were 1.0-pm pore size Zeflour filters with a diameter of 81 mm. The filter for the small sampling pump was a 0.5-pm pore size Telfo membrane filter with a diameter of 47 mm. Both types of filters were made by Gelman Science Inc. Collection of Field ETS Samples. Several public smoking environments, such as a cafeteria, a coffee house, a music club, and several restaurants in the twin cities of Champaign-Urbana were used as field sample sites. The air in a smoking car of an Amtrak train was also sampled. For ETS-free background measurements, the indoor air in a university office and a library were sampled. Two small sampling pumps were used simultaneously so that the samples could be used for either duplicate samples or different analytical procedures. The air filter in the field sample was a 0.5-pm pore size Telfo membrane filter. Each filter was preweighed and placed in a small labeled Petri dish. The filters were either preloaded in filter holders and sealed by parafiilm or loaded on site. A typical sampling time was 4-10 h, corresponding to an air volume about 1-2 m3. After being sampled, the filters were unloaded from the filter holders and placed back in the petri dish prior to reweighing and analysis. Analytical Method. Instrumental neutron activation analysis was employed in this study for cadmium determination in the filter samples. The method has been described elsewhere (10). It is a well-accepted elemental analysis technique and also provides a means for largescale multi-element determination. With epithermal neutron irradiation and Compton suppression counting techniques, the detection limit has been reduced to 1-2 ng for cadmium. The typical range of cadmium levels in the samples analyzed in this study was between 2 ng and 1pg.
Results and Discussion Characterization of the unburnt cigarette components shows that the cadmium concentrations in various American commercial cigarette tobacco do not change very much over brands with an average of 1.28 i 0.17 pg of Cd/g of tobacco, ranging from 1.0 to 1.6pglg, as shown in Figure 2. These results indicate that cadmium has a similar emission factor from various cigarette products, fulfilling criteria c of a marker of ETS. Cadmium concentrations in 1983 products were generally higher than those from 1993. By analysis of SS samples, cadmium emission factors (amount of Cd emitted from each cigarette or per gram of tobacco consumed) were measured for the brands of cigarettes tested. For each cigarette, 0.40 f 0.09 pg of cadmium was determined to be emitted in SS. This is equivalent to an emission factor of 0.65 & 0.13 pg of Cd/g of tobacco consumed (Figure 3). Combining these data allows the determination of the cadmium emission fraction (the percentage of cadmium emitted into SS or ETS). On average for the brands of cigarette tested, it was found that approximately 44% of the cadmium present in a cigarette goes to the particulate phase of SS. This percentage was VOL. 29. NO. 9 . 1 9 9 5 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 12313
Na. of cigarettes burned
I Brand of cigarette
FIGURE 2. Cadmium concentration in dinerent brands of cigantle tobacco: Ol.Vamage(93):~.Camel(93); W.MarIbom(93):W.Merit: 05. Kool(93): 06, Vantage (83):07, Camel (83);08. Marlboro (83); 09, Merit, 10, Kool (83): 11, Winston: 12, Reference 1R1; 13, Reference 1R4F.
Cd in TSP (u&)
I
FIGURE 5. Airborne cadmium concentration in an ETS chamber as a function of the number of cigarettes burned.
.. .-
0.9
z
Cd in air (ngim3)
0.8
Time (min.)
I
Cd in Air (nglm3)
Cd in TSP ("Pip)
1
FIGURE 6. Decay of airborne cadmium concentration in an ETS chamber with time (without air circulation).
FIGURE 3. Variation of cadmium emission factors in different cigarettes: 01. Vantage (93); 02. Camel (93); U3, Marlbom 193); 04, Merit,05.Kool(93);06.V~ntage(83);07.Camel(83):08,Marlboro(83): 09. Merit, 10, Kool(831; 11, Winston: 12, Reference 1Rl; 13. Reference 1R4F, Olhcrs 1 5 . 2 % ) ~
FIGURE 4. Elemental cadmium distribution in cigarate smoke (Cd in tobacco, 0.905 pglcigarette. and in cigarette paper, 0.035 pgl cigarette).
determined by using the mass balance of cadmium in a burnt cigarette (Figure 4). The results also show that, of elements measured, only cadmium has such a large emission fraction. As a comparison, 29 other elementswere analyzed to determine their particulate emission fraction,includingAs (6.8%). Br (0.8%). CI(0.9%),K(O.l%),andZn(0.6%]. AlthoughBr,CI,As,and Kare consideredvolatileelements, our experimentalresults 2314 m ENVIRONMENTAL SCIENCE &TECHNOLOGY I VOL. 29.
NO. 8.1995
show that their gas fraction emissions are less than 1096, as determined using a mass balance. It was found that these elements are retained in the ash, and therefore are poor candidates for ETSmarkers. Thus, out ofthe elements analyzed, cadmium is the most suitable marker of ETS, since its emission fractionis considerablyhigherthanthose of the other elements. As discussed above, two filters were used in MS and SS sample collectionto quantifythe small particle penetration, which was found to be approximately 5% for SS and about 2% for MS. The result of the elemental analysis shows that the penetration is about 0.6% for Cd in the second filter of the sidestream smoke sample. Elemental levels for the second filter in the MS sample were too low to measure. This indicates that Cd is not in the gas phase nor is it to be found in very small particles. The study of particulate cadmium in the controlled environmental chamber shows that cadmium concentration increases linearlywith the number of cigarettesburned &e., that it is proportional to source strength) and that the concentration of cadmium in TSP remains relatively constant (i.e., that it does not decay more rapidlythanTSP) (see Figure 5). Both of these properties are consistent with agoodETSmarker. Withoutaircirculationinthechamber, the cadmium concentration decays very slowly over time, losing only 15% of its original concentration over 5 h if the sampling loss is considered (Figure 6). Figure 6shows that
TABLE 1
Elemental Cadmium Concentrations Measured in Several Public Places Cd concn in air
Inldl
cafeteria restaurant 1 barlrestaurant coffeehouse music club 1 restaurant 2 Amtrak train car
4.0 0.6 6.0 f 1.0 6.2 1.4 7.4 f 0.9 11.4+ 1.6 21.4 2.6 24.8 2.8 37.9 1.8
