NOVEMBER 2002 VOLUME 15, NUMBER 11 © Copyright 2002 by the American Chemical Society
Perspective Acetaldehyde in Mainstream Tobacco Smoke: Formation and Occurrence in Smoke and Bioavailability in the Smoker Jeffrey I. Seeman,*,† Michael Dixon,*,‡ and Hans-Ju¨rgen Haussmann*,§ SaddlePoint Frontiers, 12001 Bollingbrook Place, Richmond, Virginia 23236-3218, British American Tobacco, Globe House, 4 Temple Place, London WC2R 2PG, England, and INBIFO Institut fu¨ r biologische Forschung GmbH, Fuggerstrasse 3, D-51149 Ko¨ ln, Germany Received July 11, 2002
A review is presented of the scientific literature on the effects of sugars (mono- and disaccharides), when used as tobacco additives, on the formation of acetaldehyde in mainstream (MS) smoke and the potential bioavailablity of MS smoke acetaldehyde derived from sugars to the smoker. The experimental data supports the following conclusions. Sugars, e.g., D-glucose, D-fructose, and sucrose, do not produce greater yields of acetaldehyde in MS smoke than are produced from tobacco itself on a weight-for-weight basis. A variety of studies suggests that natural tobacco polysaccharides, including cellulose, are the primary precursors of acetaldehyde in MS smoke. In a number of different studies using commercial cigarette brands, MS smoke yields of acetaldehyde correlate (r > 0.9) with both MS smoke “tar” and carbon monoxide. MS smoke acetaldehyde yields are affected more by cigarette design characteristics that influence total smoke production, such as filter ventilation, filtration, and paper porosity, than by reducing sugars. MS smoke acetaldehyde deposits primarily in the upper respiratory tract, including the mouth, of the smoker. Acetaldehyde is rapidly metabolized by aldehyde dehydrogenase in the blood and elsewhere in the body, including at the blood-brain barrier. Tobacco sugarderived MS smoke acetaldehyde from commercial cigarettes is unlikely to result in direct central nervous system effects on the smoker. I. Introduction II. Acetaldehyde and Carbohydrate Chemistry and Smoke Chemistry: General Considerations III. Pyrolysis Products from D-Glucose and from Cellulose
1332 1332
1335
* To whom correspondence should be addressed. † SaddlePoint Frontiers. ‡ British American Tobacco. § INBIFO Institut fu ¨ r biologische Forschung GmbH (INBIFO is a Philip Morris research laboratory).
IV. Acetaldehyde Formation in Mainstream Smoke A. General Comments on MS Smoke Precursor-Product Relationships B. Effect of Sugars in the Tobacco Blend on MS Smoke Acetaldehyde Yield C. Effect of the Addition of Sugars to Tobacco on MS Smoke Acetaldehyde Yield D. Effect of Blend Constituents Other Than Sugars on MS Smoke Acetaldehyde Yield
10.1021/tx020069f CCC: $22.00 © 2002 American Chemical Society Published on Web 11/18/2002
1335 1335 1336 1337
1338
1332
Chem. Res. Toxicol., Vol. 15, No. 11, 2002
E. MS Smoke Acetaldehyde Yields as a Function of MS Smoke Constituents V. Bioavailability of Mainstream Smoke Acetaldehyde A. General Considerations B. Analytical Problems Encountered in the Measurement of Acetaldehyde Blood Levels C. Deposition and Retention of Gas-Phase Acetaldehyde in the Respiratory Tract D. Influence of MS Cigarette Smoke on Levels of Acetaldehyde in Blood E. Factors Influencing the Levels of Acetaldehyde in the Blood F. Protein and DNA Adducts as Medium-Term Biomarkers of Acetaldehyde Exposure G. Metabolic Barriers to Acetaldehyde at the Blood-Brain Barrier H. Role of Acetaldehyde in Smoking Behavior VI. Conclusions
Perspective
1340 1342 1342 1342
1343
1344 1344 1345
1345 1346 1346
I. Introduction Acetaldehyde (1) has been identified as a cigarette smoke constituent of concern by the U.S. Consumer
Product Safety Committee (1) and the World Health Organization (2).1 An article published recently by Smith and Hansch classifies acetaldehyde as being a cigarette smoke component with significant potential biological activity and notes that it is formed in comparatively large amounts in cigarette smoke (3). In addition, acetaldehyde has been classified as an animal carcinogen (4), as “reasonably anticipated to be a human carcinogen” (5, 6), and according to a recent monograph, may be cytotoxic or genetoxic (2). Recent studies indicate that inspired acetaldehyde vapor can induce a sensory-nerve-mediated nasal vasodilatory response (7). There have also been a number of publications that consider if mainstream (MS)2 smoke acetaldehyde plays a role in human smoking behavior (8-12), for example, by interacting with nicotine in the central nervous system (CNS) and consequently being a 1A number of smoke chemistry references reported herein are from the journals Beitra¨ge zur Tabakforschung and Beitra¨ge zur Tabakforschung International. Access to these references are available in pdf format on the journal’s website: www.beitraege-bti.de. 2Abbreviations: ABC, a nonspecified tobacco blend constituent; ALD, alcohol dehydrogenase; ALDH, aldehyde dehydrogenase; CNS, central nervous system; FTC, U.S. Federal Trade Commission; IOM, US Institute of Medicine; MS, mainstream, i.e., the smoke stream issuing from the mouth end of a cigarette; MBS, Massachusetts Benchmark Study; MPP, Massachusetts puff profile, the laboratory machine smoking paradigm specified by the Massachusetts Department of Health to be used in the MBS; PREP, potentially reduced exposure product; SS, sidestream, i.e., the smoke stream that issues, not from the mouth end of a cigarette, but rather from the burning end, including through the paper or filter; “tar”, FTC tar is FTC total particulate matter (TPM) minus the mainstream smoke nicotine and water content; TPM, total particulate matter, the portion of mainstream smoke that is collected on a Cambridge filter in the machine smoking FTC method; XYZ, a nonspecified mainstream smoke constituent.
determinant for smoke exposure. Two reports suggest that MS smoke acetaldehyde is derived from sugars added in tobacco processing (11, 13) while a recent commentary briefly summarizes evidence to the contrary (14). To guide the development of potentially reduced-risk tobacco products, it is extremely important to identify and evaluate the scientific literature regarding tobacco precursor-tobacco smoke product relationships. A recent review examined the related hypothesis that ammonia compounds in smoke affect the transfer efficiency of nicotine from tobacco to smoke and nicotine’s bioavailability to the smoker (15). In this current literature review, the source of mainstream smoke acetaldehyde and the role, if any, of sugar-derived MS smoke acetaldehyde on smoking behavior are evaluated. Our goals are to (a) review the formation and occurrence of acetaldehyde in MS tobacco smoke, (b) determine the most likely precursors for acetaldehyde in MS smoke, (c) evaluate the hypothesis that sugars added to the tobacco blends of commercial cigarettes are major precursors of acetaldehyde in MS smoke, (d) examine the fate of MS smoke acetaldehyde in terms of smoker bioavailability, (e) summarize the data on the potential effect of MS smoke acetaldehyde on smoking behavior, and (f) consider the hypothesis that sugar-derived MS smoke acetaldehyde can affect smoking behavior, as defined above.
II. Acetaldehyde and Carbohydrate Chemistry and Smoke Chemistry: General Considerations Acetaldehyde is a gas at room temperature and pressure. Its boiling point is 21 °C at one atmosphere pressure (16). Acetaldehyde is miscible with water and has a vapor pressure of 58 kPa in aqueous solution at 20 °C when its concentration is 22 mol %. In aqueous solution, acetaldehyde exists as a mixture of about 50% acetaldehyde and 50% hydrate (2) in rapidly reversible equilibrium (17). The high vapor pressure of acetaldehyde in aqueous solution causes these solutions to rapidly decrease in concentration by evaporation unless they are tightly sealed.
Acetaldehyde is a highly reactive compound. It will rapidly react with a wide range of nucleophilic substances (e.g., 3; see Scheme 1) and will condense with carbonylScheme 1
containing compounds, such as with other aldehydes, ketones, and esters. Acids and bases are known to catalyze the reactions of acetaldehyde. The extent to which MS smoke acetaldehyde reacts prior to its exit from the cigarette is as yet unknown. Acetaldehyde is known to react with MS smoke constituents during trapping and storage, which can make its detection and accurate quantification difficult (18). In addition, acetaldehyde’s reactivity in biological media is well-known and is discussed in sections V.B., V.E., and V.F. below.
Perspective
Chem. Res. Toxicol., Vol. 15, No. 11, 2002 1333 Table 1. Types of Sugars Typically Used as Additives to Tobacco
a In some cases, multiple isomers can represent the substance. The actual composition in the tobacco matrix will depend on various matrix conditions, e.g., water concentration, relative concentration of acids and bases and other substances, and temperature. In general, the open chain aldehydo forms of reducting sugars are present in very low percentage of the total. See Zhu, Y., Zajicek, J., and Serianni, A. S. (2001) Acyclic Forms of [1-13C]Aldohexoses in Aqueous Solution: Quantitation by 13C NMR and Deuterium Isotope Effects on Tautometric Equilibria. J. Org. Chem. 66, 6244-6251. b Not all structural isomers illustrated. For example, carbonyl hydrates [RC(OH)2H] are not shown. c Data from Leung, A. Y., and Foster, S. (1996) Encyclopedia of Common Natural Ingredients, John Wiley, New York.
1334
Chem. Res. Toxicol., Vol. 15, No. 11, 2002
Perspective
Figure 1. Illustration of the structure and reactivity of reducing sugars. The starred carbon (*C; the anomeric carbon) is an aldehyde in the open chain form of the carbohydrate. Reducing sugars have either a hemiacetal or a hemiketal functionality, which can tautomerize upon dissolution.
Acetaldehyde has been reported to be found in low levels in tobacco (19). As previously indicated, acetaldehyde is a known constituent of tobacco smoke. Because of its high vapor pressure, acetaldehyde is found primarily (>98%) in the gas phase of mainstream tobacco smoke (18). Acetaldehyde is also known to be a constituent of sidestream (SS) tobacco smoke, with a SS/MS ratio of ca. 1.4 (18, 20) and 1.63 ( 0.86 for a limited set of 12 commercial cigarettes using the Massachusetts Benchmark data (21). Over the years, a wide range of values of MS smoke acetaldehyde (0.003-2.3 mg/cigarette) have been measured in MS smoke of U.S. commercial cigarettes using the FTC method, though typically without a Cambridge filter pad (18, 21-23). A recent survey reported 0.4-1.4 mg/cigarette in the MS vapor phase of nonfiltered cigarettes (6). Recently estimated MS smoke acetaldehyde yields for three “tar” ranges of U.S. commercial cigarettes were reported (mean value ( one standard deviation): (a) 0.234 ( 0.047 mg/cigarette for FTC “tar” range ) 0.70-7.20 mg/cigarette; (b) 0.649 ( 0.045 mg/cigarette for FTC “tar” range ) 8.10-11.6 mg/cigarette; and (c) 0.916 ( 0.028 mg/cigarette for FTC “tar” range ) 12.917.2 mg/cigarette (24). In the recent Massachusetts Benchmark Study using a modified (more intense) puff profile, the MS smoke acetaldehyde yield for 26 commercial US cigarettes was 1.458 ( 0.453 mg/cigarette, with a range of 0.596-2.133 mg/cigarette (21). A limited number of literature references report MS smoke acetaldehyde puff-by-puff data. It is well-known that total yields do not necessarily reflect puff-by-puff variations. For example, nicotine puff-by-puff increases significantly with puff number, as does “tar” yield (25, 26). In a 1968 paper with four data points, MS smoke acetaldehyde was reported to be ca. 6, 10, 9, and 13 µg/ puff in puffs number two, four, six, and eight, respectively (27). Other studies demonstrated that acetaldehyde
yields increase with puff number (28) as well as increase within the time-location of a single puff, i.e., from the beginning to end of puffs three, five, eight, and the last puff (29). Two studies published in 2001 reported new methodologies for puff-by-puff analysis (30, 31) of MS smoke gas-phase constituents. In the first, 25 MS smoke gas-phase constituents including acetaldehyde from Kentucky Reference 1R4F cigarettes were examined but quantification was reported only for ethane, formaldehyde, and butadiene (30). In the second 2001 study, MS smoke acetaldehyde from 1R4F cigarettes was quantified on a puff-by-puff basis (31). Three different analytical methods were used: conventional FTC pad, a fresh smaller size pad, and whole smoke. The major differences reported were on the first puff, though these were attributable to factors such as differences in dead volume of the two filter pad assemblies used. For the other puffs, MS smoke acetaldehyde yields were reasonably constant at ca. 60-90 µg/puff with standard deviations ca. 10-20 µg. One study also demonstrated that the amount of acetaldehyde quantified in various puffs exhibited little dependence, if any, on the amount of TPM already trapped on the Cambridge filter (28). The rate of formation of MS smoke acetaldehyde was found to be independent of cigarette circumference (32). Carbohydrates (cellulose, pectins, starch, glucosides, simple sugars, and sugar esters) can be 30% or more of the weight found in a blended cigarette (33, 34). The most abundant carbohydrates in tobacco are cellulose (4), typically ca. 10%, and pectin, ca. 6-12% (34, 35). Cellulose is a polysaccharide with its β-glucopyranosyl units 1,4-linked (compare with the β-glucopyranose isomer of D-glucose 5; see Table 1 for five isomers of D-glucose). It is important to distinguish sugars from polysaccharides, such as cellulose, pectins, and starches that are present in tobacco. Because they are polymeric substances, polysaccharides have properties different from those of
Perspective
the monosaccharide units that make up the polymer structure. In fact, there are many forms of “cellulose” differing in terms of molecular weight (i.e., size of “n” in structure 4), fine structure and superstructure.
It has been well-known for years that sugars are frequently added to the tobacco blend in the form of casings, typically to those leaf components that have reduced sugar concentrations due to losses occurring during curing of, for example, air-cured Burley tobacco (35, 36). Both reducing and nonreducing sugars serve as important flavorings and humectants in many tobacco products (34-37). Reducing sugars are those sugars that tautomerize upon dissolution in water (38, 39). See Figure 1 for structural considerations and for the chemistry used for analytical determination of reducing sugars. To be a reducing sugar, the molecule must have a readily oxidizable functional group under the reaction conditions. Hence, all monosaccharides are reducing sugars, including D-glucose and D-fructose, because they exist as either hemiacetals or hemiketals that can isomerize to aldehydes (see Table 1). Nonreducing sugars include sucrose. Sugars are known to react with natural tobacco nitrogenous compounds via Maillard browning reactions that decompose to produce pyrazines, known tobacco smoke flavorants (40-42). Depending on the blend, total sugar content in finished tobacco may be as high as 20% (43) although typical levels are ca. 10%. In this review, the term “sugars” will refer to monosaccharides, such as D-glucose (5) and D-fructose, and disaccharides, such as sucrose. Sugars added to tobacco, as well as casing materials that have sugars as part of their constitution, are described in Table 1.
III. Pyrolysis Products from D-Glucose and from Cellulose Pyrolysis studies have been conducted for many years on carbohydrates (44-47). In some cases, pyrolysis results have been used as models for cigarette smoke formation (48). A recent review by Sanders et al. examines the differences in thermal properties of D-glucose (and other hexoses and sucrose) and cellulose (49). While D-glucose and cellulose are structurally related, their pyrolysis chemistry and their smoke chemistry are considerably different. Some conclusions from the Sanders et al. review are as follows: • Pyrolysis of D-glucose, sucrose, and cellulose generally leads to the same types of reaction products, though with different relative yields at each set of reaction conditions. Among these products are low molecular weight oxygenated compounds, derivatives of furan (6) and furfural (7), and anhydro sugars such as levoglucosan (8). Furans are
Chem. Res. Toxicol., Vol. 15, No. 11, 2002 1335
relatively thermally stable and do not readily decompose to lower molecular weight oxygenated products. • Relative to pure cellulose, pyrolysis of sugars (e.g., D-glucose, D-fructose, and sucrose) appears to yield a greater percentage of furans, a lower percentage of anhydrosugars and a lesser percentage of low molecular weight carbonyl compounds such as glycolaldehyde (8) and acetaldehyde (1). Pyrolysis of cellulose in the presence of even low concentrations of salts (e.g., 0.05% NaCl) leads to major yields of hydroxyacetaldehyde (9, also called glycolaldehyde) (50).
• Pyrolysis of sucrose (10) leads to significant yields of furans as well as levoglucosan. The former reactions are due to sucrose’s polyhydroxytetrahydrofuran ring system (i.e., its furanose ring), highlighted in the structure shown.
• Pyrolysis product profiles depend on both the temperatures the materials experience as well as their residence time in those temperature regimes. The presence of acids, bases, water, and salts can affect the product distributions. When volatile products can be swept out of the thermal regions, they are more likely to survive rather than degrade further.
IV. Acetaldehyde Formation in Mainstream Smoke A. General Comments on MS Smoke Precursor-Product Relationships In the discussions below, we ask the question, “does tobacco component ABC yield mainstream smoke constituent XYZ”. A direct experimental method to answer this question is to label component ABC with, for example, 14C and determine the smoke yield of [14C]XYZ (26, 51-53). Stable isotopes such as 2H can also be used (54). A second method is to compare the MS smoke yield of XYZ from two cigarettes, a control and a control to which substrate ABC had been added to the tobacco blend. Experiments of this nature should dose ABC as high as possible, to detect subtle changes in smoke chemistry, e.g., an increased yield of XYZ, but should not change the overall properties of the tobacco or the burning characteristics of the test cigarette as expressed by, for example, total smoke delivery, puff count or static burn rate (55). Let us consider the refined question, “does tobacco component ABC influence the smoke yield of XYZ”. This
1336
Chem. Res. Toxicol., Vol. 15, No. 11, 2002
Perspective
question asks, does ABC form XYZ in a yield that exceeds the global yield efficiency (weight of smoke constituent per weight of tobacco burnt during the puffing of the cigarette) from transformations from the thousands of tobacco components themselves (19). If ABC does form XYZ in a yield equivalent to the average MS smoke yield of XYZ from the tobacco blend itself on a weight-forweight basis, then substituting ABC for an equivalent weight of tobacco blend will result in the same MS yield of XYZ. In this work, we shall interpret the conclusion that “tobacco component ABC yields mainstream smoke constituent XYZ” or “ABC is a major precursor of XYZ in MS smoke” to mean that the yield efficiency exceeds that from the tobacco blend itself on a weight-for-weight basis. In the consideration of a correlation between a tobacco constituent and a smoke constituent yield, e.g., percent reducing sugar in the tobacco blend vs MS smoke acetaldehyde yield, the yield of the MS smoke constituent may need to be normalized relative to total smoke formation, e.g., variation in total particulate matter (TPM) or “tar” yield (or carbon monoxide yield, if the examined smoke constituent is in the vapor phase) and puff count (55). (“Tar” is defined as TPM minus water minus MS smoke nicotine.) This is equivalent to determining if overall smoke yield increases (or decreases) relative to a change in the tobacco blend or if a specific smoke constituent’s yield changes. In some cases, for example, when considering the effect of cigarette circumference on formation of smoke components, the weight loss of tobacco during puffs must be used (32, 56). See the discussion in sections IV.B. and IV.E. for examples of the need to normalize the smoke data for total smoke formation. Of course, total smoke distribution must include sidestream particulate and gas-phase plus material on both the filter butt and tobacco residue (26, 57-59).
B. Effect of Sugars in the Tobacco Blend on MS Smoke Acetaldehyde Yield Phillpotts et al. (43) reported in 1975 the total MS aldehyde deliveries and sugar content of the tobaccos of 83 commercial European brands: 42 U.K. brands; seven brands from France, Italy, and Holland; nine West German brands; three Belgian brands; four Danish brands; and one brand from each of Luxembourg, Switzerland, and Norway. Mainstream smoke aldehyde delivery normalized for particulate matter yield was not correlated to sugars in tobacco, with a value of r2 ) 0.0017 (see Figure 2). Of importance in the Phillpotts et al. study was their examination of commercial cigarettes having many different tobacco types and likely varying sugar
Figure 2. Relationship between “tar” normalized total mainstream smoke aldehydes and sugar content of tobaccos from 82 commercial European cigarettes, as reported by Phillpotts et al. (43).
content. (In section IV.C., the effect of sugars added to the tobacco blend is examined.) Phillpotts et al. also examined six single grade (blend component) cigarettes, one sun-cured, one flue-cured, one air-cured Burley, and three flue-cured experimental cigarettes. No correlation (r2 ) 0.045) was found between the ratio of MS smoke total aldehydes/particulate matter and sugar content of the tobacco materials. A study published in 1982 involved 25 different experimental cigarettes (60). Three of the cigarettes contained Cytrel, a nontobacco smoking material. In all cases, the tobacco was Bright leaf and/or Bright leaf reconstituted tobacco. Because the “tar” yields in the cigarettes studied ranged from 4.14 to 26.4 mg/cigarette, it is necessary to normalize each MS smoke constituent by dividing by the “tar” yield of the individual cigarette (see section IV.A.). Table 2 illustrates this distinction for the data reported by Zilkey et al.: MS smoke acetaldehyde vs reducing sugars has an r ) 0.727, but the ratio acetaldehyde/”tar” vs reducing sugars has r ) 0.058. Hence, in this data set, when normalized for the varying “tar” yields, MS smoke acetaldehyde is not correlated to reducing sugars. In this data set, neither acetaldehyde nor acetaldehyde/”tar” was correlated to cellulose in the tobacco blend. The MS smoke acetaldehyde vs “tar” correlation has an r ) 0.786, consistent with many studies reported (see section IV.E. below) that the more “tar”, the more acetaldehyde in MS smoke. However, acetaldehyde vs CO correlation has an r ) 0.417, inconsistent with the recent Massachusetts Benchmark Study which had an excellent correlation (r ) 0.962)
Table 2. Correlation Matrix (r values) of Tobacco and MS Smoke Variables from a Series of 25 Experimental Cigarettes, Including Cytrela
lignin cellulose fiber reducing sugars “tar” CO AA AA/”tar” AA/CO a
lignin
cellulose
fiber
reducing sugars
“tar”
CO
AAb
AA/”tar”
AA/CO
1.000 0.196 0.285 -0.307 -0.291 -0.134 -0.162 0.222 -0.043
1.000 -0.403 0.151 0.334 0.519 0.227 -0.138 -0.254
1.000 -0.700 -0.808 -0.649 -0.664 0.212 -0.045
1.000 0.721 0.480 0.727 0.058 0.278
1.000 0.713 0.786 -0.278 0.154
1.000 0.417 -0.386 -0.439
1.000 0.358 0.615
1.000 0.695
1.000
Data from Zilkey et al. (60). b AA ) acetaldehyde.
Perspective
Chem. Res. Toxicol., Vol. 15, No. 11, 2002 1337
Table 3. Quantitative and Qualitative Data of Selected MS Smoke Constituents and Tobacco Componentsa Flue-cure (American Type 11a) descriptors
leaf position
cutter position
Burley (Kentucky) leaf position
Basma-M
Samsun
1.9 1.2 1 0.3 0 1 0.7 2.4 2.2 2.2
2.2 0.6 1.9 0.7 0 1.4 1 2.5 2.8 3.7
2.8 0.9 2.1 0.6 0 2.3 1.3 3.8 3.5 13
quantitative estimates of MS smoke constituents (µg/puff) 1-hydroxy-2-propanone 4.3 5 2.5 2-furfural 2.7 2.9 1.1 furfuryl alcohol 4.7 4.4 1.5 5-methylfurfural 1.9 1.8 0.4 5-hydroxymethylfurfural 11 10 0 γ-butyrolactone 2.5 2.3 1.5 cyclopentan-1,2-dione 2.2 1.9 0.9 2-hydroxy-3-methyl-2-cyclopentenone 5 5.3 2.8 4-hydroxy-2,5-dimethyl-3(2H)furanone 7.4 7.5 2.8 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one 32 28 4.2 Flue-cured
Turkish
cutter position
Burley
Oriental
componentsb
qualitative estimates of tobacco high very low low medium representative analysis of cigarette tobaccos (% of dry weight)c reducing sugars, as D-glucose 22.09 0.21 crude fiber 7.88 9.29 sugars crude fiber
a
medium low 12.39 6.63
Data from Matsushima et al. (61). b Data from Leffingwell (34). c Data from from Browne (62).
between these two smoke parameters (21). Further, the correlation between MS smoke acetaldehyde and total gas-phase aldehydes is poor, and negative, with an r ) -0.675, in contrast with the position that acetaldehyde represents the vast majority of gas-phase aldehydes. Thus, integrity of the Zilkey et al. data set (60) is questionable. Matsushima et al. quantified the MS deliveries/puff of six single blend component cigarettes, two each of fluecured, Burley, and Turkish tobaccos (61). The authors identified 10 MS smoke constituents which they considered have their origin from carbohydrate pyrolysis, and these are listed in Table 3. Also found in Table 3 are representative reference descriptions and data for sugars and fibers in Bright, Burley, and Oriental tobaccos (34, 62). The data in Table 3 are consistent with the hypothesis that tobaccos high in sugars and reducing sugars will yield higher amounts of furans and other oxygenated heterocycles when compared with tobaccos lower in sugars and reducing sugars. Crude fiber, which would include cellulose and other polysaccharides, do not appear to be related to MS smoke furan production in this data set. One tabulation reports that “reducing sugars such as dextrose (D-glucose)” are 100-fold greater in concentration in Flue-cured tobacco than in Burley tobacco and 10-fold greater than in Oriental tobacco (34), while acetaldehyde levels per cigarette from Bright (Flue-cured), Burley, and Oriental tobacco smoke are approximately the same (63). Should this type of comparison be general, it suggests that reducing sugars (e.g., D-glucose) are not a major source of acetaldehyde in MS smoke. In a recent study, Seeman et al. examined the relationship between MS smoke acetaldehyde yields and reducing sugars in the tobacco blends of a large number of U.S. commercial cigarettes (55). This data was available in the Philip Morris database for the years 1985-1989 and 1990-1993. Over this time period, the concentration of reducing sugars either stayed the same or decreased slightly. Seeman et al. also found that the “tar” normalized MS acetaldehyde yield (i.e., the ratio of MS smoke acetaldehyde/”tar”) is not correlated to reducing sugar
Figure 3. MS smoke aldehyde/”tar” vs reducing sugars (on a dry weight basis) for 264 U.S. commercial cigarettes for 1991. From Seeman et al. (55).
concentration in the tobacco blend (r2 < 0.003 for the years for which data is available) (55). An example of this lack of correlation is shown in Figure 3. The results suggest that MS smoke aldehyde and acetaldehyde yields are not related to sugar levels quantified in various fashions in the tobacco blends of different series of cigarettes.
C. Effect of the Addition of Sugars to Tobacco on MS Smoke Acetaldehyde Yield In the studies reported in section IV.B., above, the sugars were both endogeneous as well as exogeneous. In this section, known quantities of sugars were added to the tobacco blends, and either total MS smoke aldehydes or MS smoke acetaldehyde was measured. In 1968, Thornton and Valentine reported the addition of (U)14C-D-glucose to cigarettes [(U)14C-D-glucose means D-glucose uniformly labeled with 14C] (64). They determined the recovered radioactivity in MS and sidestream smoke particulate matter and gas phase. In this work, Thornton and Valentine did not identify the compounds in the MS gas phase that possessed the 14C radioactivity or their individual contributions to the total amount of
1338
Chem. Res. Toxicol., Vol. 15, No. 11, 2002
Perspective
Table 4. Delivery of Smoke Componentsa
cigaretteb control (Burley tobacco) control + 12.8% D-fructose control + 10.5% D-glucose control + 17.8% D-glucose and D-fructose control + 16.8% D-glucose a
total particulate matter (mg/cigarette)
total nicotine alkaloids (mg/cigarette)
total carbonyls (mg/cigarette)
volatile carbonyls (mg/cigarette)
volatile aldehydes (mg/cigarette)
2-furfural (µg/cigarette)
34 30 31 35
3.32 2.21 2.59 1.98
4.0 4.4 4.2 4.5
1.9 2.2 2.0 2.2
0.60 0.53 0.50 0.54
53 89 67 90
2.16
4.3
2.1
0.54
80
32 b
Data from Thornton and Massey (66). Cigarettes were selected on the basis of average weight ((20 mg). Consequently, the addition of sugars resulted in an equivalent decrease in the weight of Burley tobacco in each test cigarette.
radioactivity. Shorthly thereafter, Gager and co-workers (52, 53) determined the yield of 13 gas-phase constituents of MS smoke from cigarettes containing added (U)14C-Dglucose and (U)14C-sucrose. The organic constituents of the MS gas phase were analyzed by gas chromatography for identification and purity and by vibrating-reed electrometer for radioactivity. Acetaldehyde was among the two products reported to be formed in the highest radiochemical yields, acetone being the second. The radiochemical yield of acetaldehyde found [0.05% from (U)14C-D-glucose; 0.06% from (U)14C-sucrose] indicates that neither D-glucose nor sucrose produce greater yields of acetaldehyde in MS smoke than are produced from the tobacco itself on a weight-for-weight basis. In some studies discussed below, total aldehyde levels in MS smoke have been measured rather than levels of acetaldehyde alone. It has been estimated that 80-85% of the measured total MS smoke aldehyde delivery is due to acetaldehyde (43). More recently, acetaldehyde was found to be the major aldehyde in smoke samples of 26 commercial brands, and it made up 46-72% of the total carbonyl compounds (aldehydes plus ketones) in the MS smoke sample (65). Hence, it is reasonable to use total aldehyde levels as a surrogate for acetaldehyde in MS smoke. Thornton and Massey used this approximation in 1975 (66). They reported the addition of rather high levels of D-fructose, D-glucose, and mixtures of these two sugars at 10.5-17.8% loading levels to a single grade Burley control tobacco blend. As shown in Table 4, MS smoke yields of volatile aldehydes were not affected by the addition of large quantities of D-glucose and/or D-fructose. However, a significant increase in the delivery of MS smoke furfural (7) was observed with added sugar (see Table 4). While the yields of 7 were low, in the microgram range, other furans were not measured though likely present. The formation of furfural is consistent with the hypothesis that sugars give more furans upon pyrolysis, relative to cellulose, while cellulose results in more levoglucosan and volatile aldehydes, and related compounds upon pyrolysis, relative to sugars (49). A recent study tested cigarettes with three types of ingredient mixtures, including sugars added at two addition levels each, and compared these test cigarettes to a control cigarette without these additions. Approximately 10% of the tobacco in the test cigarettes was replaced by the ingredients. Smoke yields for a number of different constituents were measured. Relative to TPM, the acetaldehyde yields were lower in each test case. In fact, the yields were on average 9% lower in the test compared to the control cigarette (67). Stavanja et al. presented a poster at a recent Society of Toxicology meeting in which they did not see any effect
on the MS smoke acetaldehyde yield when high fructose corn syrup was added to cigarette tobacco (68). The results indicate that acetaldehyde is not formed in greater yields in MS smoke from sugars added to the tobacco blend than are produced from tobacco itself on a weight-for-weight basis. Some evidence is available which suggests that sugars can lead to an increase in MS smoke furans.
D. Effect of Blend Constituents Other Than Sugars on MS Smoke Acetaldehyde Yield In 1973, Tso, Rathkamp, and Hoffmann published correlations between MS smoke acetaldehyde and a number of smoke and leaf variables of single blend cigarettes (69). These researchers reported their analysis of a set of four different Bright tobacco cultivars, using eight stalk positions each. As shown in Table 5, acetalTable 5. Correlations between Acetaldehyde in Mainstream Smoke and Variables Examineda
leaf thickness fire-holding capacity pH of an aqueous extract of leaf tobacco leaf potassium (as K+) total volatile bases scopoletin (i) oxalic acid (ii) stigmasterol (iii)
correlation coefficient (r)
statistical significance (%)
-0.577 0.407 0.382
1 5 5
0.550 0.359 -0.456 -0.623 0.761
1 5 1 1 1
a Data from Tso et al. (69). b Twenty-three other variables examined, none of which were reported to have a statistically significant correlation with MS smoke acetaldehyde.
dehyde in MS smoke was significantly correlated with eight (out of 31) tobacco and smoke parameters examined in their study. Some of these correlation coefficients were negative. The meaning of these correlations was, and remains, unclear. Not all correlations reflect cause and effect relationships. The high positive correlation for
Perspective
Chem. Res. Toxicol., Vol. 15, No. 11, 2002 1339
Table 6. Correlation Coefficient (r) Matrix between Mainstream Smoke Acetaldehyde Yield and Selected Tobacco Leaf and Tobacco Smoke Constituent Yields for a Series of Eight Stalk Positions of Four Cultivars (SC-58, LN-38, C-139, and NC-95) of Bright Tobaccosa correlation coefficient (r) acetaldehyde in 100 g of tobacco smoke acetaldehyde in 100 g of tobacco smoke % pure cellulose % starch % holocellulose % sucrose % D-glucose % D-fructose % total free sugar % Total sugar
% pure cellulose
% starch
% holocellulose
% sucrose
% D-glucose
% D-fructose
% total free sugar
% total sugar
1 -0.070 -0.233 0.423 0.428 0.372 0.455 0.380
1 -0.337 -0.300 -0.009 0.133 -0.002 0.171
1 0.098 -0.507 -0.591 -0.518 -0.743
1 0.260 0.192 0.377 0.136
1 0.829 0.930 0.827
1 0.948 0.893
1 0.859
1
1 0.706b -0.068 -0.093 0.476 0.270 0.052 0.212 0.137
a
Data underlying these correlations are from Tso and Chaplin (71). Certain of these correlation coefficients are reported in Table 19 of a USDA Technical Bulletin (71), and some of these and others were reported in a journal article (69). In most cases, the values calculated by one of us (J.I.S.) using the data from the Technical Bulletin are very close to those reported in the original report and journal article. b In this case, the value of the reported correlation coefficient, r ) 0.403 (71), is considerably different from that calculated by one of us (J.I.S.), r ) 0.706, using the data reported in Tso and Chaplin (71). The correlation coefficient in this table uses the more recent statistical analysis. As a double check, a comparison was made between the mean values of the tobacco leaf and tobacco smoke constituents of the individual cultivars and stalk positions, and these were found to be identical to those reported (71).
Figure 4. Correlation between MS smoke acetaldehyde with cellulose from single Bright tobacco cultivars. Data from Tso and Chaplin (69).
potassium cation (K+) may have an explanation in the cellulose pyrolysis literature. Low concentration of inorganic cations are known to increase substantially the formation of low molecular weight aldehydes, in particular hydroxyacetaldehyde, in the pyrolysis of cellulose (50, 70). Data for tobacco leaf sugars, cellulose, starch, and pectin were not reported in this publication. In 1977, Tso and Chaplin authored a technical bulletin published by the U.S. Department of Agriculture (71). This bulletin contains the data underlying the previously published Tso, Rathkamp, and Hoffmann publication (69) as well as additional leaf, smoke, and biological data and simple correlations among the variables. Selected simple correlation coefficients from reanalysis of the original data set (72) are shown in Table 6. These data indicate a significant correlation (r ) 0.706) between MS smoke acetaldehyde and cellulose; that is, ca. 50% of the variation in MS smoke acetaldehyde was related to percent cellulose in the tobacco (Table 6, column 1; see also Figure 4). Neither D-glucose, D-fructose, total free sugar, nor total sugar in the tobacco explained much of the variance in the MS smoke acetaldehyde yields. For
sucrose, a correlation coefficient r ) 0.476 was found, thus demonstrating that only ca. 22% of the variation in MS smoke acetaldehyde was related to percent sucrose in the tobacco. In a 1969 study entitled “Relation between Vapor Phase Components of Smoke and Constituents of Tobacco,” Kaburaki et al. conducted sequential solvent extractions of two tobaccos and smoked cigarettes made from the resultant materials (27, 73). Three control cigarettes were examined, a Bright yellow tobacco, a Matsukawa tobacco (important Japanese tobaccos), and a cellulose cigarette. The tobaccos were sequentially extracted with diethyl ether and then with methanol. The residues most likely contained cellulose and other polysaccharides, proteins, and inorganic compounds (see Table 7). The MS smoke acetaldehyde levels from both Bright Yellow and Matsukawa tobaccos, after consecutive extractions, contained higher levels of MS smoke acetaldehyde than found in the MS smoke from cigarettes containing the original tobacco, on a microgram per puff basis. Each cigarette examined contained only the residual material following the extractions from the control cigarette, i.e., the mass of the extracted materials was not “made-up” to a constant cigarette weight. In another experiment (73), cellulose was added to Matsukawa. Compared to the Matsukawa control, nearly double the amount of acetaldehyde was analyzed in MS smoke vapor phase. These Kaburaki et al. results indicate that the primary precursors to smoke acetaldehyde are the least extractables, namely the polymeric materials including cellulose and the pectins. While total “tar” yields were not given, the values in Table 7 are micrograms per puff. In addition, the total mass of volatile components is given in Table 7. For the Bright yellow cigarette, total vapor phase volatiles decreased substantially following sequential solvent extraction; for Matsukawa tobacco, no change in total volatiles was observed. On the basis of these data, the authors concluded that the constituents whose MS yields were not decreased by the solvent extractions, including acetaldehyde, were produced mostly from the natural tobacco cellulose, hemicellulose, lignin, and denatured proteins.
1340
Chem. Res. Toxicol., Vol. 15, No. 11, 2002
Perspective
Table 7. Amounts (µg/puff) of Acetaldehyde and Representativea MS Smoke Gas Phase Constituents Following the Sequential Extraction of Tobaccoa bright yellow tobacco
constituentb total mass of vapor components acetaldehyde isoprene 1,3-pentadiene propionaldehyde furan acetone isobutyraldehyde 2-methylfuran methyl ethyl ketone butenone benzene 2,5-dimethylfuran toluene
Matsukawa tobacco
untreated (control)
ether extracted (without hydrocarbons, esters, terpenols, fatty acids, steroids)
ether, methanol extracted (also without sugars, amino acids, organic acids, polyphenols, alkaloids)
untreated (control)
ether extracted (without hydrocarbons, esters, terpenols, fatty acids, steroids)
ether, methanol extracted (also without sugars, amino acids, organic acids, polyphenols, alkaloids)
359.9
323.9
273.5
355.1
359.2
351.3
80.3 54.4 6.5 3.5 3.7 70.6 7.2 19.6 12.7 7.8 10.0 34.2 14.5
71.3 45.1 5.1 3.2 2.5 61.9 6.8 16.7 10.9 5.5 8.4 29.2 12.5
110.5 3.6 4.2 2.9 3.7 58.2 7.9 7.9 11.5 4.3 8.4 6.9 17.4
94.3 46.3 7.6 4.1 3.9 70.4 10.2 7.7 18.1 10.5 9.9 6.9 19.5
123.2 25.9 7.0 4.4 3.5 70.4 10.9 5.4 14.8 10.2 9.2 6.1 19.5
124.5 5.2 6.8 4.4 4.6 81.9 11.3 4.8 21.4 9.0 11.0 6.5 21.5
a Estimated materials removed during the extractions are listed. Data from Kaburaki et al. (27). b Twenty-two gas phase constituents were examined in total. Only those having values greater than ca. 6 µg/puff are shown.
Kaburaki et al. provided additional evidence for their conclusion. They reported the yields of acetaldehyde and 19 other gas-phase constituents of MS smoke for a 1:1 cellulose + Matsukawa tobacco compared to an allMatsukawa tobacco cigarette. (A 100% cellulose cigarette, in their hands, did not burn well and failed to give reproducible gas-phase measurements.) The Matsukawa cigarette delivered 63.3 µg/puff MS smoke acetaldehyde, while the 1:1 cellulose + Matsukawa blended cigarette delivered 131.5 µg/puff MS smoke acetaldehyde. Kaburaki et al. concluded that MS smoke acetaldehyde was produced from the cellulose (73). Others have also concluded that MS smoke acetaldehyde is formed from tobacco skeleton substances (32). The Kaburaki et al. results (27, 73) found confirmation in related pyrolysis experiments of Burton and Childs (74). The 1R1 Kentucky blend reference tobacco was extracted by various solvents (acetone, methanol, acetonitrile, and hexane). The individual resultant materials were subjected to thermolysis, and the thermal formation profiles of various low molecular weight compounds, including acetaldehyde, were obtained. Extraction of the tobacco had little influence on the temperature for maximum formation but did increase the amount of acetaldehyde formed, on a per gram basis of material pyrolyzed. This indicates that, from a thermolysis point of view, the acetaldehyde results primarily from the nonextractables, such as the cellulose. This conclusion is consistent with the smoke studies discussed above. Thus, a variety of studies suggest that natural tobacco polysaccharides, including cellulose, are the primary precursor of acetaldehyde in MS smoke.
E. MS Smoke Acetaldehyde Yields as a Function of MS Smoke Constituents Several studies have reported MS smoke nicotine, “tar”, carbon monoxide, and acetaldehyde yields for commercial cigarettes, and one reported these MS smoke yields for Kentucky Reference cigarettes. This data allows the examination of relationships between acetaldehyde and “tar” and carbon monoxide MS smoke yields.
Table 8. Simple Correlations of MPP Smoke Acetaldehyde with MS Smoke Nicotine, carbon monoxide, and “tar”, from the Massachusetts Benchmark Study (MBS)a correlation coefficient with MS smoke acetaldehyde (r) puff profile used for nicotine, “tar” and carbon monoxide
MS smoke nicotine
“tar”
MS smoke carbon monoxide
FTC puff profileb Massachusetts puff profilec
0.898 0.867
0.912 0.906
0.952 0.962
a Data from Taylor et al. (21). b One 2-s 35 mL puff/min, with no ventilation hole blocking. c One 2-s 45 mL puff/30 s, with 50% ventilation hole blocking.
Study 1 is the “1999 Massachusetts Benchmark Study” (21). This was a collaborative effort of Brown & Williamson Tobacco Corporation, Lorillard Tobacco Company, Philip Morris, USA, and R. J. Reynolds Tobacco Company. The study reported a large database of numerous smoke constituents using, in part, the FTC puffing parameters (35 mL puff volume, with 2-s puff duration, once every 60 s) as well as “Massachusetts puffing parameters” (MPP) (45 mL puff volume with 2-s puff duration once every 30 s with 50% blocking of the ventilation holes). Twenty-six U.S. commercial cigarettes and the Kentucky Reference 1R4F cigarette were examined. Excellent correlations were reported for MS smoke MPP acetaldehyde yield and FTC and MPP nicotine, “tar”, carbon monoxide yields (see Table 8). The authors noted that some MS smoke constituents, including acetaldehyde, correlated somewhat better with MS smoke CO than with either “tar” or MS smoke nicotine, consistent with acetaldehyde being a vapor phase constituent of MS smoke. Study 2 was a report commissioned by the Department of Public Health, Commonwealth of Massachusetts and performed by Labstat Inc (22). For the U.S. commercial cigarettes studied, a statistically significant correlation (r ) 0.963, P < 0.001, n ) 8; r ) 0.975, P < 0.0001, n ) 10) was observed between FTC MS smoke acetaldehyde and FTC and a Massachusetts puff profile (MPP) average “tar”, respectively. In this case, the MPP was a 45 mL
Perspective
Chem. Res. Toxicol., Vol. 15, No. 11, 2002 1341
Figure 5. Plot of FTC MS smoke acetaldehyde vs “tar” for 325 U.S. commercial cigarettes for 1991. From Seeman et al. (55).
Figure 6. Plot of the ratio MS smoke acetaldehyde/”tar” vs “tar” for 325 U.S. commercial cigarettes for 1991. From Seeman et al. (55).
puff volume with 2-s puff duration once every 34 s with 50% ventilation hole blocking. The correlations between MS smoke acetaldehyde and MS smoke carbon monoxide were slightly lower than with “tar.” Study 3 reported FTC MS smoke acetaldehyde, “tar”, and MS smoke carbon monoxide yields for a large number of US commercial cigarettes over the time period 19851988 and 1990-1993 (55). Correlations were observed between FTC “tar” and acetaldehyde (see Figure 5, for example) and carbon monoxide in the range of ca. r ) 0.90-0.95. The correlations of MS smoke acetaldehyde with MS smoke carbon monoxide were somewhat higher than with “tar,” though MS smoke carbon monoxide and acetaldehyde data were available for only 2 years. The
mean value of MS smoke acetaldehyde yield was ca. 450590 µg/cigarette from 1985 to 1988 and 1990 to 1993, with no clear trend as a function of time. Figure 6 demonstrates that the ratio of MS smoke acetaldehyde/”tar” appears to be independent of “tar” yield. Study 4 analyzed a representative sample of 29 U.S. cigarette brands and the 1R4F and 1R5F Kentucky Reference cigarettes (24). The yield of MS smoke acetaldehyde correlated with the “tar” yield with r ) 0.954. One factor that may contribute to the variance in MS smoke acetaldehyde yield as a function of “tar” is the possible effect of blend composition. In principle, lamina (Burley, Bright, and Oriental tobaccos) may each contribute differently to MS smoke acetaldehyde than stems, different types of reconstituted tobaccos, and expanded tobaccos. Two early and limited data sets suggest that Bright, Burley, a standard experimental blend, a paper process sheet, and a slurry process sheet yield approximately the same MS smoke acetaldehyde on a “per gram of tobacco burned” basis (75-77). Because stems, expanded tobaccos, and reconstituted tobaccos have significantly different density and burn rate characteristics from each other and from lamina, it is difficult to make direct comparisons of the effect of specific blend components within a blend. Indeed, smoke yield also depends on tobacco variety, stalk position, and other growing factors, causing smoke chemistry changes that can exceed those related to blend component (77). In another study, the yield of acetaldehyde and other MS smoke constituents was examined as a function of eight stalk positions of four flue-cured varieties (78). Acetaldehyde was formed in the highest amounts in the middle leaves. A recent comparison by Hoffman and Hoffman of nine smoke components, including acetaldehyde, produced from smoking single blend component cigarettes (i.e., cigarettes made from puffed tobacco, expanded tobacco, freeze-dried tobacco, and expanded stems) concluded that the levels of most components measured, including acetaldehyde, were lower relative to the control cigarette (6); however, when the acetaldehyde yields are normalized relative to either MS smoke CO or “tar”, the yields were higher in seven of the eight comparisons relative to the control. See the discussion in section IV.A. above. The effect of different puffing regimens using 1R4F and 1R5F Kentucky Reference cigarettes was recently examined (79). Table 9 lists the MS smoke acetaldehyde yields and TPM data from this data set. An excellent correlation is found between acetaldehyde and TPM. Thus, a number of studies using commercial U.S. cigarettes as well as reference cigarettes all demonstrate large and statistically significant correlations between MS smoke acetaldehyde yields and “tar” and MS smoke
Table 9. Influence of Puffing Regimen on Acetaldehyde Deliveries from Two Kentucky Reference Cigarettesa puff volume (mL)/puffs per minute/ ventilation blocking (%) 1R4F TPM yield (mg/cigt) acetaldehyde yield (µg/cigt)b MS smoke yield of acetaldehyde relative to TPM (µg acetaldehyde/mg TPM)
1R5F
35/1/0
55/2/0
55/2/100
35/1/0
55/2/0
55/2/100
10.7 674 63
26.3 1253 48
33.3 1535 46
2.53 171 68
8.8 635 72
19.8 1229 62
a Data from Rustemeier et al. (67). b A regression relationship between acetaldehyde and TPM using these data is: Acetaldehyde ) 42.5 TPM + 198; r ) 0.973, p ) 0.001.
1342
Chem. Res. Toxicol., Vol. 15, No. 11, 2002
Perspective Scheme 2
carbon monoxide yields. These correlations include cigarettes having a wide range of “tar” yields. This work suggests that MS smoke acetaldehyde formation is more related to overall MS smoke formation, generally a function of cigarette design characteristics such as filter ventilation, filtration and paper porosity than reducing sugars. In other words, the MS smoke yield of acetaldehyde varies proportionately as total MS smoke yield changes, i.e., “tar”, or as a surrogate for MS smoke gasphase changes, i.e., MS smoke carbon monoxide. Cigarette design factors which, for example, decrease total MS smoke such as filter ventilation and cigarette paper permeability, will also simultaneously decrease both MS smoke acetaldehyde and MS smoke carbon monoxide.
V. Bioavailability of Mainstream Smoke Acetaldehyde A. General Considerations The pharmacology of acetaldehyde has been researched for many decades. Most of the experimental results dealing with the pharmacological properties of acetaldehyde have arisen from research involving the intake, metabolism, and pharmacokinetics of ethanol. Acetaldehyde is a metabolite of ethanol and is formed by the oxidation of ethanol (11), catalyzed mainly by alcohol dehydrogenases (ADH) (80). Acetaldehyde is oxidized to acetate (12), primarily by aldehyde dehydrogenase (ALDH) enzymes (81). See Scheme 2. In recent years, there have been discussions concerning (1) the possible formation of MS smoke acetaldehyde from sugars added to tobacco and (2) the role of MS smoke acetaldehyde in human smoking behavior (8-11, 14). Point 1 has been addressed above. Regarding point 2, before one could conclude that acetaldehyde delivered in cigarette smoke could affect behavior in smokers as a result of interacting directly in the central nervous system (CNS) of smokers, four conditions would have to be met: 1. Acetaldehyde delivered in cigarette smoke must be retained within and absorbed from the respiratory tract during the inhalation of cigarette smoke. 2. Levels of acetaldehyde in the systemic circulation must be elevated following the inhalation of cigarette smoke. 3. Acetaldehyde must reach and then pass through the blood-brain barrier. 4. Acetaldehyde must reach its putative CNS target in biologically relevant concentrations.
B. Analytical Problems Encountered in the Measurement of Acetaldehyde Blood Levels In some of the sections that follow, various estimates on the contribution of smoking-related acetaldehyde to blood acetaldehyde levels are presented. Indeed, analysis of acetaldehyde levels in vivo are key to the evaluation
of bioavailability of MS smoke acetaldehyde. In this section, we review the literature on acetaldehyde analysis in blood. The literature indicates that these analyses may be hampered by the use of different analytical techniques for blood acetaldehyde, which produce a wide range of values. For example, reported values for blood acetaldehyde range from
