In the mid-1980s, each major US cigarette manufacturer prepared a list of those ingredients added at that time to its cigarette products. The lists were combined into one and submitted to the US Office of Smoking and Health in 1986. It comprised 599 entities. On the basis of extensive literature survey and examination of much unpublished data from the Tobacco Industry members on the chemistry and toxicology of the ingredients, a panel of eminent toxicologists assessed the safety of each listed ingredient with regard to its pyrolysate components and its possible effect when added to cigarette tobacco on the chemical and biological properties of the cigarette mainstream smoke. Subsequently, Doull et al. listed the 599 ingredients and summarized the conclusions of the panel on their effect on the chemical and biological properties of cigarette smoke.
In addition to the panel and Doull et al., other investigators have noted that many of the compounds used as ingredients in cigarette tobacco blends are identical with or similar to identified components of tobacco and/or tobacco smoke. The validity of this statement is obvious when the compounds in the Doull et al. list are cataloged as in Table 1. Those tobacco ingredients that are not individual compounds but are naturally-occurring oils, resins, etc. or extracts of naturally-occurring materials not only contain many of the compounds listed by Doull et al. as tobacco additives but also contain many of the same compounds present in tobacco.
Detailed examination of the literature on the chemical and biological properties of the recently used tobacco ingredients listed by Doull et al. plus a massive amount of chemical and biological data generated during the past several decades indicates that not only does none of the Doull et al. listed ingredients contribute any significant adverse chemical properties to cigarette mainstream smoke (MSS) but also none affects adversely the biological properties of the MSS.
The chemical factors examined included: a) The effect on MSS composition of the ingredients added to cigarette tobacco at the usual use level or several times that. In two major series of studies, one by Carmines et al. and one by Baker et al., the effect of the added ingredient on the concentration in mainstream smoke of specific components defined as toxicants was determined. The Carmines et al. study involved analysis of the smoke components suggested by the US Consumer Product Safety Commission and of concern to the International Agency for Research on Cancer (IARC). The Baker et al. study involved analysis of the so-called ‘Hoffmann analytes’ in cigarette smoke. b) The nature of the pyrolysis products generated during the smoking process or during pyrolysis of an individual ingredient under conditions approximating those in the cigarette pyrolysis zone. In many instances when the added ingredient is a compound, a significant percentage of it is transferred unchanged to the MSS and sidestream smoke (SSS). The small percentage not transferred intact to the smoke is seldom converted to an MSS component possessing significant toxic properties.
The extensive biological studies that showed no significant adverse effect of the MSS from ingredient-containing cigarettes included: a) The specific tumorigenicity to laboratory animal skin of the mainstream cigarette smoke condensate (CSC) from ingredient-containing cigarettes vs. the mainstream CSC from ingredient-free cigarettes. b) Exposure of laboratory animals via inhalation to the MSS from ingredient-containing cigarettes vs. the MSS from ingredient-free cigarettes. c) Determination in a variety of tests of the in vitro genotoxicity of the mainstream particulate phase and/or vapor phase.
In addition, the results of non-tobacco-related studies are available in which many individual compounds on the Doull et al. list were assayed for mutagenicity in the Ames test with several strains of Salmonella typhimurium. An excellent example is the 1984 study by Ishidate et al. who examined the mutagenicity of many compounds included as additives in Japanese foods. Over 40 of the compounds exhibiting non-mutagenicity also occur on the Doull et al. tobacco ingredient list.
Assessment of the total chemical and biological data cited herein provides a noteworthy contradiction to the much repeated assertions - with no data supporting them - that the ingredients added to cigarette tobacco result in significant adverse changes in the chemical and biological properties of the cigarette MSS.
Examination of extensive laboratory data collected during the past four decades, particularly considerable unpublished data generated between the mid-1950s and the late 1970s, indicates that none of the materials used as flavorants on smoking tobacco products, particularly cigarettes marketed by a US manufacturer, imparts any significant adverse chemical or biological properties to the mainstream smoke (MSS) from flavorant-treated tobacco, a conclusion reached by Doull et al. (1) in their assessment of available information on nearly 600 ingredients variously used as cigarette tobacco additives in the US Tobacco Industry. In a more recent detailed assessment of the chemical and biological properties reported in the published literature for the MSS from cigarettes fabricated with tobacco with or without one or more additives, Paschke et al. (2) reached a similar conclusion; namely, that in general, no significant increase in the biological activity (carcinogenicity, mutagenicity, and cytotoxicity) of tobacco was reported from cigarettes containing added ingredients. Many flavorful tobacco additives listed by Doull et al. are structurally identical with or similar to highly polar, volatile components identified in the aqueous alcohol-soluble portion of cigarette MSS and tobacco. In the late 1950s, nearly two decades before the precise nature of the aqueous alcohol-soluble components of tobacco was defined, it was determined that their addition to cigarette tobacco produced no significant increase in the cigarette MSS of either the total polycyclic aromatic hydrocarbon (PAH) content or the benzo[a]pyrene (B[a]P) content, MSS components of considerable interest at that time.
During the period of tobacco smoke research from the early 1950s to the mid-1960s it was repeatedly asserted that a) tobacco and many tobacco components were involved in the pyrogenesis of polycyclic aromatic hydrocarbons (PAHs), several of which were reported to initiate tumors on the skin of laboratory animals and b) tobacco additives (flavorants, casing materials, humectants) were highly likely to be similarly involved in PAH pyrogenesis. Extensive knowledge on PAHs was deemed highly necessary because of their claimed importance in the smoking-health issue. The numerous assertions about the generation of PAHs in cigarette mainstream smoke (MSS) triggered extensive and intensive research both within and outside the Tobacco Industry to define the nature of the PAHs, their per cigarette MSS delivery amounts, their precursors, etc. It was not until 1960 that VAN DUUREN et al. (1) reported three specific aza-arenes in cigarette MSS that were asserted to be involved in smokers’ respiratory tract cancer. As noted in a recent Letter to the Editors (2), the presence of these three aza-arenes in tobacco smoke has never been confirmed. Between 1960 and 1965, other MSS components (phenols as promoters, polonium-210, N-nitrosamines, ciliastatic compounds) were asserted to be responsible for smoking related diseases. However, no major assertions were made that phenols, polonium-210, or the N-nitrosamines were derived from flavorants, casing materials, or humectants. Some investigators did report that several ciliastats were derived from added sugars and glycerol. The ciliastat proposal was drastically diminished in importance by the findings in the 1960s that only a relatively small proportion of the ciliastats reached the smoker's cilia. During that time, pertinent skills and competencies in research on tobacco smoke composition, particularly the PAH fraction, have been developed. Such skills permitted the isolation in crystalline form of 14 PAHs and the quantitation of these and many other PAHs. They were also used to put in perspective the pyrogenesis of PAHs from a) specific tobacco components, b) additives, and c) processed tobaccos (reconstituted tobacco sheet [RTS], expanded tobacco). R.J. Reynolds Tobacco Company (RJRT) pioneered the use of RTS (1953) and expanded tobaccos (1969) in cigarette blends and generated much previously unpublished data on the effect of such processed tobaccos on MSS composition.
Examination of extensive laboratory data collected during the past four decades, particularly those unpublished data generated in the 1950s and 1960s, indicates that none of the materials used as casing materials (sugars, licorice, cocoa) and humectants (glycerol, propylene glycol, other glycols) on smoking tobacco products, particularly cigarettes, imparts any significant adverse chemical or biological properties to the mainstream smoke (MSS) from cased and humectant-treated tobacco, a conclusion reached by Doull et al. (1) in their assessment of available information on nearly 600 flavorant, casing material, and humectant ingredients variously used as cigarette tobacco additives in the U.S. Tobacco Industry. Addition of casing materials and humectants to the cigarette tobacco blend produced no significant increase in the cigarette MSS of either the total polycyclic aromatic hydrocarbon (PAH) or the benzo[a]pyrene (B[a]P) content, MSS components that have been of considerable interest for many years. Examination of the transfer of humectants from the humectant-treated tobacco to cigarette MSS indicates that the humectants act as significant diluents to the remaining MSS particulate-phase components generated from the tobacco during the smoking process. This dilution decreases the effects observed in several bioassays, e.g., mutagenicity determined in the Ames Salmonella typhimurium test.
Since the mid-1960s, various investigators, agencies, and institutions have disseminated lists of cigarette mainstream smoke (MSS) components reported to be tumorigenic on the basis of laboratory bioassays conducted under conditions significantly different from those encountered by the smoker during exposure to the components in the cigarette MSS aerosol. Since 1990, numerous lists of cigarette MSS components, defined as significant tumorigens, have been compiled by American Health Foundation personnel, Occupational Safety and Health Administration (OSHA), Fowles and Bates, and R.J. Reynolds R&D personnel. The purpose of most of the reports was to define human risk assessment and to dissuade smokers from smoking. Various investigators and agencies have frequently cited the earlier and/or the more recent lists of tumorigenic entities. The recent compilations, involving nearly 80 MSS components, suffer from serious deficiencies including: a) Use of per cigarette delivery ranges for specified components which often include analytical data from cigarettes manufactured in the 1950s and 1960s which are not comparable to lower-'tar’ yield cigarettes manufactured since the mid-1970s. b) Absence of standard analytical procedures for most of the listed components. c) Methodological considerations regarding bioassays used to determine tumorigenicity of the listed MSS components. d) Difficulty in extrapolating in vivo bioassay data obtained by non-inhalation modes of administration of a single compound to the human smoking situation involving inhalation of a complex aerosol containing that compound. e) Inhalation data inadequacies regarding the tumorigenicity of many of the components. f) Several tobacco smoke components are listed despite the fact their presence has not been confirmed, their MSS level has not been defined, or their MSS level is no longer relevant. g) Insufficient consideration of inhibitors of tumorigenesis and mutagenesis found in MSS. h) Difficulty in extrapolation of inhibition/anticarcinogenesis/antimutagenesis observed in a one-on-one in vivo situation to the complex MSS aerosol situation. j) Alternate exposures to many of the listed smoke components. k) Discrepancies among the lists. l) Discrepancies within the lists.A more appropriate use of the listing process is the identification of potential chemical targets for removal from, or inhibition in cigarette MSS.
The draft initial list of harmful or potentially harmful tobacco and/or smoke components prepared by the Constituent Subcommittee of the Tobacco Products Scientific Advisory Committee (TPSAC) differs significantly from the similar lists prepared by Hoffmann and colleagues who had over four decades of experience and knowledge in tobacco and/or tobacco smoke components and their chemical and biological properties. The draft list comprises 106 components, 60 of which were included in the recent Hoffmann et al. lists but does not include nine of the Hoffmann-listed components. All of the 106 components appear in a list by Rodgman and Green of 162 tobacco and/or tobacco smoke components, each of which was defined as biologically adverse at one time or another over the previous years by one or more investigators. As with the Hoffmann et al. lists, the list by the TPSAC Constituent Subcommittee contains numerous anomalies.
· Three harmful components (dibenz[a, j]acridine, dibenz[a, h]acridine, 7H-dibenzo[c, g]carbazole) first reported in tobacco smoke in the 1960s that were not confirmed over the next forty years by many talented investigators in Japan, Germany, and the USA, including several at the U.S. Department of Agriculture (USDA).
· Two harmful components (arsenic, N-nitrosodiethanolamine) the levels of which have decreased significantly because their precursors have not been used in tobacco agronomy for over three decades.
· The many water-soluble components that reach the lung at a much reduced level to exert their ciliastasis.
· A component (chrysene) that the International Agency for Research on Cancer (IARC) has removed from its tumorigenicity listing, a decision accepted by Hoffmann et al. who removed chrysene from their more recent tobacco/tobacco smoke listings of adverse components.
TPSAC gives no indication of the following:
· The relationship between the per cigarette delivery of some of the harmful components and their level of permissibility by Occupational Safety & Health Administration (OSHA).
· The components on its list that significantly offset the adverse biological activity of several others of its listed components.
· Many components in tobacco and/or tobacco smoke not listed by TPSAC have been reported to significantly reduce the adverse biological effect of several components on the TPSAC list plus several others.
The analytical methods developed since the 1950s to determine the “smoke pH” of cigarette mainstream smoke (MSS) are reviewed. Most of the methods involve averaging the values obtained with all the puffs from a cigarette or averaging values from the total number of puffs from several cigarettes plus solution of the MSS in various quantities of water prior to pH determination. “Smoke pH” values thus obtained have little relevance to the situation involving a smoker who smokes a cigarette one puff at a time. In the human biological situation, the smoke components must traverse a layer of buffered fluid separating the smoke-filled respiratory tract cavity and the underlying tissue. Seldom is the effect of either the volume or the pH of the buffered fluid taken into account in determining the “smoke pH” of cigarette MSS taken one puff at a time. While numerous recent discussions have dealt with the effect of pH on the nature (degree of protonation) of nicotine in cigarette MSS and its rate of absorption by the smoker, seldom is the volume and/or pH of oral cavity saliva or of the fluid coating the lung surface considered in the assessment of either the nature or absorption of nicotine by the smoker. If the degree of protonation of nicotine be pH dependent, then the degree of protonation of nicotine as it traverses the film of fluid separating the respiratory tract cavity and the underlying tissue must be dependent on the pH of the buffered fluid and not on the “smoke pH” per se. Statements that adjustment of the “smoke pH” by ammoniation of one or more tobacco blend components increases both the level of MSS nicotine and its rate of absorption, thus resulting in greater consumer acceptance and continuation of smoking, fail to take into account the other changes that occur in the MSS composition to enhance its consumer acceptability. Inclusion of modest percentages of ammoniated tobacco in the blend increases the “smoke pH” of the MSS but does not necessarily increase MSS nicotine level. Not only are the MSS levels of several low molecular acids, aldehydes, and ketones known to be respiratory tract irritants substantially reduced but also the MSS levels of several highly flavorful pyrazines are substantially increased. These MSS composition changes yield an MSS classified as milder and more flavorful and whose consumer acceptability is considerably enhanced. Until the pH of MSS is determined on a per puff basis in a system simulating that in the oral cavity or the lung of the smoker, the nature of specific smoke components at the time of contact and their reactivity with respiratory tract tissue remain highly speculative. “Smoke pH” value for MSS, like the “tar” value for MSS, provides little, if any, useful information about the chemical composition of the source of the MSS or about its chemical and biological properties.
Tobacco and tobacco smoke are both complex mixtures. We previously reported 8430 unique chemical components identified in these complex mixtures but two years later our updated number was 8889. Addition of unlisted isomers raised these numbers to 8622 and 9081, respectively. Our previous number of 4994 identified tobacco components is now 5229; our previous number of 5315 identified tobacco smoke components is now 5685. An operational definition of a complex mixture is as follows: A complex mixture is a characterizable substance containing many chemical components (perhaps thousands) in inexact proportions.
Detailed knowledge of the amount and type of each component within the substance is uncertain even with today's analytical technology. Although it has been estimated that as many as 100000 components are present in these complex mixtures, their analyses indicate that the vast majority of the mass of each of these complex mixtures accounts for the 8430 compounds reported previously. Over 98.7% of the mass of tobacco has been accounted for in terms of identified components in tobacco. Greater than 99% of the mass of whole smoke has been accounted for based on identified chemical components. Certainly, many more tobacco and tobacco smoke components are present in these complex mixtures but the total mass of these components obviously is quite small.
One of the significant challenges we face as a scientific community is addressing the problems of determining the risk potential of complex mixtures. Many issues are associated with toxicological testing of the complex mixture of tobacco smoke. Conducting valid experiments and interpreting the results of those experiments can be quite difficult. Not only is the test agent a complex mixture but also the tests are performed on species that have complicated life-processes. Interpretations of test results are often paradoxical. Significant progress has been made in the toxicological evaluations of complex mixtures in the last 80 years. The challenges we face in terms of testing the biological properties of tobacco smoke are substantial. The statement by DIPPLE et al. in their summary of the research on polycyclic aromatic hydrocarbons from the 1930s through 1980 is equally true today for the cigarette smoke situation:
…many important questions remain unanswered
…many questions persist despite the considerable progress that has been made.