3.1.1. Mouse

A number of early investigations of the effects of mainstream tobacco smoke on tumour development in mice were conducted using the C57BL strain. This strain is relatively resistant to the development of both spontaneous and chemically induced lung tumours (Shimkin & Stoner, 1975). [Therefore, the lack of tumour response in the first few investigations described below may have been due to the choice of this strain.]

Groups of 100 male and 100 female young adult C57BL mice were exposed (nose only) for 12 min/day to a mixture of fresh cigarette smoke and air (1/39, v/v) every other day for life. [The cigarettes were made from a composite blend of flue-cured tobaccos typical of the major brands of untipped cigarette smoked in the United Kingdom (Harris et al., 1974).] The concentration of nicotine in the mixture was about 0.1 mg/mL and that of carbon monoxide, 0.064% (v/v). The nicotine content in the lungs of mice that died during exposure was 14–17 µg. Groups of 100 males and 100 females served as controls. No lung tumours, either adenomas or carcinomas, were found in control mice, but lung tumours (alveologenic adenocarcinomas) were found in 4/100 male and in 4/100 female mice exposed to tobacco smoke [p = 0.06]. Some of the adenocarcinomas were transplantable. Mice exposed to smoke often showed emphysema and marked peribronchial and perivascular infiltration with lymphocytes, independent of the presence of tumours (Harris & Negroni, 1967).

A total of 126 C57BL and 126 BLH mice [sex and initial group sizes unspecified] were exposed to the gas phase of 12 cigarettes [type unspecified] puffed for 2 s/min. The gas phase was generated by passing the smoke through a Cambridge filter that retained the particulate matter. The mice were exposed in a 200-L chamber for 90 min daily (approximately the maximal tolerated dose) until death (approximately 27 months). The survival rates for animals that received more than 12 months of treatment were not affected by treatment in either strain of mice [numbers not provided]. The percentages of mice with lung adenomas were 5.5% and 32% (7/126 and 40/126) in exposed C57BL and BLH mice and 3.4% and 22% (3/90 and 19/60) in C57BL and BLH controls, respectively. The percentages of BLH mice with lung adenomas that survived for at least 10 months were 37% of the treated group and 31% of the controls. The difference in incidence of adenoma between the control and treated groups was stated not to be statistically significant (Otto & Elmenhorst, 1967). [The Working Group noted the incomplete reporting of the experiment.]

A group of 50 male C57BL6/Mil mice, 6 weeks of age, was exposed to cigarette smoke from 85-mm unfiltered cigarettes [type unspecified] in a vacuum chamber for 15 min, five times per week, for 63 weeks. The nicotine content in the lungs of mice that were killed ranged from 14 to 27 µg. When the nasal cavity and respiratory tree were examined microscopically, the incidences of hyperplasia and metaplasia were found to be increased in exposed mice. These conditions were considered to be secondary to inflammation. No tumour was found (Wynder et al., 1968). [The Working Group noted the relatively short duration of the study.]

Groups of Snell's mice, 3–4 months of age at the start of treatment, were exposed to two puffs (15-mL puff volume, 2-s duration, 58-s interval between puffs) of either whole fresh cigarette smoke [type unspecified] (160 males and 118 females) or to the gas phase (100 males and 89 females); 117 males and 83 females served as controls. Animals were exposed in individual containers once a day (except during holidays and weekends) for life. When body weight loss was 4 g or more, animals were withdrawn from exposure treatment for periods varying from 2 days to 2 months. Two popular brands of unfiltered cigarettes were used; each cigarette was smoked to a butt length of 23 mm. The gas phase was produced by passing the smoke through a Cambridge filter. The efficiency of exposure was assessed by determining the carbon monoxide concentration in blood and the nicotine content of the lungs (about 5 µg). Survival after 12 months of age was distinctly reduced by treatment, being about 92% (184/200), 60% (172/278) and 40% (88/189) in the control, whole smoke-treated and gas phase-treated groups, respectively. After 12 months of age, the occurrence of pulmonary tumours was earlier in exposed mice than in controls. At the end of the experimental period (26 months), the numbers of mice that survived 12 months or more that had pulmonary adenomas were 8/106 and 1/78 male and female controls, 7/107 and 2/65, respectively (p = 0.475), male and female animals exposed to whole smoke and 1/44 and 3/44, respectively (p = 0.15), male and female animals exposed to the gas phase. The proportions of mice with pulmonary adenocarcinomas were 5/106 male and 3/78 female controls, 11/107 (p = 0.15) male and 5/65 (p = 0.35) female animals exposed to whole smoke and 10/44 (p = 0.005) male and 5/44 (p = 0.15) female animals exposed to the gas phase (Leuchtenberger & Leuchtenberger, 1970).

Groups of 100 male and 100 female C57BL mice, aged 14–18 weeks, were exposed (nose only) to a mixture of fresh cigarette smoke and air (1/39, v/v) for a 12-min period, generally on alternate days, for life. Groups were exposed to the whole smoke of two different types of cigarette, one made from flue-cured and the other from air-cured tobacco. The latter more closely resembled flue-cured cigarettes than typical air-cured cigarettes which contain, for example, burley tobacco. A further group of animals was exposed to the gas phase only of smoke from the flue-cured cigarette; this was generated by passing the whole smoke through a Cambridge filter to remove the particulate matter. A group of 200 males and 200 females served as controls. The experiments with whole smoke were repeated under similar conditions with another group of 100 male and 100 female mice. Mean survival time in eight of 10 groups of mice exposed to whole smoke was 4–14 weeks longer than that in controls. The incidences of lung tumours in controls were 3/160 males and 1/159 females; those in mice exposed to the whole smoke of cigarettes made from flue-cured tobacco were 9/162 males and 7/164 females [p = 0.07 and 0.04], respectively; the incidences in mice exposed to the gas phase of cigarettes made from flue-cured tobacco were 3/81 males [p > 0.05] and 2/88 females [p > 0.05]; and those in mice exposed to the whole smoke of cigarettes made from air-cured tobacco were 7/189 males [p > 0.05] and 0/173 females. The majority of the lung tumours were adenomas (Harris et al., 1974).

A group of 117 female BALB/c mice, 9 weeks of age, was exposed to smoke from ‘high-tar’ (16 mg tar, 1.1 mg nicotine) cigarettes diluted with air (7:1, air:smoke) for 7–8 min per day on 5 days per week for 95 weeks; the exposure was discontinued for 3 weeks starting between the 48th and 49th week of treatment. An untreated group of 130 females served as controls. Groups of approximately 16 control and 16 exposed mice were killed after 56, 64, 72 and 80 weeks of treatment. Twenty control and 10 exposed mice survived until termination of the experiment after 95 weeks. The incidence of bronchial adenomas was calculated by the authors to be 3.8 times higher in the exposed groups of mice than in the control groups after 83 weeks, but to be similar in the two groups after 95 weeks of exposure. The authors also reported an increase in the incidence of lymphomas after both 83 weeks (2.6 times) and 95 weeks (3.2 times) of exposure (Keast et al., 1981). [The Working Group noted that insufficient numerical data were provided in relation to the number of animals at risk.]

Two thousand and fifty-three (C57BL/Cum × C3H/AnfCum)F₁ mice were exposed (nose only) to cigarette smoke from University of Kentucky reference 2R1 cigarettes on 5 days/week for 110 weeks and observed until death. In addition, 1014 mice were sham-exposed and 449 mice were held as cage controls. The deposition of smoke particulates was estimated to be about 125–200 µg total particulate matter/lung/day. The only lung cancers observed were diagnosed as alveolar adenocarcinomas. These adenocarcinomas were observed in 19/978 smoke-exposed mice and 7/651 sham-exposed mice. The difference between the incidence in smoke- and sham-exposed groups was not significant, but the data suggested that the tumours occurred with a shorter latency in the smoke-exposed group (p = 0.10). Other changes associated with exposure to smoke were increased incidence of pigmented alveolar macrophages, otitis media and head and neck fibrosarcomas (Henry & Kouri, 1986). [The Working Group noted that both parent strains are relatively resistant to the development of spontaneous and chemically induced lung tumours.]

3.1.2. Rat

A group of 408 female Wistar rats, 10 weeks of age, was exposed (nose only) to a mixture of smoke and air in a ratio of 1:5 generated from T29 cigarettes (especially manufactured from a composite blend of flue-cured tobaccos, representing the major untipped cigarette brands smoked in the United Kingdom during 1967–68), for 15 s/min during 11 min, twice a day on 5 days per week for life. After 1 year, the rats received 5% carbon dioxide in air instead of air for 5 min before the start of each smoke exposure session and also for the 45-s/min interval during which smoke was not generated. Measurement of an arsenious sulfate tracer showed that 8.5% and 17.5% of the delivered tar was recovered in the lungs of rats subjected to the air and the air/carbon dioxide regimens, respectively. Two control groups of 102 animals were kept, one sham-exposed and one untreated. The smoke-exposed and sham-exposed animals had lower body weights than the untreated controls. About 25% of the treated rats were still alive at 100 weeks compared with 73% of 102 untreated controls and 66% of 102 sham-exposed controls. All treated animals had died by 140 weeks; the control animals had all died by 160 weeks. Three of the smoke-exposed rats had pulmonary squamous neoplasms of uncertain malignancy, and one had an invasive squamous-cell carcinoma of the lung. No lung tumour was observed in the sham-treated or untreated controls. The incidence of benign mammary tumours was lower in the smoke-exposed rats than in the sham-exposed or untreated controls (Davis et al., 1975a). [The Working Group noted that the daily exposure period was very brief.]

A group of 18 female Wistar rats, 13–14 weeks of age, was exposed (nose only) to the gas phase of smoke (produced by passing the smoke through a Cambridge filter) from a standard British reference cigarette diluted 1:5 with air. Exposure took place for 15 s followed by exposure to air for 45 s during 11 min, twice daily on 5 days per week for life. A control group of 16 animals was kept. Average period of survival was 100 weeks for treated animals and 114 weeks for controls. No lung tumour was reported in either the exposed or the control rats (Davis et al., 1975b). [The Working Group noted the small number of animals used and the short duration of the period of exposure each day.]

A group of 80 female Fischer 344 rats, 12–14 weeks of age, was exposed (nose only) to a mixture of smoke and air in a ratio of 1:10 for 28–30 s/min followed by air for 30 s during standard smoking of a US reference untipped cigarette (average, 8.4 periods of exposure/cigarette). The mean smoke particulate and nicotine levels recorded during chamber monitoring were 18.4 mg and 0.89 mg per cigarette, respectively. The animals were exposed to 1 cigarette/h, 7 cigarettes per day, on 5 days per week for 128 weeks, followed by observation for a further 6 months. The mean pulmonary particulate deposition during exposure was 0.25 mg/cigarette (total, 1.75 mg/rat per day). A group of 63 rats served as untreated controls and 30 rats were sham-exposed. The length of survival of smoke-treated, sham-treated and untreated controls was not significantly different, but smoke-and sham-treated animals had lower mean body weights. Because the tumour incidences in untreated and sham-treated controls were similar, the two groups were combined (93 animals). One alveologenic carcinoma was observed in one control rat and one pulmonary adenomatoid lesion in two other control rats. Ten respiratory tract tumours were observed in seven smoke-exposed rats: one nasal adenocarcinoma, one nasal squamous-cell carcinoma, five pulmonary adenomas, two alveologenic carcinomas and one squamous-cell carcinoma (p < 0.05). Of the smoke-exposed animals, 21 developed subcutaneous sarcomas at sites of ulceration on the forelimbs (p < 0.05), four rats developed benign tumours of the oral tissues (p < 0.1) and four developed adrenal gland tumours (one adenoma and three carcinomas) (p < 0.1). No such tumours occurred in controls. The incidences of tumours of the pituitary gland, uterus and ovary, haematolymphatic system and mammary gland were lower in smoke-exposed rats than in controls (Dalbey et al., 1980).

Three groups of 80 female Fischer 344 rats were exposed to a mixture of smoke and air in a ratio of 1:10. The smoke was generated from three research cigarettes [source not specified] containing 25.4 mg tar, 0.16 mg nicotine; 13.3 mg tar, l.06 mg nicotine; or 25.7 mg tar, 1.91 mg nicotine, respectively (see also Griest et al., 1980). Each rat was exposed to smoke for 28 s/min followed by air for 30 s during standard smoking of the cigarettes, to provide 10–11 exposures/cigarette. The animals were exposed to eight cigarettes per day during a 16-h period, on 7 days per week for 96 weeks, at which time all survivors (50–70% of animals) were killed. Groups of 80 animals served as untreated and sham-treated controls. The occurrence of squamous metaplasia of the laryngeal and tracheal epithelium was significantly increased in smoke-exposed rats. One squamous-cell carcinoma of the lung was observed in a rat exposed to ‘low-tar medium-nicotine’ smoke; the incidences of other tumours were similar in treated and in control animals (Wehner et al., 1981). [The Working Group noted that, in contrast to the study described previously, all surviving animals were killed at 104 weeks of age and that the method for sampling for histopathology was not fully reported.]

In an experiment in which combined exposure of CDF^®(Fischer 344)/CrlBR rats to mainstream tobacco smoke and ²³⁹PuO₂ was studied, two groups of rats (6 weeks of age) were subjected to whole-body exposure for 6 h/day, 5 days per week, for 126 weeks to mainstream cigarette smoke diluted to concentrations of either 100 or 250 mg total particulate matter/m³ (LCS and HCS groups). A significant increase in the occurrence of lung tumours was found in females exposed to smoke only but not in males. The incidences of benign and/or malignant lung tumours in female rats were 0/113, 4/145 (3%) and 6/83 (7%; p < 0.05; one-sided Yates test) in the groups treated with filtered air alone, and the LCS and HCS groups, respectively. The corresponding tumour incidences for males were 3/119 (2.5%), 3/173 (2%) and 7/78 (8%), respectively (Finch et al., 1995).

3.1.3. Syrian hamster

In a study involving 4440 hamsters, three groups of 80 male and 80 female Syrian golden hamsters, 8 weeks of age, were exposed to a mixture of smoke and air in a ratio of 1:15. The smoke was generated during a 7–10-min period from 30 German reference cigarettes, once, twice or three times a day on 5 days per week until death. Two groups of 80 males and 80 females were exposed either to the smoke of black (dark air-cured)-tobacco cigarettes twice a day, or to the vapour phase of German reference cigarettes, generated from 30 cigarettes during a 10-min period twice a day. Eight hundred animals served as untreated controls. The average length of survival was 52–65 weeks for treated males and 41–49 weeks for treated females. When compared with untreated controls, a shorter survival time and loss of body weight were noted in animals exposed to smoke. In all treated groups, ‘the most remarkable and severe alterations' were observed in the larynx, and their severity depended on duration of treatment and dosage. The incidence of laryngeal leukoplakias ranged from an average of 11.3% in animals exposed to the reference cigarette for 10 min to 30.6% in those exposed for 30 min; the incidence in animals exposed to smoke from black-tobacco cigarettes was 10%, and slight leukoplakia was observed in 1–5% of control animals. The incidences of laryngeal carcinomas were 0.6–10.6% in animals exposed to smoke from the reference cigarette and 1.25% in animals exposed to smoke from black-tobacco cigarettes. No such tumour was observed in control animals or in those exposed to the vapour phase (Dontenwill et al., 1973).

Similar results were obtained in a study involving 2160 hamsters exposed to the smoke from German reference cigarettes or a modification thereof (Dontenwill et al., 1977a).

A modification of the method of Dontenwill (above) was used in a study of inbred Syrian golden hamsters. Groups of 102 male BIO^® 87.20 and BIO^® 15.16 hamsters (13 weeks of age) were exposed to smoke (1:5 in air) from University of Kentucky 1R1 reference cigarettes twice a day on 5 days per week for up to 100 weeks. Groups of 60 animals served as sham-exposed or cage controls for each strain. Exposure to smoke for up to 100 weeks had no effect on length of survival; however, a reduction in body weight was noted. Over 90% of the smoke-exposed animals of both strains showed hyperplastic or neoplastic changes in the larynx; and benign squamous-cell papillomas of the larynx were observed in both strains. Laryngeal cancer occurred nearly five times more frequently in strain BIO^® 15.16, however, and two animals of this strain developed nasopharyngeal tumours. The incidences of tumours at locations other than the respiratory tract were similar in the two strains (Bernfeld et al., 1974).

Groups of 35–64 male BIO^® 15.16 inbred Syrian golden hamsters, 60–70 days of age, were exposed to concentrations of 11% or 22% smoke from commercial British filter-tipped cigarettes or filter-tipped cigarettes composed of tobacco and Cytrel^® (a tobacco substitute), or cigarettes made of 100% Cytrel^® twice a day (12-min sessions of 27 smoke exposures/min) on 7 days per week for up to 74–80 weeks. Groups of 40 sham-exposed animals and 51 caged animals served as controls. Survival rates were markedly reduced in all groups after 59 weeks of age; a reduction in body weight was noted in treated animals when compared with controls. Laryngeal carcinomas occurred in 47.4% (27/57) of the animals exposed to smoke from the British filter-tipped cigarettes; the first tumours occurred after 59 weeks in the hamsters treated with the 22% smoke concentration. The incidence of this type of tumour was dose-related and decreased as the proportion of Cytrel^® in the blends increased. No such tumour was observed in controls. Laryngeal papillomas, laryngeal epithelial hyperplasia and a few tracheal papillomas were also observed in treated animals. Histopathological findings at other sites did not appear to be related to smoke inhalation (Bernfeld et al., 1979).

A group of 51 male Syrian golden hamsters, 2 months of age, was exposed three times a day for 10 min, 5 days per week, for life, to smoke from University of Kentucky 1Rl reference cigarettes. Another group of 51 male hamsters served as sham-exposed controls. The smoke-exposed animals survived longer (mean, 19.6 months) than the sham-exposed animals (mean, 15.3 months). The smoke-exposed animals had significantly higher incidences of epithelial lesions of the larynx than the sham-exposed controls (22% versus 0%, p < 0.01) and also had a significantly higher total number of tumours than sham-exposed controls (28% versus 6%; p < 0.05). The laryngeal changes in the smoke-exposed group ranged from inflammatory conditions, with growth abnormalities of the epithelium, to squamous-cell papilloma formation (Wehner et al., 1974, 1975a,b, 1976).

3.1.4. Rabbit

Thirty rabbits were exposed daily in individual compartments to the smoke from 20 cigarettes (2 puffs/min, 2 cigarettes at each treatment period) for up to 66 months. No tumour that could be related to the exposure was observed in any of the exposed rabbits or in 31 controls that survived for 24 months or more (Holland et al., 1958, 1963). [The Working Group noted that details of the types of cigarettes, method of smoke generation and doses delivered were not provided.]

3.1.5. Dog

Groups of male beagle dogs, 1.7–3.3 years old, trained to inhale cigarette smoke through tracheostomata, were exposed to one of three different treatments: (1) smoke from seven untipped cigarettes containing 34.8 mg tar and 1.85 mg nicotine per day (62 dogs), (2) smoke from seven cigarettes containing the same tobacco, but with filter tips, containing 17.8 mg tar and 1.17 mg nicotine (12 dogs) or (3) smoke from 3.5 untipped cigarettes containing the same tobacco per day (12 dogs). Eight non-exposed tracheotomized dogs served as controls. The body weights of the dogs in the various groups did not differ significantly. Between day 57, when the training period ended, and day 876, when killing of the surviving dogs began, 28 dogs died: 24 in the first group, two in the second, two in the third and none in the control group. Extensive examination of the lungs and bronchial tree at autopsy and microscopically was carried out. Lesions of the lung (reported as tumours), described as ‘bronchiolo-alveolar, invasive and noninvasive', were found in 23/62 of dogs in group 1; two of the lesion-bearing dogs in this group also had small bronchial lesions. Non-invasive bronchiolo-alveolar lesions were also found in 4/12 dogs in group 2, 7/12 dogs in group 3 and 2/8 control dogs. The bronchiolo-alveolar lesions tended to be multiple, with as many as 20 per lung lobe, and were found in 41 of the 203 lung lobes from the 29 dogs with such lesions. No distant metastasis was found (Auerbach et al., 1970; Hammond et al., 1970). [The Working Group noted that no data were given on specific smoking parameters or measures of exposure. They also noted the small number of control dogs used and the unusually high incidence of lung lesions in these animals; these are lesions that rarely occur spontaneously in dogs. The focal inflammatory lesions usually found in the lungs of animals exposed to smoke were not mentioned in the report of this study. Examination of the upper respiratory tract and other organ systems was not reported. The authors' interpretation of the photomicrographs as representing neoplasia was not considered to be entirely convincing.]

As part of a study on the combined effects of radon daughters or uranium ore dust and cigarette smoke on the induction of respiratory tumours (see below), 19 male and female beagle dogs, 24–30 months of age, were exposed to cigarette smoke only. The dogs were exposed to the smoke of 10 University of Kentucky 1R1 reference cigarettes per day (three in the morning, four at midday and three in the afternoon) on 7 days per week for 48–60 months; individual masks permitted smoke to be inhaled at every 10th breath. Eight dogs served as sham-exposed controls. Carboxyhaemoglobin levels in smoke-exposed dogs increased with the duration of the experiment, from 2% at 19 months to 4.7% after 50 months; the corresponding mean values for sham-exposed dogs were 1.4% and 1.8%, respectively. Smoke-exposed animals killed after 49 (six dogs), 60 (six dogs), 64 (six dogs) and 65 months (three dogs) were available for analysis. No significant respiratory lesion and no lung tumour was found in either group (Cross et al., 1982). [The Working Group noted the small sizes of the groups and the limited evidence of smoke delivery.]

3.2.1. Mouse

3.2.2. Rat Selected studies are summarized in Table 3.2.

3.2.3. Syrian hamster

Selected studies are summarized in Table 3.2.

Table 3.2

Selected studies of carcinogenicity in response to exposure to mainstream tobacco smoke in conjunction with exposure to known carcinogens or other agents in rat and hamster.

3.2.4. Dog

Nineteen male and female beagle dogs, 24–30 months old, were exposed by inhalation to a combination of radon (105 nCi/L [3900 Bq/L]), radon progeny (605 WLM) and uranium ore dust (12.9 mg/m³) for 4 h/day, on 5 days per week, for 54 months. A second group of 19 beagle dogs was exposed to the smoke from 10 cigarettes/day on 7 days per week at intervals between exposures to radon progeny and uranium ore dust. Lifespan was shorter in both groups than in controls. Eight dogs exposed to radon alone had nine respiratory tumours (two nasal carcinomas, six pulmonary carcinomas, one pulmonary fibrosarcoma); and 2/19 dogs in the group that received radon plus cigarette smoke had respiratory tumours (one nasal carcinoma and one pulmonary carcinoma) (Cross et al., 1982). [The Working Group noted the small sizes of the groups and the incomplete reporting.]

3.3.1. Skin application

(a) Mouse

CSC produces both benign and malignant tumours on mouse skin. The carcinogenic potency of the CSC depends on tobacco variety, composition of cigarette paper and the presence of additives (see, for example, Wynder et al., 1957; Davies & Day, 1969; Dontenwill et al., 1972; Gargus et al., 1976; Gori, 1976; Dontenwill et al., 1977b; Gori, 1977, 1980). The tumours induced are usually of epidermal origin. Ohmori et al. (1981) reported a low, but significant incidence of mastocytomas in a series of experiments in which mouse skin was treated with CSC.

An example of mouse skin studies is given in a series of four publications (see Gargus et al., 1976; Gori, 1976, 1977, 1980) that reported the results of skin-painting experiments in which more than 100 variables of CSC were tested in female ICR Swiss mice. These data permitted an evaluation of the overall carcinogenicity of CSC and of intralaboratory variation in bioassay results over time, because, in these studies, University of Kentucky 1R1 and SEB cigarettes stored at −20 °C were tested in four series of studies begun in 1970, 1972, 1974 and 1975, and the results (Table 3.4) are representative of the tumour response found in such studies. The purpose of the studies was to investigate possible relationships between biological activity and cigarette design as well as the chemical characteristics of cigarette smoke. The CSC derived from reference cigarettes and specially modified cigarettes was applied generally at two dose levels, usually 25 mg and 50 mg per 0.1-mL application, 6 days a week, for 78 weeks. Acetone was used as the solvent. In some instances, 3-, 6- and 12.5-mg doses were also given, and, for a single type of CSC, doses of 10 mg and 20 mg were given twice daily or five times a week for 78 weeks. Solvent-treated and untreated controls were kept. Test group sizes usually comprised 100 animals, but up to 800 controls were used; 6400–9200 mice were used for each of the four sets of experiments. A skin tumour was observed in 3/800 acetone-treated controls; otherwise, no tumours and no carcinomas were found in control animals. Skin tumours, frequently malignant, were found in every CSC-treated group. [These extensive studies reported differences in the activity of CSCs from cigarettes of different design. The Working Group of the IARC Monographs on tobacco smoking (IARC, 1986) noted that, although the trends of the differences were often consistent, different statistical procedures were used to analyse the data, and insufficient data were provided to permit a uniform analysis for quantitative comparison of these data.]

Table 3.4

Skin tumour incidences in ICR Swiss mice after skin painting with smoke condensate from reference cigarettes^,.

Cigar and pipe smoke condensates have also been tested on mouse skin. In one such study, a 1:1 (w/v) acetone solution of nicotine-free cigar or pipe tar was painted three times a week on the skin of female Swiss mice. A 1:1 acetone solution of whole CSC was also employed. At the end of 19 months, skin papillomas were produced in 65% and carcinomas in 41% of 46 mice treated with the nicotine-free cigar tar, papillomas in 69% and carcinomas in 33% of 45 mice treated with the nicotine-free pipe tar, and papillomas in 47% and carcinomas in 33% of 86 mice treated with CSC. No tumour was produced in 23 acetone-treated controls. At 9 months, when the first tumours appeared, 78% of the controls and 70–91% of the tar-treated animals were still alive (Croninger et al., 1958). [The Working Group recognized that the nicotine-free cigar and pipe tars were prepared in a manner that would cause unknown changes in their composition.]

3.3.2. ‘Initiation–promotion’ skin-painting studies

3.3.3. Topical application to oral mucosa

3.3.4. Intrapulmonary administration

3.3.5. Gavage study