Some tobacco additives are toxic, whilst others produce toxic substances during combustion and pyrolysis when the tobacco is burned, see Section 12.4.2.1 and 12.4.2.2.
Table 12.6.3.1 lists many tobacco additives that are toxic, or suspected to be toxic, in their unburned state (labelled TUB), or that produce toxic products after burning (labelled TAB). Titanium dioxide, diacetyl and guaiacol are examples of additives with known or suspected toxic activity in their unburned state. Three examples of chemicals with toxicity after burning are 1) sorbitol, which forms acetaldehyde and formaldehyde after pyrolysis, 2) glycerol, which forms acrolein, and 3) b-damascone, which forms benzene, toluene, anthracene and phenanthrene.1 The toxicity and carcinogenicity (cancer-causing activity) of chemicals such as formaldehyde, acrolein and benzene are discussed in detail in Section 12.4.3.
Many tobacco additives, particularly flavourings, are regarded as safe for consumption in food. The US Food and Drug Administration (FDA) designates food additives as GRAS: Generally Recognized As Safe, after assessment by experts.2 Tobacco additives such as guar gum, liquorice and diacetyl are designated GRAS.3 However, these three additives, as well as others that are designated GRAS, are considered potentially toxic additives in tobacco. This discrepancy comes about because food additives are not burned or inhaled into the lungs. It is during burning that many of the toxic chemicals in tobacco emissions are created. Chemicals that are eaten in foods are processed by the gastrointestinal system, including exposure to acid in the stomach. Select nutrients are taken up into the blood stream from the gut, after being processed by enzymes. However, in the lungs, chemicals from the smoke quickly enter the blood stream without the same processing that occurs in the gut. That a chemical is designated GRAS by the FDA for food consumption certainly does not mean that it is safe to breathe in the emissions of this chemical that are created in a burning cigarette.1
Many of the additives with known toxicity are also naturally present in tobacco product emissions. Examples are aldehydes produced by sugars when burned, as well as diacetyl (naturally present in tobacco plants) and ammonia in smoke, which are made from compounds naturally present in tobacco as well as added during processing.1,4 However, establishing whether these additives increase the harm to health from smoking is complex.
Toxicity of a chemical, such as a tobacco product additive, can be measured in a variety of ways, such as using cells grown in a laboratory, skin tests and other toxicity tests using rodents. Smoke inhalation experiments in rodents aim to emulate the exposure in humans.5 There is a lack of high-quality experimental data from testing of the toxicity of individual additives in cigarettes and other tobacco products. The reasons why these experiments are challenging are detailed in a 2017 review.6 Given that tobacco smoke is already a highly toxic mix of thousands of chemicals, and individual additives are often at low concentrations, any additional toxicity from an additive will likely be small in magnitude and difficult to detect. Very large numbers of samples or animals will be necessary to detect differences. Both unburned and burned products need to be tested, but the burned products of complex mixes such as natural extracts are mostly uncharacterised, making testing them difficult in animal experiments. Furthermore, many diseases caused by smoking, such as cancer, take a considerable length of time to become apparent, so short-term testing, such as rodent toxicity tests, are insufficient to examine the risks of these diseases. Such tests in animals for the chronic effects of toxicity are expensive, time-consuming and technically challenging. In addition, inhalation tests of individual additives would ideally require the use of tobacco products made with and without that additive, but that were otherwise identical—a task that is very difficult for independent researchers. The authors comment that ‘tobacco smoke is highly detrimental to health irrespective of whether burned tobacco contains additives or not. It is hard to justify killing a large number of animals merely to demonstrate that tobacco additives enhance, do not change or slightly attenuate the inherent toxicity of smoking.’6 Animal testing of tobacco products is also banned in several countries—see Section 10.14.5.
Regardless of the lack of direct evidence that a specific additive leads to health effects, it is well-established that additives such as menthol and flavours increase the uptake of smoking and make it harder to quit–thereby increasing the exposure of people to the multitude of toxic chemicals in tobacco smoke that are the cause of countless diseases and millions of deaths.6,7
References
1. Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR). Final opinion on additives used in tobacco products. European Commission, Health & Food Safety, Directorate C: Public Health 2016. Available from: http://ec.europa.eu/health/scientific_committees/emerging/docs/scenihr_o_051.pdf
2. Food & Drug Administration. Generally recognized as safe (GRAS). FDA, 2019. Available from: https://www.fda.gov/food/food-ingredients-packaging/generally-recognized-safe-gras
3. Food & Drug Administration. Part 184 - direct foods substance affirmed as generally recognized as safe. FDA, Available from: https://www.ecfr.gov/current/title-21/chapter-I/subchapter-B/part-184
4. Talhout R, Opperhuizen A, and van Amsterdam JG. Sugars as tobacco ingredient: Effects on mainstream smoke composition. Food and Chemical Toxicology, 2006; 44(11):1789-98. Available from: https://www.ncbi.nlm.nih.gov/pubmed/16904804
5. Scientific Committee on Health Environmental and Emerging Risks. Opinion on additives used in tobacco products (opinion 2) tobacco additives ii. European Commission, 2016. Available from: https://ec.europa.eu/health/sites/default/files/scientific_committees/scheer/docs/scheer_o_001.pdf
6. Paumgartten FJR, Gomes-Carneiro MR, and Oliveira A. The impact of tobacco additives on cigarette smoke toxicity: A critical appraisal of tobacco industry studies. Cadernos de Saúde Publica, 2017; 33Suppl 3(Suppl 3):e00132415. Available from: https://www.ncbi.nlm.nih.gov/pubmed/28954055
7. Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR). Addictiveness and attractiveness of tobacco additives. Brussels, Belgium 2010. Available from: http://ec.europa.eu/health/scientific_committees/emerging/docs/scenihr_o_031.pdf