What is heat-not-burn?
Absence Of Combustion
Combustion In Conventional Cigarettes
Combustion is a defining characteristic of conventional cigarettes. When a cigarette is lit, the combination of tobacco and paper (fuels) with oxygen (oxidant) and heat (energy) generates a self-sustaining combustion, an exothermic process that consumes the tobacco. This process results in the formation of ash and smoke that contains solid particles and high levels of harmful and potentially harmful constituents (HPHCs). During the natural smoldering of a cigarette (between puffs), temperatures between 600°C and 800°C (approx. 1,100°F and 1,500°F respectively) occur in the center of the burning cone. During a puff, fresh air (and therefore oxygen) is drawn through the lit-end of the cigarettes, and hence the temperature increases to more than 900°C (over 1,650°F) at the periphery of the burning zone.
Two indicators are required to detect unambiguously biomass and/or tobacco combustion:
1. Presence of relevant amounts of nitrogen oxides in the gaseous products, which are not formed from the decomposition of nitrates already present in the original biomass/tobacco substrate.
2. Clear evidence of an exothermic process.
Tobacco Temperature in a Lit Conventional Cigarette
Evidence Of Absence Of Combustion In THS
In contrast to lit-end cigarettes, which generate smoke, Philip Morris International’s novel Tobacco Heating System (THS) is designed to reduce the formation of HPHCs by electronically heating the tobacco, which generates an aerosol.
The tobacco does not ignite or burn. The electronically-controlled heating prevents combustion from occurring. The temperature of the heating blade is carefully controlled and the energy supply to the blade is cut if its operating temperature exceeds 350°C (662°F). The operating temperature of the THS product is substantially lower than that required to cause ignition and combustion of tobacco, and the temperature measured in the tobacco does not exceed 300 °C (572°F).1
This absence of combustion, because of controlled heating, is designed to reduce significantly the formation of HPHCs by the THS product compared with conventional cigarettes. Indeed, the levels of 18 HPHCs required to be reported to FDA were measured in the aerosol generated by THS and found to be over 90% lower, on average, than those in cigarette smoke from a 3R4F reference cigarette.2
The results for 37 HPHCs representing a broad range of chemical compounds from different variants of THS (version 2.2) tobacco sticks (two versions of the THS2.2 Regular: THS2.2 FR1 and THS2.2 D2, and two versions of the THS2.2 Menthol: THS2.2 FR1 M and THS2.2 D1 M) compared to the mainstream smoke from the 3R4F reference cigarette (constituent levels set at 100%) on a per-unit basis under the Health Canada Intense (HCI) machine-smoking regimen are shown in the table below. The table excludes HPHCs that were below the level of quantification in the THS aerosol.3
PMI has generated several lines of evidence demonstrating that no combustion takes place in THS:
1. Temperature: The highest observed temperature of the tobacco in the Tobacco Stick is approximately 300°C (572°F) and cannot exceed 350°C (662°F) (the programmed maximum temperature of the heater). This is well below the temperature required for tobacco combustion (known to be in excess of 400°C (752°F); in fact, the temperature of most of the tobacco in Tobacco Stick is significantly below 250°C (482°F). Indeed the temperature of the tobacco reaches <230°C (446°F) at 0.5 mm from the heater blade, <190°C (374°F) at 1.7 mm from the heater blade and < 120°C (248°F) at 3.4 mm from the heater blade.
Temperature profiles were measured at the interface between the heater and the tobacco substrate (blue line) and in the tobacco plug (at 0.2 mm from the heater) during product use (brown line). The reported temperatures are the average of five replicates. The inset shows further temperature profiles that were measured at 0.5 mm, 1.7 mm, and 3.4 mm from the heater.
2. Exothermic process: Contrary to the increase in temperature that occurs when a puff is taken with a lit-end cigarette, there is a significant drop in the temperature of the tobacco in the Tobacco Stick when a puff is taken (figure above). Furthermore, when the energy source is switched off, the temperature of the tobacco begins to decrease (figure below). Because combustion is a self-sustaining process, the decrease in temperature indicates an absence of combustion. These facts demonstrate the absence of an exothermic process.
Temperature profile measured 0.5 mm from the heater. The heater was turned off after 300 seconds of operation.
3. Ash: Since combustion does not occur, the structural integrity of the Tobacco Stick is retained after use. The tobacco is not consumed, as it is in a cigarette, and no ash is formed.
4. Oxygen: Oxygen is the necessary oxidant of tobacco combustion. PMI tested THS in a chamber with air and in a chamber filled only with nitrogen, where one of the essential elements of combustion (oxygen) was absent. The aerosol generated by THS in an atmosphere of pure nitrogen (where combustion cannot occur) contained equivalent levels of HPHCs as the aerosol generated in air (21% oxygen). The aerosol was equivalent under both atmospheres supporting the view that combustion does not occur during THS use.
5. Solid particles: Combustion of tobacco in cigarettes generates solid ultra-fine particles with a median diameter below 100 nm,4 which have been shown to be cytotoxic.5 PMI analyzed both THS aerosol and cigarette smoke for the presence of solid particles by stripping them of their volatile constituents. This was achieved by passing the aerosol and smoke through a commercial Dekati thermo-denuder operating at 300°C (572°F). The analysis of the materials collected during this process by scanning electron microscopy revealed that the THS aerosol does not contain solid particles, and confirmed that cigarette smoke does (each 3R4F cigarette contains approx. 1012 ultra-fine particles).6
6. Carbon monoxide and nitrogen oxides: The THS aerosol contains substantially lower levels of HPHCs compared with cigarette smoke. Importantly, nitrogen oxides (NOx) and carbon monoxide (CO), two important combustion markers, were reduced by over >96%.7 Furthermore, their levels were shown to be equivalent when THS was operated in a nitrogen atmosphere, demonstrating that their origin is not linked to combustion.
7. Reduced exposure to CO: The reduction in CO formation by THS led to the reduction in exposure measured in clinical studies. The levels of blood COHb (carboxyhemoglobin, a biomarker of exposure to CO) in smokers who switched from cigarette smoking to THS use was indistinguishable from the level of blood COHb in smokers who abstained from smoking for the duration of these studies. This demonstrates that the drastically reduced levels of CO measured in laboratory-based aerosol chemistry analyses were confirmed in clinical studies.
Use of a confinement study design ensures that the compliance to arm allocation is fully controlled, and therefore provides useful information on the maximal possible reduction of exposure under such controlled conditions.
Abbr.: CC = Combustible Cigarettes; SA = Smoking abstinence; THS = Tobacco Heating System
1. Average yield reductions of an investigational variant of THS compared to the 3R4F reference cigarette, calculated as an average of the reductions of individual HPHCs, which could be reliably quantified in the study. Aerosol was collected using Health Canada’s Intense Smoking Regime. All yields were taken on a mass-per-stick basis. Reduction calculations exclude nicotine.
2. Cozzani V, McGrath T, Smith M, Schaller J-P, Zuber G. Absence of combustion in an electrically heated tobacco system – an experimental investigation. 21st International Symposium on Analytical and Applied Pyrolysis. Nancy, France. 9th-12th of May 2016.
3. Schaller J-P, Keller D, Poget L, Pratte P, Kaelin E, McHugh D, Cudazzo G, Smart D, Tricker AR, Gautier L, Yerly M, Pires RR, Le Bouhellec S, Ghosh D, Hofer I, Garcia,E, Vanscheeuwijck P and Maeder S. Evaluation of the Tobacco Heating System 2.2. Part 2: Chemical composition, genotoxicity, cytotoxicity, and physical properties of the aerosol. Regul Toxicol Pharmacol. 2016. pii: S0273-2300(16)30290-2. [Epub ahead of print].
4. Pratte P, Cosandey S, Goujon Ginglinger C. Investigation of solid particles in the mainstream aerosol of the Tobacco Heating System THS2.2 and mainstream smoke of a 3R4F reference cigarette. Hum Exp Toxicol. 2016 Jan 1:960327116681653. doi: 10.1177/0960327116681653. [Epub ahead of print]5. Fariss MW, Gilmour MI, Reilly CA, Liedtke W, Ghio AJ. Emerging mechanistic targets in lung injury induced by combustion-generated particles. Toxicol Sci. 2013; 132:253-67.
6. Pratte P, Cosandey S, Goujon Ginglinger C. Investigation of solid particles in the mainstream aerosol of the Tobacco Heating System THS2.2 and mainstream smoke of a 3R4F reference cigarette. Hum Exp Toxicol. 2016 Jan 1:960327116681653. doi: 10.1177/0960327116681653. [Epub ahead of print]
7. Schaller J-P, Keller D, Poget L, Pratte P, Kaelin E, McHugh D, Cudazzo G, Smart D, Tricker AR, Gautier L, Yerly M, Pires RR, Le Bouhellec S, Ghosh D, Hofer I, Garcia,E, Vanscheeuwijck P and Maeder S. Evaluation of the Tobacco Heating System 2.2. Part 2: Chemical composition, genotoxicity, cytotoxicity, and physical properties of the aerosol. Regul Toxicol Pharmacol. 2016. pii: S0273-2300(16)30290-2. [Epub ahead of print].
8. Haziza, C., G. de La Bourdonnaye, D. Skiada, J. Ancerewicz, G. Baker, P. Picavet and F. Lüdicke (2016). Evaluation of the Tobacco Heating System 2.2. Part 8: 5-Day randomized reduced exposure clinical study in Poland. Regul Toxicol Pharm, in press.
9. Haziza, C., G. de La Bourdonnaye, S. Merlet, M. Benzimra, J. Ancerewicz, A. Donelli, G. Baker, P. Picavet and F. Lüdicke (2016). Assessment of the reduction in levels of exposure to harmful and potentially harmful constituents in Japanese subjects using a novel tobacco heating system compared with conventional cigarettes and smoking abstinence: A randomized controlled study in confinement. Regul Toxicol Pharmacol 81: 489-499.
PMIScienceUSA.com is operated by Philip Morris International for the purpose of publishing and disseminating scientific information about Philip Morris International’s efforts to develop and assess products that have the potential to reduce individual risk and population harm associated with tobacco use.
This site is for use by scientists, the public health and regulatory communities, and other stakeholders with an interest in tobacco policy. The purpose of this site is not for advertising or marketing. It is not intended for use by consumers.