Polonium-210 and lead-210 in heated tobacco smoke| Tabaccologia

Abstract

It is known that conventional tobacco cigarettes, among other carcinogens, contain two radionuclides, Polonium-210, a potent alpha-emitting carcinogen, and Lead-210, its precursor, weakly alpha, beta and gamma radioactive. Recently, a study by the University of Lausanne, in addition to confirming the presence of Polonium-210 and Lead-210 in conventional tobacco cigarettes, also unveiled their presence in heated tobacco product sticks (IQOS HEETS). The levels of these radionuclides are lower than those released from conventional cigarettes because only 15% of the tobacco in an IQOS stick is actually heated to the target temperature of 330°C, nevertheless the impact on health is significant. Results show that conventional smoking of one pack per day induces a dose to the lung of about 0.3 mSv/year. This dose decreases by a factor of ten (0.03 mSv/year) for the IQOS system. The presence of alpha radioactivity in conventional and IQOS heated tobacco cigarettes may be an important factor in increasing the motivational baggage to quit.

Introduction

Polonium-210 (Po-210) and Lead-210 (Pb-210) are naturally occurring radionuclides belonging to the Uranium-238 (U-238) decay series, present in tobacco plants [1-5]. These radionuclides can be absorbed by the tobacco plant through three main pathways, variously combined [1,3,6].

1. The main source of Po-210 and Pb-210 in tobacco comes from the deposition on leaves of Radon-222 (Rn-222) progeny, specifically Pb-210 and Po-210. These radionuclides are captured by leaf trichomes, glandular structures capable of retaining heavy metals [3,7-10].
2. A smaller fraction comes from uptake through the roots thanks to a particular rhizosphere [8,11].
3. A further contamination route is represented by fertilisers based on calcium polyphosphates from soils rich in Pechblenda and Apatite, containing Uranium and its decay products [12,13]. These fertilisers help to enrich the natural amount of radionuclides absorbed by the roots.

Both radionuclides are present in the smoke of conventional cigarettes [12].

Carcinogenicity of Polonium-210s

Po-210 is an energetic emitter of α particles and has therefore been recognised as a potential carcinogenic component of tobacco smoke [10,11,14].

As early as 1964, Radford and Hunt hypothesised that the presence of Po-210 in tobacco smoke and its preferential localisation in the bronchial epithelium could be a cause of lung cancer [5]. Studies have confirmed the link between exposure to low doses of Po-210 and the occurrence of lung cancer [14,15]. Despite this, the tobacco industry has never taken measures to remove or reduce the presence of Po-210 and Pb-210 in tobacco products [2,7,13]. Furthermore, studies on the role of Rn-222 and its progeny have shown a synergistic effect with tobacco smoke in lung cancer risk [4,5,16]. Po-210 present in tobacco smoke may be responsible for some lung cancer cases otherwise attributed to radon [2,4,5,16-20].

A mere 0.1 mg of Po-210 emits the same number of α particles as 5 grams of radium, making it a highly harmful agent for humans [21]. A Canadian study conducted on cattle by Thomas and co-workers showed that exposure to α-radiation from Po-210 is 7 to 14 times more harmful than exposure to X-radiation in causing cellular damage. Both radionuclides are present in traditional cigarette smoke [22].

α radiation can trigger oncogenic mutations through methylation mechanisms that inactivate the oncosuppressor gene p16(INK4a) [23].

Radioactivity of heated tobaccos

Heated tobacco products (HTP) represent a new consumption mode in which tobacco is heated at lower temperatures than a conventional cigarette. The most popular of these is IQOS, developed by Philip Morris International (PMI), which heats tobacco to 330°C. At this temperature, the volatilisation of Po-210 and Pb-210 is still a matter of debate. A group of researchers from the University of Lausanne conducted a study to determine the activity of Po-210 and Pb-210 in heated tobacco smoke compared to conventional tobacco smoke [24]. The researchers also analysed the development of these radionuclides at different temperatures between 50 and 600°C.

The results show that conventional smoking of one packet per day induces a dose of α radiation to the lung of about 0.3 mSv/year, whereas with IQOS this dose is reduced to about 0.03 mSv/year. However, this reduction does not result from specific industry countermeasures, but rather from the fact that only 15% of the tobacco in an IQOS cigarette is actually heated to the target temperature of 330°C. If tobacco were heated uniformly at 300°C, 80% of the Po-210 would be released, leading to similar doses of α-radiation to the bronchopulmonary system as in conventional cigarettes.

Implications

The public is generally aware of the presence of toxic chemicals in tobacco products, but few know that they also contain radioactive particles that are deposited in the lungs when inhaled. Since radioactivity is one of the most feared threats to human health [25,26], this awareness could be an additional incentive to stop smoking and using heated tobacco.

The World Health Organization Framework Convention on Tobacco Control (WHO-FCTC) recommends informing the population, and in particular smokers, about the risks of smoking through awareness-raising campaigns, a key strategy to reduce tobacco demand [27]. Although there is no conclusive evidence on the efficacy of these interventions in promoting smoking cessation, providing smokers with feedback on their current or future biomedical effects (e.g. measurement of exhaled carbon monoxide, lung function, arterial vessel patency, genetic susceptibility to lung cancer) could increase their motivation to quit [28].

An even stronger approach could be to provide smokers with information on their personal level of urinary Po-210 radioactivity, accompanied by a motivational counselling intervention. This strategy could be particularly effective, considering the recent increase in HTP use during the COVID-19 pandemic, as observed in Italy [29,30].

Conclusions

The Swiss study confirmed the presence of Po-210 and Pb-210 in tobacco smoke, both conventional and heated. Although IQOS releases lower levels of these radionuclides than conventional cigarettes, their impact on health remains significant.

Properly informing smokers of both conventional tobacco and HTP could be a valuable tool to increase motivation to quit smoking or avoid starting, with a positive impact on overall public health.

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