Introduction

Natural radioactive nuclides (primarily 238U series nuclides, 232Th series nuclides, and 40K nuclides), which exist in soil and air, are absorbed by various agricultural products through roots and leaves. Because tobacco leaves are familiar products and consumers have a high affinity for them [1, 2], newspaper and magazine articles have highlighted the issue regarding an effective dose of natural radioactive nuclides contained in them. Although the effective dose of radionuclides to smokers was reviewed by Iwaoka and Yonehara [3] in 2010, some original studies related to the effective dose to smokers have recently been reported. This article reports the effective dose of radionuclides to smokers based on the results from a review of various recent parameters related to dose estimation for smokers with the inclusion of the data from the past. From among the different types of tobacco products such as cigarettes, cigars, and snuff, this article deals with cigarettes.

Parameters for dose estimation

Natural radioactive nuclides such as 238U, 232Th, 210Po, and 210Pb are contained in cigarettes [4–6]. Among these radionuclides, 210Po and 210Pb are volatilized at the temperature of a burning cigarette and are taken into the human body [7, 8]. Therefore, activity concentration of 210Po and 210Pb in cigarettes, transfer factors of 210Po and 210Pb from cigarettes to mainstream smoke, and dose conversion factors of 210Po and 210Pb are important parameters for dose estimation.

Activity concentration of cigarettes

There are two basic pathways of absorption of 210Po and 210Pb by plants: direct deposition of 210Po and 210Pb in the atmosphere and absorption of 210Po and 210Pb through the roots in the ground. Regarding tobacco leaves, the direct deposition is main pathway [8–11]. 210Po and 210Pb in tobacco leaves reach radioactive equilibrium, while tobacco leaves are processed for the production of cigarettes [10, 12–15]. The activity concentration of 210Po and 210Pb in cigarettes is about 8.0–24 mBq per cigarette with arithmetic mean of 14 mBq per cigarette (Table 1). Cigarettes were categorized into those just sold at origin and those produced at origin (Table 1). Even if cigarettes are produced at origin, cultivation of tobacco leaves and producing may not occur in the same origin, such as in Japan where tobacco leaves are imported [29].

Table 1 The activity concentration of 210Po and 210Pb in cigarettes (mBq/cigarette)
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Transfer factor of 210Po and 210Pb

210Po and 210Pb in cigarettes are volatilized at the temperature (600–800 °C) of a burning cigarette and transfer from cigarettes to smoke [7, 8]. The transfer factor of 210Po from cigarettes to mainstream smoke has been reported in various studies [15, 16, 18, 21, 25, 28, 30–34]. The transfer factor of 210Po from cigarettes to mainstream smoke is about 0.09–0.49 with arithmetic mean of 0.18 (Table 2). The transfer factor of 210Po from cigarettes to ashes, cigarette butts, and sidestream smoke is 0.09–0.21, 0.29–0.37, and 0.25–0.61, with arithmetic means of 0.16, 0.32, and 0.37, respectively (Table 2). The number of reports on transfer factors of 210Pb from cigarettes to mainstream smoke is limited, among which, reports from Ferri and Christiansen [35], Sakanoue et al. [34] and Schayer et al. [15] are particularly useful. The transfer factor of 210Pb from cigarettes to mainstream smoke is 0.08–0.18 with arithmetic mean of 0.12 (Table 3). The transfer factors of 210Pb from cigarettes to ashes, cigarette butts, and sidestream smoke have not yet clearly perceived.

Table 2 The transfer factor of 210Po from cigarettes to mainstream smoke, ashes, cigarette butts, sidestream smoke, and total smoke
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Table 3 The transfer factor of 210Pb from cigarettes to mainstream smoke
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Dose conversion factors for 210Po and 210Pb

In some studies, effective dose conversion factors provided by International Commission on Radiological Protection (ICRP Publ. 72) [36] have used for dose estimation. These dose conversion factors are calculated by using standard inhalation conditions, which are an inhalation speed through the nose of 0.45–3.0 m3 h−1 and an aerosol particle size of 1 μm (activity median aerodynamic diameter; AMAD). Since smoking conditions are not the same as standard inhalation conditions, dose conversion factors calculated by using smoking conditions are preferable to dose conversion factors provided by ICRP Publ. 72. The inhalation speed through the mouth for smoking will be 31.5 m3 h−1, assuming 4 s for a puff (inhale for 2 s, then exhale for 2 s) and using internationally recognized standards [37] (35 ml/puff). Regarding definite particle sizes of mainstream smoke, a study by Chen et al. [38] reported a size of 0.45 μm (mass median aerodynamic diameter; MMAD) and a study by Hinds [39] reported a size range of 0.37–0.52 μm (MMAD). If it is assumed that 210Po and 210Pb are equally dispersed in aerosol particles on mainstream smoke, the MMAD and AMAD are equal and about 0.4 μm. Based on these smoking conditions, effective dose conversion factors for 210Po and 210Pb calculated by the LUDEP [40] were 1.4 × 10−5 and 1.4 × 10−6 Sv Bq−1. These dose conversion factors are several times larger than those in ICRP Publ. 72 (Table 4). Specific parameters for LUDEP are shown in Table 5.

Table 4 Effective dose conversion factor
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Table 5 Specific parameters for LUDEP
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Dose estimation of smokers

Arithmetic means of activity concentration of 210Po and 210Pb in Table 1, arithmetic means of 210Po and 210Pb transfer factors from cigarettes to mainstream smoke in Tables 2 and 3, and effective dose conversion factors based on smoking conditions in Table 4 are used for dose estimation. A consumption of 20 cigarettes per day as the standard was used for dose estimation [41]. The annual effective dose to a smoker calculated by using those values and Eqs. (1)–(3) was 0.27 mSv year−1, which was lower than an intervention exemption level (1 mSv year−1) given in ICRP Publ. 82 [42]:

$$ E_{\text{po}} = \, A_{\text{popb}} \times F_{\text{po}} \times D_{\text{po}} \times T \times C $$
(1)
$$ E_{\text{pb}} = A_{\text{popb}} \times F_{\text{pb}} \times D_{\text{pb}} \times T \times C $$
(2)
$$ E_{\text{popb}} = E_{\text{po}} + E_{\text{pb}} $$
(3)

where E popb is the annual effective dose (mSv year−1), E po is the annual effective dose of 210Po (mSv year−1), E pb is the annual effective dose of 210Pb (mSv year−1), A popb is the activity concentration (mBq/cigarette) of 210Po (210Pb) in a cigarette, F po is the transfer factor of 210Po from cigarettes to mainstream smoke, F pb is the transfer factor of 210Pb from cigarettes to mainstream smoke, D po is the effective dose conversion factor (Sv Bq−1) of 210Po, D pb is the effective dose conversion factor (Sv Bq−1) of 210Pb, T is 365 (day year−1); and C is the daily consumption of cigarettes. Furthermore, a reverse calculation of the number of cigarettes consumed per day so that the effective dose became 1 mSv year−1, yielded about 70, an extremely rare number.

Conclusions

The annual effective dose to smokers was much lower than the intervention exemption level of 1 mSv year−1. However, there is a need to evaluate aggregate effects to health, because smoking involves introduction of harmful elements (cancerous particles or cancer-inducing particles such as dimethylnitrosamine and formaldehyde) and radioactive material. Furthermore, there is a need to assess effective doses in sidestream smoke; thus, continued study of missing parameters is needed.