Naturally occurring primordial radionuclides of the uranium-radium series, such as ²¹⁰Pb and ²¹⁰Po have long been associated with tobacco plants [1]. Tso et al. (1966) [2] stated that the principal mechanism of incorporation involves root uptake from soil and phosphate fertilizers. Francis et al. (1968) [3] suggested that the deposition of ²¹⁰Pb by rainfall is the principal mechanism of ²¹⁰Pb entry in plants. The properties and distribution of trichomes (hairs) on tobacco leaf surfaces suggest that they are effective collectors of small Aitken (nuclei) particles (<0.1 μm diameter) by means of diffusive deposition due to Brownian motion of the particles [1, 4, 5]. As about 85 % of tobacco trichomes have glandular heads coated with a sticky exudates mixture of organic compounds, trichomes may retain the small atmospheric particles which are deposited on the glandular heads throughout the period of plant growth.

So, ²²⁶Ra (²³⁸U-series), ²²⁸Ra (²³²Th-series) and ⁴⁰K of the naturally occurring primordial radionuclides which are abundant in soil and in most fertilizers (uranium-238 is associated with phosphate fertilizers [6]) follow root uptake, whereas airborne ²¹⁰Pb, a long-lived (T_1/2=22.3 y) decay product of radon-222, resides on particles <0.3, μm diameter. Papastefanou and Bondietti [7] reported activity median aerodynamic diameters, AMADs for ²¹⁰Pb associated with aerosol particles ranging from 0.28 to 0.49 μm, averaging 0.37 μm, and is captured on tobacco trichomes.

Cesium-137 and -134 of Chernobyl reactor accident origin, followed both routes, that is, during May 1986 when tobacco plants were planted, both radionuclides were in high concentration in air (1.95 and 0.98 Bq m⁻³, respectively [8]) and these radionuclides were deposited onto the plant leaves and captured on tobacco trichomes. In the years which followed, cesium and all fallout radionuclides after dry and even wet deposition onto the ground were mixed and migrated in the soil to deeper layers below the ground surface, suggesting root uptake.

The issue of radioactivity in tobacco smoke has received much attention in the scientific press and, increasingly, in the medical press [9]. Besides this, Martell and Sweder [10] reported that radon decay products naturally found in the environment are altered when they pass through burning cigarettes into mainstream smoke. Martell [11] stated that the cumulative alpha-, beta- and gamma-radiation dose, particularly that from alpha radiation, from inhaled radionuclides deposited in small volumes of bronchial tissue may be an important factor in the initiation of bronchial (lung) cancer in smokers. The smokers are subjected to alphas radiation in the bronchial epithelium from three sources: (1) from indoor radon and thoron decay products inhaled between cigarettes, (2) from ²¹⁴Po, ²¹²Po and ²¹²Bi in large mainstream smoke particles and (3) from ²¹⁰Po which grows from decay of ²¹⁰Pb-enriched particles that persist at bronchial bifurcations [12].

Radford Jr and Hunt [13] at Harvard School of Public Health (Boston, MA,) reported that for an individual smoking two packages of cigarettes a day, the radiation dose to bronchial epithelium from ²¹⁰Po inhaled in cigarette smoke probably is at least seven times that from background sources, in localized areas may be up to 10 Sv (1000 rem) or more in 25 y. Besides, Winters and Di Franza [14] at the University of Massachusetts (Boston, MA) much later reported that in a person smoking one and a half packs of cigarettes (i.e. 30 cigarettes) per day, the radiation dose to the bronchial epithelium in areas of bifurcation is 80 mSv y⁻¹ (8,000 mrem) – the equivalent of the dose to the skin from 300 X-ray films of the chest per year. This figure was comparable with total-body exposure to natural background radiation containing 0.8 mSv y⁻¹ (80 mrem) in someone living in the Boston area.

Takizawa et al. [15] reported that the range of ²¹⁰Po contained in the tobacco grands in Japan varied from 13.0 to 20.1 Bq kg⁻¹ (mean 15.4 Bq kg⁻¹), 50 % of ²¹⁰Po present in tobacco was transferred into the smoke and the other 50 % remained in the ash and butt. One pack-a-day smoker inhaled 24 mBq d⁻¹ of ²¹⁰Po through smoking and the annual inhalation was 8.8 Bq. Peres and Hiromoto [16] reported that ²¹⁰Po in (dry) tobacco in Brazil ranged from 10.9 to 27.4 Bq kg⁻¹ and ²¹⁰Pb from 11.9 to 30.2 Bq kg⁻¹. The collective committed effective dose resulting from the use of cigarettes produced in Brazil per year was estimated to be 1.5x10⁴ man Sv, considering an annual production of 5x10⁸ kg of cigarettes in Brazil and the committed effective dose of 0.16 mSv y⁻¹ of cigarette smoking.

Colangelo et al. [17] reported ²¹⁰Po in tobacco in Argentina ranged from 10 to 80 Bq kg⁻¹ and the lung dose due to the use of tobacco varied from 75 to 600 μSv y⁻¹. Khater [18] reported that the range of ²¹⁰Po in cigarette tobacco in Egypt ranged from 9.7 to 22.5 mBq per cigarette (average 16.6 mBq per cigarette). The average percentages of ²¹⁰Po content in fresh tobacco plus wrapping paper that were recorded by post-smoking filters, ash and smoke were 4.6, 20.7 and 74.7, respectively. Cigarette smokers, who are smoking one pack (20 cigarettes) per day, are inhaling on average 123 mBq d⁻¹ of ²¹⁰Po and ²¹⁰Pb each. The mean values of the annual effective dose for smokers (one pack per day) were estimated to be 193 and 251 μSv y⁻¹ from ²¹⁰Po and ²¹⁰Pb, respectively.

Approximately 10 L d⁻¹ is inhaled through 40 cigarettes-this is about 1/2000 of the amount of air usually breathed per day (20 m³ d⁻¹ [10, 19]. Published radiochemical data for inhaled mainstream smoke showed on average ²¹⁰Po content of 1.33 mBq (0.036 pCi) per cigarette or 96.2 Bq kg⁻¹ smoke (2.6 pCi ²¹⁰Po per g smoke) [20], with a ²¹⁰Pb: ²¹⁰Po ratio of 0.66 ± 0.23 [21].

This paper reports data on the radioactivity of tobacco leaves after the collection from tobacco fields before cigarette production in order to find any association in the uptake of the naturally occurring radionuclides and of cesium radioisotopes of Chernobyl origin, and to estimate the effective dose from cigarette tobacco as the cumulative α-, β- and γ-radiation dose, particularly that from α-radiation, from inhaled radionuclides deposited in small volumes of bronchial tissue may be an important factor in the initiation of bronchial (lung) cancer in smokers [11].

Seventeen different samples of tobacco leaves produced in the year 1990 at different locations in Greece (Figure 1, Table 1) were examined a year later for radioactivity using γ-ray spectrometry. The spectrometric system consisted of two different high-purity Germanium low-background detectors. One planar Ge detector of active area 2000 mm², thickness 20 mm, Be window 0.6 mm and energy resolution (FWHM) 400 eV for 5.9 keV γ-rays (⁵⁵Fe) and 700 eV for 122 keV γ- rays (⁵⁷Co), was appropriate for γ-rays ranging from 5 to 186 keV for determination of ²¹⁰Pb (47 keV), ²²⁶Ra (186 keV) and ²³⁸U via its decay product ²³⁴Th with 63 and 93 keV γ-rays. A second coaxial Ge detector (p-type), crystal size 155 cc, resolution (FWHM) 1.9 keV at 1.33 MeV (⁶⁰Co) and 900 eV at 122 keV (⁵⁷Co), peak-to-Compton ratio 55:1 and efficiency 42 %, was appropriate for γ-rays ranging from 240 to 2614 keV for determination of ²²⁶Ra via its decay products ²¹⁴Pb and ²¹⁴Bi, ²³²Th via its decay products ²²⁸Th, ²²⁸Ac and ²²⁸Ra, ⁴⁰K (1460 keV), ¹³⁷Cs (662 keV) and ¹³⁴Cs (605 keV).

The samples were dried before radioactivity measurement for 3–4 days at a temperature of 30°C to avoid any moisture adsorption. After that, the samples were cut into very small pieces using a blender and were mixed with active charcoal and then sealed appropriately for about one month to reduce leaching and to attain radioactive equilibrium between radon and thoron decay products [22], as eight half-lives of ²²²Rn (T_1/2=3.82 d), the decay product of ²²⁶Ra and precursor of ²¹⁴Pb and ²¹⁴Bi, correspond to 1 month. About eight half-lives of ²²⁴Ra (T_1/2=3.66 d), the decay product of ²²⁸Ra, are also 1 month.

The samples were measured for radioactivity in two geometries, that is, in a standard geometry 40 g plastic can of 6 cm diameter and a Marinelli beaker of 1 L (volume). The overall efficiency of the counting system was known to an accuracy of better than 5 % for the plastic can geometry and about 12 % for the Marinelli beaker. The +− errors in Table 1 are the experimental errors of each measurement. The collecting time was 120,000 s.

Gamma-ray spectra obtained with both HPGe detectors are presented in Figure 2a for the planar detector and Figure 2b for the coaxial detector.

The radioactivity in tobacco leaves collected from 15 different regions of Greece before cigarette production has been studied in order to find any association between the uptake of the naturally occurring primordial radionuclides and the radioisotopes of cesium of Chernobyl origin. The activities of the radioisotopes of radium, ²²⁶Ra and ²²⁸Ra in the tobacco leaves would be either through root uptake or through the fertilizers used for cultivation of tobacco plants in the fields, as the air concentrations of the radioisotopes of radium are extremely low. Lead-210 originating from air was deposited onto the tobacco leaves and trapped by the trichomes. Potassium-40 in the tobacco leaves was due to root uptake either from soils or from fertilizers. The radioisotopes of cesium, ¹³⁷Cs and ¹³⁴Cs, in the tobacco leaves were due to root uptake and not to deposition onto the leaf foliage as they still remained in soil four years after the Chernobyl reactor accident but were absent from the atmosphere because of the rain washout (precipitation) and gravitational settling.

In estimating the radiation dose induced from smoking, it was concluded that the annual effective dose to lungs due to inhalation for adults (smokers) averaged to 80 μSv for ²²⁶Ra, 67 μSv for ²²⁸Ra and 105 μSv for ²¹⁰Pb, that is 252 μSv in total. The annual effective dose due to ¹³⁷Cs of Chernobyl origin averaged to about 200 nSv, that is three orders of magnitude lower than that of the naturally occurring radionuclides. The effective dose of 252 μSv per year must be compared with the average worldwide exposure to natural radiation sources due to inhalation 1.26 mSv y⁻¹.