Robustness of HPHC Reduction in THS 2.2 Aerosol Relative to 3R4F Reference Cigarette Smoke under Extreme Climatic Conditions

The Tobacco Heating System 2.2 (THS 2.2), developed by Philip Morris Products S.A. and commercialized under the brand name IQOS®, uses a precisely controlled heating device into which specially designed tobacco sticks (Heat-Sticks) are inserted and heated to generate an aerosol. When a tobacco substrate is heated instead of being burned, the levels of harmful and potentially harmful constituents (HPHCs) released into the aerosol are significantly reduced, and average reductions in the range of 90–95% on a per-stick basis relative to cigarette smoke have been observed (1). By exposing users to lower levels of HPHCs, THS 2.2 has the potential to offer a less harmful alternative for adult smokers who would otherwise continue to smoke, as has been widely published (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12). Since THS 2.2 is a product designed to be made available to all markets in the world where cigarettes are currently sold, it is therefore important to understand how the product behaves under different climatic conditions, in terms of its ability to maintain reductions in the emission of recognized HPHCs. It is known that the moisture content of a cigarette tends to equilibrate with ambient conditions once the packet has been opened, and the effect of moisture on the composition of cigarette smoke has been comprehensively presented in a study published by ZHA and MOLDOVEANU (13). Therefore, it is reasonable to assume that ambient temperature and humidity conditions could also affect the composition of heated tobacco aerosol when such products are used under different climatic conditions.

The aim of this study was to characterize the performance of THS 2.2 in its ability to reduce the levels of known HPHCs in aerosol relative to cigarette smoke, represented by the 3R4F reference cigarette over a range of different climatic conditions. Considering that the THS 2.2 heating element is positioned in the center of the tobacco substrate plug during use, and that the temperature is controlled electronically, the average tobacco temperature is lower in a reduced temperature environment when compared with standard conditions, resulting in lower HPHC yields in THS 2.2 aerosol. Accordingly, our focus was upon climatic conditions that were higher in temperature than standard ISO conditions (14), with either increased or decreased humidity, to best represent worst-case scenarios for product usage. Relative reductions using the average of all investigated HPHCs, and also the levels of reduction for each HPHC individually with respect to predefined “robustness” limits, were determined.

Analytical smoking machines capable of controlling climatic conditions were used to generate aerosol and smoke under different conditions of temperature and humidity. Tobacco sticks and cigarettes were conditioned under the same conditions prior to aerosol generation in order to evaluate how short-term exposure to extreme climatic conditions could influence the performance of THS 2.2 in terms of reducing the levels of HPHCs compared with cigarette smoke. While the duration of common stability studies may extend over several months - for example, when determining a product’s shelf life - in this instance, the goal was to assess the performance of tobacco sticks taken from a freshly opened pack, which are then exposed to extreme climatic conditions for a few days before being used to generate aerosol under the same conditions. To compare the performance of THS 2.2 sticks under extreme scenarios, two climatic conditions representing zones identified in the World Health Organization (WHO) guidelines (15) as “Hot and Dry” climate (30 °C and 35% RH) and “Hot and Very Humid” climate (30 °C and 75% RH) were introduced. In place of the WHO-defined conditions for “Subtropical and Mediterranean” climates (25 °C and 60% RH), temperature and humidity conditions set forth in ISO 3402, which has a lower temperature stipulation (22 °C), were used. Atmospheric pressure was excluded from climatic parameters, hence the range 86–106 kPa as required by ISO 3402 was assumed for all climatic conditions investigated.

Analysis of an established list of principal aerosol/smoke constituents and 54 relevant HPHCs was used for comparative assessment of levels between THS 2.2 aerosol and mainstream smoke of the 3R4F reference cigarette (9). As applied in a previous investigation (16), our approach was again to use puff volume adjustment to express the yields for each individual constituent. Although the total puff volume is fixed for THS 2.2, we anticipated that the combustion rate of the tobacco rod of the 3R4F reference cigarette could change with climatic conditions, and therefore influence the total number of available puffs. In such an eventuality, this volume-adjusted approach offers a means to compare the two products on the basis of equivalent total puff volume. Moreover, we intentionally retained the same approach for determining the average HPHC reduction levels across climatic conditions, which not only considers yields expressed on a total puff volume basis, but also yields adjusted in relation to nicotine delivery (nicotine-adjusted yield, expressed as HPHC yield per milligram of nicotine), which is commonly used to express the level of toxicants to which cigarette smokers would be exposed (17–18). Furthermore, we used the concept of including a threshold of robust reduction, which was also introduced in our previous article (16).

The dataset of aerosol/smoke constituent yields is available in Supplementary Information Table S2. The descriptive statistics for the concentrations of the investigated constituents generated in THS 2.2 aerosol and 3R4F cigarette mainstream smoke are compiled in Supplementary information Table S3 and Supplementary information Table S4, respectively. The means and standard deviations of the yields of the constituents investigated in mainstream aerosol/smoke were calculated among the replicates analyzed for each product and condition and are reported in Appendix A. The means and standard deviations calculated from nicotine-adjusted and volume-adjusted yields are reported in Appendixes B and C, respectively. These tables summarize the results for 57 analytes, including nicotine, nicotine-free dry particulate matter (NFDPM), and glycerol, and 54 HPHCs for all tested conditions.

From respective means calculated over all test replicates and reported in Table 2, the changes of puff numbers in 3R4F calculated relatively to ISO conditions decreased by 13% under “Hot and Dry” conditions, whereas the number increased by 16% under “Hot and Very Humid” conditions. The relative reductions in HPHC concentrations in THS 2.2 aerosol were calculated as percentages of the concentrations in 3R4F cigarette mainstream smoke. Out of the 54 HPHCs investigated in THS 2.2 aerosol, 14 presented various proportions of determined values falling below the LOQ (1-aminonaphthalene, 2-aminonaphthalene, 3-amino-biphenyl, 4-aminobiphenyl, arsenic, benzo[a]pyrene, dibenz[a,h]anthracene, cadmium, chromium, lead, nickel, quinoline, selenium, and vinyl chloride). These cases required particular attention because of computational limitations. Supplementary Information Table S5 presents the imputed compounds and the proportions of > LOQ cases per product. The preponderance of imputed values resulted in a computational optimization shortage, such that the 0.95 confidence intervals of reduction were not calculated. Instead, the results reported in Supplementary Information Table S6 for 11 of these compounds are the minimum and maximum reduction means observed over the combinations. Because more than 90% of the values were below the LOQ for both THS 2.2 and the 3R4F cigarette, the calculation of reduction was not applicable for chromium, nickel, and selenium. Excluding these three HPHCs, the average reductions in HPHC concentrations were calculated across all smoking-machine climatic conditions used to generate THS 2.2 aerosol and 3R4F smoke. For lead and quinoline, no reductions were calculated for conditions where more than two out of five 3R4F replicates had values below the LOQ. Consequently, the number of HPHCs used for calculating the average reductions varied among the conditions (n = 50 or 51). Figure 2 (reprinted with permission from (1)) summarizes the average HPHC reductions calculated for each combination of climatic conditions and is based on puff volume-adjusted yields (range 89.6–94.2%). Figure 3 (reprinted with permission from (1)) summarizes the average HPHC reductions calculated for each combination of climatic conditions and is based on nicotine-adjusted yields (range 87.4–90.3%).

Figure 2

Figure 3

For aerosol generated by THS 2.2 under the three investigated ambient conditions, the average reduction in puff volume-adjusted HPHC concentrations relative to 3R4F was in the range of 92.8–93.4% when compared only against the ISO ambient conditions used for the 3R4F cigarette (range 87.4–90.3%, by using nicotine-adjusted data).

The lowest reduction performance achieved for individual HPHCs across all climatic conditions was used to define the threshold for robust reduction. The 0.95 confidence intervals for reductions used to calculate the low boundaries of reduction associated with the lowest conditions are reported in Supplementary Information Table S7. In the cases where it was not possible to calculate confidence intervals, for each imputed compound, we used the lowest mean value of reduction observed within the compared conditions.

The threshold of reduction was derived from volume-adjusted yields. Table 5 presents a summary of the thresholds of robust reduction of THS 2.2 tested under various climatic conditions, but compared with 3R4F tested under ISO climatic conditions. The results are presented for 51 compounds, the calculation of reduction for three compounds being not applicable. Comparing with 3R4F under ISO climatic conditions showed that the reduction of HPHC levels in THS 2.2 aerosol was greater than 90% for 39 compounds. The thresholds of reduction for the other compounds ranged from 40% to 85%.

Using 3R4F values generated under ISO conditions as a reference, the seven compounds presented in Table 3 were characterized by changes of threshold values when THS 2.2 processed under ISO climatic conditions was compared with extreme climatic conditions. For HCN, m-cresol and toluene the threshold values changed from 99% to 95%. For the three compounds, the diminution of relative reduction means did not exceeded 0.3%. The changes of relative reduction means were more prominent for acetone (−0.5%), acetamide (−1.2%), mercury (−3.5%) and ammonia (−6%). In these five compounds, ammonia showed the highest diminution of threshold value with a difference of 10% whereas the three remainders equally decreased by 5%. “Hot and Dry” condition dominantly influenced ammonia, HCN and mercury whereas the effect of “Hot and Very Humid” conditions was more pronounced for m-cresol, toluene, acetone and acetamide.

In contrast to Table 5, Table 6 includes results that show the influence of 3R4F subjected to extreme climatic conditions. It presents a summary of the thresholds of robust reduction resulting from non-inferiority tests conducted over the five climatic combinations considered in the comparison of THS 2.2 and 3R4F. Here, 38 compounds demonstrated a robust reduction greater than 90%. In the low range, mercury, acrylamide, and phenol demonstrated a reduction not exceeding 40%. The remaining compounds showed threshold reduction values distributed between 50% and 85%.

Reduction threshold values decreased for fifteen HPHCs with the introduction of comparisons with differing 3R4F conditions. For this group of constituents, percent relative differences derived from volume-adjusted yields were calculated in “Hot and Very Humid” relative to ISO climatic conditions. Results in Table 4 show significant decreases of means in the range of 11–76% for 3R4F smoke. In the low range, values can be associated with the volume-adjustment corresponding to the relative difference of 16% in puff number, whereas higher values can be predominantly attributed to the change in 3R4F smoke composition. Focusing on THS 2.2, when the uncertainty of the mean equaled or exceeded the mean value, which was observed for nine aerosol constituents, then the statistical resolution did not lead to a reliable conclusion about variations within the calculated relative differences. To lower this uncertainty and obtain more accurate results, additional replicates would have been needed. Nevertheless, for ethylene oxide, methyl ethyl ketone, o-cersol, p-cresol and phenol, the statistical resolution was found acceptable to conclude that changes in reduction thresholds resulted from volume-adjusted yields decreasing in 3R4F smoke and increasing in THS 2.2 aerosol. Regardless statistical resolution, the high levels of relative differences found for the six other HPHCs suggested a dominant influence of 3R4F in decreases of reduction thresholds. The greatest differences between reduction threshold categories linked to the change in 3R4F reference conditions were observed for acetamide (−10%), formaldehyde (−20%), and nitrobenzene (−20%). In a group of compounds falling below the threshold of 40%, important changes were also observed for acrylamide (−40%) and mercury (−15%). Phenol was the most impacted with a threshold dropping from 85% down to below 40% with the introduction of varied 3R4F conditions. The overlap of the confidence intervals of the means (CI_95%) of the volume-adjusted yields between THS 2.2 (2.8 ± 0.3 ng/mL) and 3R4F (5.7 ± 3.2 ng/mL) explained the substantial drop in reduction threshold value for phenol.

The examples for individual HPHCs combined in Figure 4 present graphics of volume-adjusted yields placed contiguously to graphics of respective relative reductions. Data are extracted from Supplementary Information Tables S 3, S 4 and S 7. Means and 0.95 confidence intervals of volume-adjusted yields for THS 2.2 and 3R4F are also included to show the diversity between products and conditions and to identify cases that would dominantly influence changes in relative reductions. This representation can be used for the identification of factors, across climatic conditions, which had dominant influences on volume-adjusted yields and relative reductions. The figures provide visual evidence of the substantial influence of extreme climatic conditions on the levels of volume-adjusted yields of acetamide, acryl-amide, formaldehyde, nitrobenzene, mercury, and phenol in 3R4F smoke. Also, the relative reduction means and the threshold lines presented in the corresponding graphs clearly highlighted the influence of climatic conditions on relative reduction which can be linked to the respective volume-adjusted yields.

Figure 4

The present study has demonstrated the robustness of THS 2.2, a heat-not-burn tobacco product, in reducing the levels of HPHCs in the aerosol produced under extreme temperature and humidity conditions corresponding to actual climatic zones where the product could be used. The study was designed to evaluate the robustness of the product used in normal conditions as opposed, for instance, to assessment of its shelf life. Therefore, the test setup allowed us to simulate situations where THS 2.2 and 3R4F reference cigarettes were used in extreme temperature and humidity conditions after freshly opened packs were exposed for at least 48 h to the same experimental conditions. We used two climatic zones identified in the WHO guidelines as “Hot and Dry” climate (30 °C and 35% RH) and “Hot and Very Humid” climate (30 °C and 75% RH) to simulate extreme climatic conditions. In the context of this study, we accepted that the temperature and humidity used in accordance with ISO 3402 (22 °C and 60% RH) were sufficiently close to the defined “Subtropical and Mediterranean” climate zone (25 °C and 60% RH), to simulate a third category of climatic conditions. For both THS 2.2 and 3R4F reference cigarettes, the puffing intensity used was in accordance with the HCI standard, which was modified with regard to the temperature and humidity conditions considered in this study.

Evaluation of the robustness of reduction over all HPHCs in THS 2.2 aerosol in proportion to the levels measured in 3R4F smoke indicated that the volume-adjusted approach was comparable with the nicotine-adjusted approach. The general point to be made is that using volume-adjusted yields is an appropriate approach for comparing products of different categories, regardless of the possible nicotine levels. Therefore, volume-adjusted yields allow comparison of actual HPHC yields between different products and conditions, while nicotine-adjusted values more specifically address the users’ adaption of puffing with regard to nicotine yields. The effectiveness of the approach presented here means that its application could confidently be envisaged within the broader scope of the evolution of this category of product.

Machine-smoking testing is useful for characterizing tobacco product emissions for design and regulatory purposes. The demonstration of product robustness substantiates the evidence package provided in the due diligence assessment of THS 2.2. In this context, the study demonstrated that, overall, the concentrations of investigated HPHCs were reduced by at least 90% under HCI and extreme climatic conditions, and this was established with a conservative approach including various climatic conditions for generating 3R4F cigarette mainstream smoke, which was used as a reference.

The concept of thresholds applied for robust concentration reduction was introduced in a previous study (16) to categorize each HPHC investigated in THS 2.2 as a function of its demonstrated level of robust reduction. The same methodology was adopted in this study, and, as expected, THS 2.2 showed a robust performance in terms of HPHC reductions.

More interestingly, we should highlight that the thresholds were, in all cases, defined by the statistically determined lowest level of reduction among the three climatic conditions applied in this study. This approach, combined with the level of confidence of the non-inferiority test (0.975) used in the statistical computations, clearly affirms that, for the three climatic zones - which include extreme temperature and humidity conditions - we have minimized the risk of overestimating the performance of THS 2.2.