3.1. Scope Planning
Six main categories were considered under the rubric of “scope”: the tested nicotine delivery devices, the hazards, the sources of the hazards, the routes of exposure, the health effects, and the target population. Analysis was restricted to two nicotine delivery systems: ECs and CCs. In total, 12 EC products and 50 CC products were assessed in the two key reviewed studies.
Based on our literature review, 12 toxicants in ECs and CCs emissions are regarded as the most significant hazards. These 12 toxicants were selected for assessment. Of these, acetaldehyde (Ace), acrolein (Acr) and formaldehyde (Frm) are chemicals from the carbonyl compounds group [12
]. Diethylene glycol (DEG) and propylene glycol (PG) are from the diols group. Arsenic (As), cadmium (Cd), chromium (Cr) and nickel (Ni) are heavy metals [14
]. Carbon monoxide (CO) is found in CC emissions [15
], and is a toxic gas. Finally, N
′-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) are tobacco-specific nitrosamines (TSNAs).
The emissions of the tested nicotine delivery devices were considered to be the major sources of hazards of ECs and CCs [16
] with inhalation being the sole route of exposure [18
]. The scope of the health impact was restricted to the categories of cancer, cardiovascular diseases (CVD) and respiratory diseases as these are considered the greatest health issues associated with tobacco use as reported by the United States Centres for Disease Control and Prevention (U.S. CDC) [20
The estimates of exposure are based on current smokers, defined as people having smoked more than 100 cigarettes in their lifetime and at least one cigarette in the last 28 days [21
]. The average daily CC consumption of current New Zealand (NZ) smokers is 11 CCs per day [21
]. Goniewicz and colleagues equate one CC smoked to one EC vaping session consisting of 15 puffs, making the average daily EC consumption of NZ current smokers 11 vaping sessions (15 puffs/session × 11 sessions = 165 puffs) [12
3.2. Hazard Identification
In this step, toxicokinetic and toxicodynamic data were collected from the literature for each chemical identified.
Most of the identified hazards are typically absorbed by inhalation, although PG and DEG are usually found in products that are ingested [23
]. All identified hazards can travel to different parts of the body once absorbed. There is limited information about where inhaled DEG is distributed in the human body [23
]. Some metabolites are harmful while some do not pose a significant health risk. Cd and Ni usually accumulate instead of going through any specific metabolism processes [25
]. Most identified hazards are excreted by the renal system. Some hazards (Acr, Frm, As, and Cd) are excreted via multiple pathways (renally, by defecation or by exhalation) [27
]. CO is excreted by exhalation [15
All identified hazards may contribute to different types of adverse respiratory outcomes. Only CO and Cd have been identified as having a significant impact on the cardiovascular system [18
3.2.3. Carcinogenicity Classification
The carcinogenicity classification of the identified hazards is based on the IARC monograph [31
]. Of the 12 identified chemicals, seven (Frm, As, Cd, Cr, Ni, NNK and NNN) are classified as Group 1 carcinogens (carcinogenic to humans), one (Ace) is a Group 2B carcinogen (possibly carcinogenic to humans), one (Acr) is a Group 3 agent (not classifiable as to its carcinogenicity to humans), and the rest (DEG, PG, CO) are not classified as they are not considered to be carcinogenic [31
3.2.4. Detection in Tested Devices
Nine out of the 12 hazards of concern have been detected in the emissions of ECs: Ace, Acr, Frm, DEG, PG, Cd, Ni, NNK, and NNN. For CCs, eight out of the twelve hazards of concern were detected in the emissions of CCs: Ace, Acr, Frm, As, Cd, CO, NNK, and NNN.
3.4. Exposure Assessment
In this study, the frequency of exposure was assumed to be 165 puffs for ECs and 11 CCs per day (equivalent to the NZ daily average consumption amongst current smokers); the duration of exposure is assumed to be one year [11
]. The data considered in the exposure assessment are based on two key studies that were reviewed (as noted in Section 2.3
Based on these two key studies, nine (Ace, Acr, Frm, DEG, PG, Cd, Ni, NNK and NNN) out of the 12 identified hazards were mentioned in relation to EC emissions. DEG is a special case. DEG was not identified by the key reviewed studies (as noted in Section 2.3
), but was identified in testing by the U.S. Food and Drug Administration (U.S. FDA). Referring to the test results, one of the 18 tested ECs was found to have 1% DEG content in its liquid [42
]. Another toxicant, PG, was also not identified by the key reviewed studies (as noted in Section 2.3
), but was identified in the study done by Geiss et al. In Geiss’ study, the hazards in EC emissions were measured by analysing the chemical composition of EC mainstream vapour on simulated indoor environments under controlled conditions using a 30 m3
emission chamber [43
]. Referring to the study results of 12 tested ECs, the average PG exposure level in ECs emissions is 12.12 mg per vaping session (133.28 mg in 11 vaping sessions) and the maximum exposure level is 12.90 mg per vaping session (141.90 mg in 11 vaping sessions).
As and Cr were both detected in levels lower than those considered to pose a significant risk. Therefore, for the purpose of this study, they were regarded as not being significant hazards associated with the use of ECs. Since ECs are designed to deliver nicotine without combustion, neither of the reviewed EC studies took CO into consideration when assessing the hazards associated with EC emissions.
On the other hand, ten (Ace, Acr, Frm, As, Cd, CO, NNK and NNN) hazards were identified in CC emissions mentioned by the two key studies. Because the analytical limits of Cr and Ni were not revealed, the exposure levels of Cr and Ni in CC emissions are simply indicated as being “below the analytical limit”.
PG and DEG are not identified as hazards in CC emissions. This is because CCs deliver nicotine through the burning of tobacco leaf, while ECs heat a solution potentially containing PG and DEG. Therefore, PG and DEG are unlikely to be detected in CC emissions.
3.5. Risk Characterisation
Based on the findings above, using ECs exposes users to nine out of the 12 hazards identified as being of concern in CCs. Five of them are Group 1 carcinogens (Frm, Cd, Ni, NNK, and NNN), one is a Group 2B carcinogen (Ace), one is a Group 3 carcinogen (Acr), and two are non-carcinogens (DEG and PG). These nine hazards have been correlated to two major types of health effects: cancer and respiratory effects [20
Assuming the average 11 vaping sessions per day of ECs, exposures for four (Acr, DEG, PG, and Cd) out of the nine hazards reach the maximum safe exposure levels. These hazards’ maximum exposure levels are from 1.2 times to 170 times higher than the corresponding guideline levels.
CCs expose users to eight out of twelve hazards identified as of concern; five are Group 1 carcinogens (Frm, As, Cd, NNK, and NNN), one is a Group 2B carcinogen (Ace), one is a Group 3 carcinogen (Acr), and one is a non-carcinogen (CO). These eight hazards are linked to all three major types of adverse health: cancer, respiratory effects and cardiovascular effects. Assuming a daily consumption of 11 cigarettes, all identified chemicals’ maximum exposure levels (Ace, Acr, Frm, As, Cd, CO, NNK and NNN) would exceed the guideline levels by factors ranging from 1.58 to 4500.
3.6. Risk Comparison
This step in the process uses standardised values to represent the health risk of using the tested nicotine delivery devices being compared. It includes: (1) risk comparison of EC and CC risk profiles, (2) risk comparison of maximum hazard exposure levels (Figure 1
), and (3) risk comparison of average hazard exposure levels (Figure 1
Based on a risk comparison, using ECs exposes users to nine hazards (seven of them classified as carcinogens) and two are considered to result in adverse health effects. In comparison, using CCs results in exposure to eight hazards (seven of which are carcinogens) and three at levels considered to leads to adverse health effects. The maximum exposure levels to the four hazards found in EC emissions are higher than the guideline levels. In comparison, the maximum exposure levels to the seven hazards in CC emissions are higher than the guideline levels. The average emissions are higher than the guideline level for two of the hazards in EC emissions and seven of the hazards in CC emissions.