Over view
This is the first study using controlled laboratory conditions that compares: (1) the contribution of collection vessels to elements/metals in the analyte, (2) the elements/metals in aerosols produced by different tank-style EC, (3) the efficiencies of two methods of aerosol collection, and (4) different topographies for aerosol production. Acid presoaking of the glassware used for aerosol collection was necessary to remove elements that leach from glass and could add to the concentrations of elements measured in aerosols. Of 19 elements/metals screened, three (cobalt, silver, titanium) were not detected in any samples. It is likely that some of the elements in tank aerosols, such as aluminum, calcium, chromium, copper, iron, lead, magnesium, nickel, silicon, tin, and zinc, were from components in the atomizing units. The total concentrations of elements/metals in aerosols collected with the cold trap method (1,226 to 6,767 µg/L) was higher than that for the impinger method (43 to 3,138 µg/L). The impinger method had the advantages of being faster to perform, collecting some elements not found with the cold trap method, and avoiding surfaces, such as tubing, that could contribute elements to the aerosols leading to an overestimation of total concentrations. For total concentrations of individual elements averaged for all brands, occasional differences were observed with different topographies, but in general concentrations were similar across topographies, as well as with the two methods of collection. The concentrations of some elements, such as lead, were significantly higher in aerosols produced at high voltages. When comparing individual elements across brands, results were again remarkably similar. For example, with the exception of one brand, zinc appeared in all aerosols irrespective of topography. Lead appeared in all aerosols, except those made with Clone, which had a simple atomizer and overall fewer elements in its aerosols. These data provide a useful benchmark for element/metal concentrations in aerosols made from a range of tank-style EC used with different topography parameters.
Leaching of elements from glassware
Other EC reports have not addressed leaching from glassware as a possible source of contaminants that affect concentrations of elements in EC aerosols, but leaching from filters used in cigarette smoke analysis has been reported41. Our data demonstrate the importance of establishing that elements do not leach into the aerosol solution from surfaces used in collection and taking this into account when computing final concentrations of elements. Acid corrosion can occur in glass by creating pores in the silica scaffold thereby leaching the alkali components of the glass and bringing them into solutions42, which could explain why there was some potassium in the impinger acid solutions even after 5 days of soaking. In addition to pretreating glassware, all plasticware should be pretreated with acid to seal it43. It is also important to minimize the amount of time a sample is stored before analysis, as elements could leach during storage and contribute contaminants to the aerosol solutions.
Methods of aerosol collection
There is currently no standard method for EC aerosol collection for metal analysis4. Therefore, labs have used various methods, such as glass washing bottles with methanol in dry ice, quartz filters, and condensation using pipette tips and narrow tubing31,44,45; however, these have been used without examining how the method affects the element concentration in aerosols. As our study shows, element concentrations can vary with the method of collection. The total concentration of elements in the cold trap high voltage low air flow rate group was about 3.5 times higher than the continuous impinger method. This could be due to: (1) leaching of elements in the cold trap method from the peristaltic pump tubing or plastic storage tubes, which were not pretreated in acid, (2) more efficient collection of all aerosol with the cold trap method, (3) the longer time (6 minutes) between puffs with the cold trap may have enabled more complete collection of the aerosol, and (4) the cold trap was a better method of collection for silicon and calcium, which contributed to the higher total concentration. It is also important to note that the cold trap method was better at collecting the alkali (sodium and potassium) and alkaline earth metals (magnesium and calcium) and metalloids (silicon, boron), but not as efficient as the impinger method at collecting the transition (heavy) metals (chromium, iron, nickel, zinc, and copper). Although we do not know the reason for these different efficiencies, these data clearly show that the method of collection can affect concentrations and that not all elements were affected in the same way. The use of two different methods provides insight into ranges of elements in EC aerosols and may help understand differences in values reported in prior literature.
Aluminum, boron, iron and nickel were present in higher concentrations in aerosols collected with the impinger method than with the cold trap. This may be due to better mixing of the aerosols with the larger volume of solvent in the impinger or loss of some elements in plastic storage tubes that were not acid sealed in the cold trap method. We recommend the use of the impinger method in conjunction with presoaking the impingers in nitric acid until leaching stops and storing aerosols in acid pre-sealed tubes with analysis as soon as possible after collection.
Effects of topography
Some elements were only present in samples prepared using specific topographic parameters. With the cold trap method, aluminum, copper, and lead were generally detected in samples prepared using high voltage, suggesting that the EC must heat high enough to drive these elements/metals into the aerosol. These same three elements were detected in all impinger samples, which were all prepared using high voltages. In cases in which an element was present only in aerosols created at low voltage (e.g., low air flow rate for aluminum with impinger method – Fig. 5B) or only in aerosols created with continues puffing (e.g., aluminum and sodium Fig. 3B), it is possible that the element was part of a coating that was released during the initial use of the EC and no additional aluminum was available for aerosolization with the subsequent topographies.
The impinger method results are generally similar for each element within a brand. Aerosols created with the continuous puffing protocol usually contained more elements than aerosols made with the interval method, while the interval puffing protocol produced aerosols that generally had somewhat higher concentrations of individual elements (e.g., lead) than those produced by the continuous protocol (Fig. 3B). Although the reason for the higher concentration with interval puffing is not known, the cycling of the filament through hot and cold temperatures could make it more friable and prone to release more elements. The interval puffing protocol is more similar to the way a consumer would use the product and probably better represents actual user exposure.
Dominant elements
Those elements/metals that were dominant in the aerosols, i.e. appeared in all or almost all samples (aluminum, calcium, chromium, copper, iron, lead, magnesium, nickel, silicon, sodium, tin and zinc) have been reported previously in the atomizing units of cartomizer and disposable EC products28,29,30,46, and it is likely that they originated in the atomizers. The Clone had the fewest metal parts and the fewest types of metal in the atomizer, and also had the fewest number of elements in its aerosol. The elements that are present in aerosols from the Clone are in similar concentrations to those in other products. These data suggest that reducing metal components in atomizers will decrease metals in aerosols, in support of our prior study29. It is also possible that some elements/metals in aerosols originated in the e-fluid, as one prior study reported26. Although not included in the current study, we have unpublished data on elements in a spectrum of EC fluids. Only sodium was high enough in some fluids we used to affect the data in this study. In fact, the difference seen in sodium in Fig. 3B is likely due, at least in part, to a high level of sodium in the refill fluid used for the continuous but not the interval puffing.
Source of elements/metals in aerosols: The concentration of elements/metals in e-fluids is higher after an EC has been used26, supporting the idea that metals in aerosols come from heated components in the atomizers. Some elements, such as lead, potassium, sodium, and zinc, have relatively low melting points (321 °C, 64 °C, 98 °C, 420 °C respectively) that would facilitate their transfer into aerosols when ECs heat up to 320 °C (Supplemental Table 1)25. Zinc was commonly found in aerosols, suggesting these devices heat up to over 320 °C. The atomizing units of the ECs used in this study did not contain lead46 nor did the refill fluids. Thus the source of the lead has not yet been determined for these products, but could be the glass or metals components of the tank/reservoir.
Number and concentration of elements are affected by model and method
The number of elements in the aerosols varied with method of collection and also with the model of the EC. The interval method produced a significantly higher concentration of copper and zinc in the aerosols from Aspire and Smok products than the continuous method. This is important since it more closely resembles how an EC would actually be used. The higher concentrations of chromium, copper, and iron in the impinger aerosols of Smok and Tsunami suggest that the sub-ohm batteries and newer tanks deliver more metals into the aerosols than the older models of tank-style EC.
Comparison to prior data
The range of total concentration of elements/metals in the aerosols of tank-style EC in the current study (374 to 3,028 µg/L) was similar to that found previously in disposable EC (973 to 2,296 µg/L)30. A group recently screened 15 elements in the aerosols from different brands of tank-style EC using a condensation method of collection26. For the subset of eight elements (aluminum, chromium, copper, iron, lead, nickel, tin, zinc) that were present in the current and preceding studies, the total median concentrations were 670.04 µg/L (tanks – condensation collection)26 101.172 µg/L (disposable -cold trap collection)30, 161.44 µg/L (tanks – cold trap collection- current study), and 441.30 µg/L (tanks – impinger collection- current study). For this subset of elements, the median concentrations of the impinger (current study) and the Olmedo et al. 2018 study are in reasonable agreement. However, the subset medians for both cold trap methods are lower than that for the tank condensation and impinger collection methods. These differences could be due to less efficient collection of certain elements using the cold trap method, lower concentrations of elements in the aerosols produced by the lower voltage disposable models, the use of different EC models/brands in each study, or a combination of these factors. The importance of voltage/power is shown by the observation that some elements (aluminum, boron, copper, iron, lead, sodium) were only produced at the higher voltage.
Comparison to cigarette smoke
The total concentration of elements/metals in the aerosol of tank-style EC (226–6,767 µg/L) was higher than that found in cigarette smoke prepared using the International Organization for Standardization (ISO) (2,690 µg/L), Health Canadian Standard (HCS) protocols (1,103 µg/L)30. Of the 19 elements screened in this study, four (boron, iron, silver, titanium) were present in cigarette smoke and not in EC aerosol prepared using the cold trap method. However, some elements (aluminum, cadmium) were present in EC aerosol and not in cigarette smoke. Four elements (copper, lead, nickel, zinc) were present in both EC aerosol and cigarettes smoke, and both lead (407 µg/L) and zinc (36 µg/L) were found in higher concentrations in EC aerosol than in cigarette smoke (ISO – 0.126 µg/L, HCS – 1.252 µg/L)30. The concentration of copper and nickel in cigarette smoke was within the range in EC aerosol (nickel: ISO – 0.655 µg/L, HCS – 2.769 µg/L, EC – 0.074–2.3 µg/L, copper: ISO – 80 µg/L, HCS – 170 µg/L, EC – 19–200 µg/L)30. Other studies have reported that individual metals in cigarette smoke prepared using the HCS usually had a higher concentration of metals than samples prepared using the ISO protocol41,47,48,49. For example, the concentrations in Marlboro Red cigarette aerosols were two to three times higher in samples prepared using the HCS47.
Potential health effects of EC elements/metals
The potential health effects of elements and metals in EC aerosols have recently been reviewed34,50,51. Chromium, lead, and nickel are of particular concern as they are known carcinogens32. Prolonged exposure to chromium from EC aerosol could cause gastrointestinal effects, nasal and lung cancer, respiratory irritation, and lung function impairment34,52,53,54. Tank-style EC deliver higher concentrations of nickel than previous EC models28,29,30. Nickel inhalation can cause lung disease, damage to the nasal cavity, lung irritation, lung inflammation, hyperplasia in pulmonary cells, and fibrosis53,55,56. Prolonged exposure to lead, which has been found in varying concentrations in all styles of EC, could produce vomiting, diarrhea, cardiovascular effects, and lung cancer34. Olmedo et al. 2018 also reported that concentrations of chromium, lead, and nickel are high enough in EC aerosols to be a health risk26. Likewise the concentrations of some elements (chromium, copper, lead, nickel, zinc) reported in our study exceed the proposed Occupational Safety and Health Administration, permissible exposure limit (OSHA PEL)34. For example, the OSHA PEL for chromium is 5 × 103 ng/m3 34, and the concentration of chromium found in one brand of tank-style EC (Tsunami 2.4) was 3.3 × 107 ng/m3, which is much higher than the OSHA PEL. Because, OSHA values are for occupational not recreational exposure, our values may underestimate potential harm to EC users.
Speciation
Since most methods of measuring metals in aerosol samples only report concentration and not speciation, it is not yet known if the species of chromium, nickel and lead would be harmful. For example, chromium (III) is an essential nutrient in the human diet and not readily absorbed by cells, but its reduction to Cr(VI) could cause oxidative stress, DNA adducts, DNA-protein crosslinks, and damage to lipid bilayers in cells57,58. In addition, exposure to Cr(VI) is a respiratory irritant and could lead to nasal, sinus, and lung cancer54.
