Vaping is a term relating to heating a liquid and inhaling the “vapor” produced. Vaping first became popular as a means of inhaling tobacco without combusting it. Electronic cigarettes consisting of a battery, heating coil, and liquid reservoir usually containing nicotine were developed and went through several “generations” of modification. The third generation consists normally of a vape pen, holder containing the battery and heating oil, and a liquid cartridge with or without nicotine. The liquid consists of glycerin and ethylene glycol typically in amounts of 30–70%. A third liquid is also included at about 1/3 of the total amount. Some terpenes for taste and odor and nicotine may also be included. The vaper heats the cartridge for a few seconds , inhales the aerosol and exhales it into the room or ambient air. Many studies of secondhand exposure to e-cigarette aerosol have been published.Recently, vaporization of marijuana has emerged as a popular method of delivering marijuana. This method heats marijuana liquid to the point of vaporization, avoiding combustion.Shortly after this method was introduced, Earley wine and Van Dam reported on four subjects who smoked marijuana and agreed to switch to vaping for a short period. All four refused to return to smoking marijuana at the end of the experiment. Eight years later, these early adopters of vaping have been joined by millions of persons worldwide. For example, 3.8%, 10.5%, and 13.8% of about 25 million US high school students reported vaping marijuana in 2016, 2017, and 2018, respectively . Abrams et al. showed that vaping marijuana produced similar levels of THC in blood as smoking marijuana cigarettes,hydro tray without the increase in carbon monoxide in exhaled breath associated with combustion.
Essentially all passive exposure to marijuana smoke from vaping is from exhaled breath since there is no side stream smoke. The aerosol emerging from exhaled breath will be different in many respects from the inhaled aerosol, due to lung deposition, humidification, growth, and coagulation, so it is important to test exposure under real-world conditions using human vapers. One recent study has focused on indoor air concentrations of particles due to various indoor sources, including both marijuana and tobacco smokers . This study included 193 persons, of whom about 22% and 15% smoked tobacco and marijuana, respectively. It may be the first study to look at passive exposure to marijuana from human smokers in their own homes. The authors found that nonsmokers exposed to persons smoking either tobacco or marijuana cigarettes had roughly twice the exposure to fine particles as non-exposed nonsmokers. The Dylos monitors used in this study were not calibrated by comparison to gravimetric levels, so the investigators could not estimate PM2.5 exposures or source strengths. Exposure models depend on the vaping “topography” including frequency of use, depth or time of inhalation, and time retained in the lungs. McClure et al. studied 20 heavy users who were allowed to smoke marijuana cigarettes freely over 4 days. On average, they smoked 12 cigarettes per 9-h day, taking 13 puffs from each cigarette. The volume per puff ranged from 51 to 61 ml. A second study of 98 Dutch adolescents found that they smoked an average of 2.5 joints per day of use, and 21 days of use per month . Another required parameter in an exposure model is the air exchange rate. Chan et al. used leakage information to show that rates in the US vary in log-normal fashion from about 0.1 to 2 h-1. The rate is affected by the indoor-outdoor temperature difference and by wind direction and speed .
One of the strongest effects on air exchange rates is window-opening behavior . The volume of the home is another necessary parameter, statistics for which can be obtained for the US from the US Census Bureau . Although these previous studies are useful in developing exposure models, we believe that no studies have sufficiently characterized the two crucial ingredients of such models: source strengths and decay rates of real-world aerosols from vaping marijuana liquids by human subjects. In this study, we use more than 100 controlled experiments involving human vapers in rooms within inhabited homes to provide information on source strengths and decay rates, from which models of exposure can be built. The main objective of the study was to measure, under real-world conditions, the two main parameters affecting secondhand PM2.5 exposure to marijuana aerosol from vaping: the source strength and removal rate from the air. The source strength is the mass of PM2.5 emitted and is generalizable to other locations and situations. The removal rate for nonvolatile particles not subject to coagulation is the deposition rate k . In the case of vaping marijuana, the deposition rate may be augmented by evaporation, so the removal rate = k + evaporation rate . In carrying out this objective, we evaluated the calibration factors for the 3 p.m. instruments measuring vaping aerosol. For the Side Pak and Piezo balance monitors, the CFs were determined directly using gravimetric techniques . Since the Purple Air monitors in this study were collocated with the Side Pak monitors, the CF for them was determined by direct comparison with the Side Pak readings. A secondary objective was to compare the performance of a low-cost monitor to research-grade instruments . If the low-cost monitor performed sufficiently well, it could be adopted in future studies of exposure. It offers the opportunity of a much broader sample of homes and participants. It is able to monitor continuously with no maintenance. In the case of Purple Air, there is also an existing network on the internet that can be sampled at will by a researcher.
Three main particle monitor types were used in the study: an optical monitor with a PM2.5 impactor ; a monitor employing a piezoelectric crystal , also with a PM2.5 impactor; and a low-cost optical monitor providing estimates of PM1, PM2.5 and PM10 . The Piezobalance is manufactured by Kanomax, Inc. Japan, and has previously been licensed for sale in the US by TSI Inc., Shoreview, MN. Piezobalances used in this study included models from both Kanomax USA Inc. , and TSI, Model 8510. For the Piezobalances used in this study, a special connector has been added by the factory, which allows the 1-min average frequency counts to be output to a computer where they can be logged. The instrument employs a vibrating quartz crystal exposed to a steady flow rate that has passed through a PM2.5 impactor. As the exposed crystal collects particles, its frequency changes due to the piezoelectric principle,planting table and within a certain frequency range the change in frequency is proportional to the amount of material collected on the exposed crystal. The frequency change during each measured time interval is then multiplied by the factory-set calibration factor to give an estimate of the amount of mass collected during the time interval. The SidePak is an optical particle monitor. It uses a laser to sense particles as they pass through a chamber. The scattered light is collected and calculated as a volume determined by applying Mie scattering formulae. The SidePak is calibrated at the factory using ISO 12103-1 Test Dust . As with all optical monitors, it is recommended that the particular aerosol mixture being studied be analyzed using gravimetric methods, so that a calibration factor can be determined for that aerosol. For example, the calibration factor for the SidePak has been found to be about 0.32 for tobacco smoke . We have been able to determine a calibration factor for the marijuana aerosol produced by vaping . The PurpleAir instruments use a laser of ~650 nm wavelength to sort particles into one of six size categories . There are two lasers in each monitor, providing the opportunity to detect departures from normal operation and calculate internal precision. The monitors have a small inaudible fan to counter “starvation” of air at the face of the monitor. They operate off line current and have no battery. Every 80 s they upload observations directly to the Web.
The monitors provide the number per deciliter of particles in each of the six categories. Also, they provide two data series, identified as CF1 and CF ATM for PM1, PM2.5, and PM10. The manufacturer of the sensor is a Chinese company . The company does not provide details on the calibration aerosol used, such as the density, or any correction factors employed in calculating PM1, PM2.5 or PM10. Therefore, we chose not to use the CF1 or CF ATM data series provided by Plan tower. Instead we adopted a standard method for determining PM2.5 from the particle numbers provided for the three size categories up to 2.5 μm. We chose an intermediate particle diameter within each category to represent all particles in the category, calculated the resulting particle volume, and determined a reference mass by adopting a density of 1 g cm-3. This approach succeeded in improving the limit of detection from about 2 μg/m3 using the CF1 or ATM to ~1 μg/m3 . The calibration factors for the Piezobalance and SidePak response to vaping marijuana were studied using gravimetric procedures by Zhao et al. . The Piezobalance CF was 0.97 . The SidePak CF was 0.44 . A carbon monoxide monitor was employed to estimate the air exchange rate of each of the two experimental rooms. A known amount of carbon monoxide was released using a flow rate regulator attached to a 107-liter cylinder containing 10% CO . The air exchange rate was determined as the negative slope of the background-corrected logarithm of the CO concentration. A correction was made for the temperature based on observations of the variation of the Langan monitor readings with temperature. Tests were carried out between May 21, 2018 and May 25, 2019. Each test was conducted in a room in a home. Two rooms were used, one in Santa Rosa and one in Redwood City . Rooms were sealed off from the remainder of the home. The HVAC system was off and floor registers sealed. Usually no fan was employed. In some experiments, one or two table fans were employed to test the effect of a fan on measured decay rates. Two Piezobalances, two PurpleAir monitors, and 2–3 SidePaks were employed for each experiment. They were situated at two well-separated locations at heights between 0.9 and 1 m. A single Langan electrochemical device was set at a central location in the room to monitor CO. CO was released using the cylinder discussed above. A target peak concentration above 5 ppm was set. Background PM2.5 concentrations were collected for 5–10 min before the experimenter took one or more puffs of the heated marijuana oil. A battery operated device was used to vaporize marijuana oil from a 500 ml cartridge . Oils with different CBD/THC ratios were tested: 18:1 ; 8:1 ; 7:1 ; and 2:1 . The amount of CBD and THC was listed for each cartridge. For example, the 8:1 CBD/THC ratio included 336 mg CBD and 40 mg THC. The missing 124 ml liquid was not identified. The other formulations also included about 2/3 marijuana liquid and 1/3 unidentified liquid. Only one person at each location was the vaper. He sat or stood at a location roughly equidistant from the two locations for the instruments. After completing the protocol for heating the cartridge , the experimenter left the room. Two protocols for heating the cartridges were adopted to study the effect of different heating times on the amount of vapor produced. Protocol I consisted of heating the oil for 3 s by pressing the power button on the vaping pen and then inhaling for three additional seconds while still pressing the power button. Protocol II involved heating for 12 s before the 3-s inhale, a total of 15 s of heating compared to 6 s in Protocol I. Source strengths were calculated as follows. Since the initial peaks registered by the monitors occurred under conditions of poorly-mixed air, the true estimated peak concentration assuming perfect mixing was determined by calculating decay rates after good mixing was attained . The line of best fit could then be extended “backward” in time to the time of the puff.