We developed a device-independent test method to investigate carbonyl emissions from different e-cigarette liquids under precisely controlled temperatures. PG and GL were identified to be the main sources of toxic carbonyl compounds from e-cigarette use. GL produced much more formaldehyde than PG. Besides formaldehyde and acetaldehyde, measurable amounts of acrolein were also detected at ≥270°C but only when GL was present in the e-liquid. At 215°C, the estimated daily exposure to formaldehyde from e-cigarettes, exceeded United States Environmental Protection Agency (USEPA) and California Office of Environmental Health Hazard Assessment (OEHHA) acceptable limits, which emphasized the need to further examine the potential cancer and non-cancer health risks associated with e-cigarette use.

Significant amounts of formaldehyde and acetaldehyde were detected at reactor temperatures ≥215°C for both PG and GL. Acrolein was observed only in e-liquids containing GL when reactor temperatures exceeded 270°C. At 318°C, 2.03±0.80 μg of formaldehyde, 2.35±0.87 μg of acetaldehyde, and a trace amount of acetone were generated per milligram of PG; at the same temperature, 21.1±3.80 μg of formaldehyde, 2.40±0.99 μg of acetaldehyde, and 0.80±0.50 μg of acrolein were detected per milligram of GL.

Introduction

E-cigarettes, as battery-powered nicotine delivery devices with many different configurations, have increased in popularity in recent years, especially among youth. According to the California State Health Officer’s 2015 report [1], e-cigarette use among teens surpassed the use of traditional cigarettes for the first time in 2014, and the prevalence of such use tripled in one year from 2.3% to 7.6% in 2013 for young adults (18–29 years old, including 20% who have never tried traditional cigarettes). Nationally, 8.1% of adults have tried this new device, with 1.4% being current users in 2012 [2]. In order to fuel the rapidly growing market for e-cigarettes, manufacturers advertise these new devices as healthier and safer alternatives to traditional tobacco smoking or as a tool for smoking cessation. However, in addition to the questionable premise that e-cigarettes assist in smoking cessation, increasing scientific evidence indicates that e-cigarettes produce more than “harmless water vapor” during “vaping”. Immediate adverse physiological effects after short-term use of e-cigarettes have been reported. For example, Vardavas et al. [3] observed pulmonary effects similar to those seen with tobacco smoking after using e-cigarettes for five minutes. Flouris et al. [4] found that e-cigarettes caused measurable (but smaller) changes in lung function compared to combusted tobacco, while short-time e-cigarettes use produced a similar nicotinergic impact. The potential human health risks posed by the long-term use of e-cigarettes directly on vapers and indirectly on bystanders, from second-hand smoking as well as third-hand exposures, remain largely unknown [5]. Therefore, as use of e-cigarettes increases rapidly, more studies on e-cigarettes are urgently needed to ensure that consumers’ (and bystanders’) health is safeguarded.

Although e-cigarettes have a very wide variation in terms of device design and component functionality, they typically share basic common features, which include an aerosol generator (heating element), a flow sensor, a battery, and a storage tank for a nicotine-containing solution (usually called e-liquid or e-juice) [6]. In order to generate aerosols, the vaper inhales at the mouthpiece of e-cigarettes to allow e-liquid, generally containing PG or GL (or a mixture of both) as nicotine solvent, along with or without nicotine, water, and flavorants added for various tastes, to pass through the heating element and to be vaporized. Of increasing concern are the thermal breakdown products from e-liquids at high temperature, due either to the device design or to unintentional overheating. For example, in some “voltage adjustable” e-cigarette devices, users can raise the battery output voltage to generate more aerosols and to increase nicotine delivery through increased internal temperature. In fact, the achievable temperature inside e-devices ranges up to 350°C [7, 8], sufficiently high to decompose PG or GL. Diaz et al. [9] studied the homogeneous oxidation reaction of PG in a 10% O 2 /He mixture at different temperatures and concluded that PG can be easily oxidized at as low as 127–227°C to form formaldehyde, acetaldehyde, and carbon dioxide via oxidative C-C cleavage and to form acetone via a dehydration route. The dehydration of GL to yield acrolein is another well-known reaction in organic chemistry. Acrolein can undergo further degradation to form formaldehyde and acetaldehyde or other small molecules depending on circumstances [10]. For instance, carbon monoxide, acetaldehyde, and acrolein were detected as the initial products by pyrolysis of GL through a laminar flow reactor [11].

Among the reported thermal breakdown products from PG and GL, low molecular weight carbonyls such as formaldehyde, acetaldehyde, and acrolein have drawn much attention due to their known toxicity. Formaldehyde is a Group 1 human carcinogen as classified by the International Agency for Research on Cancer (IARC), and acetaldehyde is regarded as a Group 2B possible human carcinogen [12]. Acrolein is toxic, is a strong irritant for the skin, eyes, and nasal passages, and is on the original list of hazardous air pollutants from the United States Environmental Protection Agency (U.S. EPA) [13]. To date, a number of papers [7] have reported the presence of low molecular weight carbonyl compounds and two additional potentially harmful compounds, propylene oxide and glycidol, by Sleiman et al. [14] from e-cigarette aerosols. Very recently, Geiss et al. [15] studied the correlation of volatile carbonyl yields with the heating coil temperature using a third generation of e-cigarette device, an increasingly popular model with adjustable output power. However, because of limited understanding of the role played by critical variables, e.g., device type, e-liquid composition, and user’s puffing behavior, in terms of emitted toxic compounds, the measured carbonyl concentrations from e-cigarettes have a very large variability among different studies. Some factors that could affect measured amount of carbonyls are summarized in Table 1. Furthermore, it is quite challenging to accurately determine the operating coil temperature during actual e-cigarette vaping. A temperature difference as high as 100°C was observed by Geiss et al. [15] between the first and the last puffs in five consecutive puffs. Given the differences in commercial e-liquids and e-cigarette design and use, a standardized method is necessary to evaluate carbonyl emissions from e-cigarette vaping at specific temperatures.

In this study, we employed a convenient tube reactor to mimic e-cigarette vaping under precisely controlled temperatures. The aim was to investigate how the two main e-cigarette solvents—propylene glycol and glycerol—modulate thermal breakdown to toxic carbonyls in emitted aerosols, without using any specific e-cigarette device type and in the absence of other additives, e.g., nicotine and flavoring. We hypothesize that any differences between thermal breakdown products from PG and GL would contribute to our fundamental understanding of the source of toxic carbonyls from e-cigarette use. Knowing the link between amount of generated carbonyls from different e-cigarette solvents and precisely measured temperature will provide important insights of toxicological risk exposure assessment from e-cigarette use.