Characteristics of Electronic Cigarette Aerosol and Its Impact on Indoor Air Quality
In recent years, electronic cigarettes (e-cigs) have gained increasing popularity among adolescents and young adults while emerging data raise concerns about their health effects. Essentially different from conventional tobacco cigarettes (t-cigs), e-cigs produce an aerosol via vaporization of the e-liquid, which is typically made of propylene glycol (PG), vegetable glycerin (VG), nicotine, water, and flavoring compounds. E-cig aerosols mainly consisting of liquid droplets were found highly dynamic and volatile. However, knowledge on the transport and transformation of e-cig aerosols and the related exposure is still limited. This dissertation investigated the characteristics of e-cig aerosols and evaluated the impacts of e-cig use on indoor air quality focusing on the effects of e-liquid and environmental factors. This work is subdivided into the following seven chapters: an overview (Chapter 1), a literature review (Chapter 2), three chapters of original research (Chapters 3-5), the conclusions of the work (Chapter 6). We first conducted a literature review to summarize the effects of e-cigs on indoor particulate matter (PM) pollution, chemical compositions of e-cig aerosols, and associated respiratory and cardiovascular effects. Many studies have reported high levels of ultrafine particles (UFPs, particles with an aerodynamic diameter ≤ 100 nm) and PM2.5 (particles with an aerodynamic diameter ≤ 2.5 μm) due to e-cig use (i.e., vaping) indoors, which were comparable to t-cig smoke. Although the concentrations of toxic chemical compounds in e-cig aerosols are generally lower than those in t-cig smoke, a substantial amount of vaporized PG, VG, nicotine, and hazardous substances, such as aldehydes and heavy metals, has been reported. Evidence confirms that e-cig use impairs indoor air quality and poses bystanders for risk of secondhand exposure. To explore the effects of e-liquid compositions on the e-cig aerosols, we investigated how PG/VG ratio and nicotine content affect e-cig aerosol emissions and dynamics. We found that adding nicotine to the e-liquid altered the particle emission factors. For nicotine-free e-liquids, increasing the PG/VG ratio resulted in increased particle loss rates measured by particle number concentration (PNC) and PM2.5. This pattern was not observed with nicotine in the e-liquids. The particle loss rates, however, were significantly different with and without nicotine. In real-world settings, we assessed the impacts of e-cig use on indoor air quality in vape shops and their neighboring indoor spaces in multiunit buildings. The mean (SD) PNC and PM2.5 concentration in the studied vape shops were 2.8�10^4 (2.3�10^4) particles/cm3 and 276 (546) �g/m^3. Out of the six studied pairs, PNCs in five vape shops and PM2.5 in two vape shops were significantly correlated with those in their neighboring businesses. Nicotine was detected in the air of all the studied vape shops [mean (SD) 2.59 (1.02) �g/m^3] and neighboring businesses [mean (SD) 0.17 (0.13) �g/m^3]. We also identified major factors such as dilution, ventilation, and vaping density (puffs/h/100m^3) that influenced the behaviors of exhaled e-cig aerosols, and evaluated the transport of exhaled e-cig particles from the vape shop to its outdoor environment. Compared to mainstream e-cig aerosols measured in laboratory chamber studies, larger particle sizes (mode ~ 250 nm) were observed at the end of vaping in vape shops. We developed a ventilated artificial lung system to better understand how the human lung environment alters the inhaled e-cig aerosols. We used this system to assess the effects of breathing, temperature, and relative humidity (RH) on characteristics of inhaled e-cig aerosols in the last Chapter. Increasing the respiratory rate (from 10 to 15 BPM) and breath volume (from 480 to 720 ml) resulted in enhanced particle decay rates from 32 to 80 h^-1 in the ambient room environment. The decay rates under all breathing patterns increased by ~ 11 – 20% when changing the temperature and RH from ambient room to simulated lung conditions. The warm and humid lung environment resulted in hygroscopic growth of inhaled particles that significantly enhanced the presence of supermicron (> 1 �m) e-cig particles. Overall, the use of e-cigs that leads to high levels of PM2.5, UFPs, and gas-phase nicotine degrade the indoor air quality, raising concerns regarding secondhand exposure to e-cig aerosols. E-cig aerosols that are highly dynamic in the environment, especially small particles, can travel farther away and penetrate into neighboring indoor spaces. The characteristics of e-cig aerosols and related exposures can be affected by a wide range of factors including, but not limited to, e-liquid compositions, air exchange rate, ventilation, dilution, vaping density (puffs/h/100m^3), distance, inhalation and exhalation process in the human lung, and environmental conditions with varying temperature and humidity. These data can guide exposure assessment and mitigation strategies.