Introduction Secondhand smoke (SHS) from combustible cigarettes causes numerous diseases. Policies have been developed to prevent SHS exposure from indoor cigarette use to reduce health risks to non-smokers. However, fewer policies have been implemented to deter electronic cigarette (ECIG) use indoors, and limited research has examined the impact of secondhand exposure to ECIG aerosol.

Methods Indoor air quality was measured at a 2-day ECIG event held in a large room at a hotel. Fine particulate matter (PM) was measured using 2 devices that measured concentrations of PM 2.5 μm aerodynamic diameter or smaller (PM_2.5). Measurements were taken before the event, over 2 days when the event was ongoing, and the day after the event. PM_2.5 measurements were also taken from the restaurant at the hotel hosting the event and a restaurant at a nearby hotel.

Results During 6 time points when the event was ongoing, between 59 and 86 active ECIG users were present in the event room (room volume=4023 m³). While the event was ongoing, median PM_2.5 concentrations in the event room increased from a baseline of 1.92–3.20 μg/m³ to concentrations that ranged from 311.68 μg/m³ (IQR 253.44–411.84 μg/m³) to 818.88 μg/m³ (IQR 760.64–975.04 μg/m³).

Conclusions PM_2.5 concentrations observed at the ECIG event were higher than concentrations reported previously in hookah cafés and bars that allow cigarette smoking. This study indicates that indoor ECIG use exposes non-users to secondhand ECIG aerosol. Regulatory bodies should consider establishing policies that prohibit ECIG use anywhere combustible cigarette use is prohibited.