The use of electronic cigarettes (e‐cigarettes), electronic shishas (e‐shishas), and new “smokeless” tobacco products as substitutes for the traditional tobacco cigarette, and as a lifestyle product, has increased significantly in recent years. Several studies showed that the consumption of e‐cigarettes causes the release of undesired substances, which might affect indoor air quality. Therefore, the question now arises whether, analogous to passive smoking, passive vaping also exists.

With an electronic cigarette, the operating fluids—known as “e‐liquids”—are vaporized through heating and then inhaled. Commercially available liquids usually contain the carrier substances propylene glycol and glycerol, various flavorings, water, and in most cases nicotine.1, 2 The vaporization process is activated by pressing a button and by drawing on the mouthpiece. Release processes such as those of the smoldering tobacco cigarette via sidestream smoke are negligible for vaping. The aerosol is formed when the consumer activates the e‐cigarette. The substances from the liquid therefore only enter room air via fugitive emissions, and in exhaled breath.

Propylene glycol and glycerol provide the visible clouds of vapor when exhaling. From these substances, which are supersaturated in the respiratory air, fine and ultrafine liquid particles are formed, whose size distribution and number concentrations are similar to those of tobacco smoke.3 The indoor air concentrations of fine (PM_2.5) and ultrafine particles (UFP) increase when e‐cigarettes are used.2 The ultrafine particles penetrate into the alveoli of the lungs.4

In a room with intensive e‐cigarette consumption, PM_2.5 concentrations in room air can rise to values measured in restaurants with permitted cigarette consumption.5 From epidemiological studies, it is known that ambient PM_2.5 causes respiratory and cardiovascular diseases.6 No safe level of exposure has been established. Although the particles in e‐cigarette aerosols differ in their composition from those outdoors and also from those of tobacco smoke, it can be assumed that they can impair health as a result of their chemical composition. In cell cultures and in animal experiments, it has been recently shown that particles in the aerosol of e‐cigarettes trigger oxidative stress and inflammatory reactions.7

In extreme cases, the aerosol composition can have dramatic acute health effects. A deadly outbreak of lung injuries associated with vaping was reported during the second half of 2019, which resulted in more than 50 deaths and 2500 hospitalizations.8 The US Centers for Disease Control and Prevention (CDC) named this newly identified lung disease EVALI (e‐cigarette or vaping product use‐associated lung injury). According to Layden et al,9 a total of 84% of the patients reported having used tetrahydrocannabinol (THC) products in e‐cigarette devices. The chemical(s) responsible for this acute effect and the physiological mechanisms have not yet been fully elucidated, but a recent CDC report points to the presence of vitamin E acetate in bronchoalveolar lavage fluid extracted from EVALI patients as a key factor associated with the disease.10 Vitamin E acetate is an illicit additive in some vaping products that contain THC.

While propylene glycol and glycerol are approved as food additives and are safe for oral use, no long‐term studies exist on potential health effects from human inhalation. Employees in the entertainment industry, who are regularly exposed to propylene glycol‐containing aerosols such as theatrical mist, suffer more often from respiratory irritations and restricted lung function than unexposed persons.11 Nebulized propylene glycol triggers irritation of the eyes and the respiratory tract, even after only short‐term exposure.12 It is due in particular to the widespread application of the compound in e‐cigarettes that an indoor guideline value was derived by the German Committee on Indoor Guide Values.13 Schober et al2 showed that the frequent use of e‐cigarettes leads to room air concentrations of propylene glycol, which clearly exceed the provisional health precaution guideline value of 0.06 mg/m³.2 Especially in small spaces like car interiors the consumption of tobacco products and e‐cigarettes can be of particular concern.14

The aerosol of e‐cigarettes also contains a broad spectrum of other organic substances. Those include solvent impurities (eg acetol), and flavorings such as diacetyl, vanillin and menthol.1, 2 In addition, other carcinogenic and/or irritating chemicals are formed by thermal decomposition of propylene glycol and glycerol during the evaporation process, including formaldehyde, benzene, glycidol, acrolein, and acetaldehyde.15–17

Several studies have determined the presence of nicotine in indoor air when nicotine‐containing e‐cigarettes had been consumed. According to a Bavarian study, nicotine in aerosols leads to an increase in nitrogen monoxide (NO) in the respiratory air of consumers after only five minutes of e‐cigarette use.2 After evaluating all scientific findings, the German Indoor Hygiene Commission (IRK) has reached the conclusion that e‐cigarettes are no less harmful than conventional cigarettes and that nicotine‐containing and nicotine‐free e‐cigarettes contribute toward indoor air pollution through gaseous organic compounds. Therefore, “passive vaping” can endanger the health of third parties.18 Nicotine from vaping can deposit on indoor surfaces, leading to thirdhand exposures as described for tobacco smoke.19 Considering the impact of other emitted compounds on indoor air quality, Logue et al20 found that air concentrations of formaldehyde and acrolein could exceed reference exposure levels recommended by the State of California under different scenarios.

More recently, a new generation of heated tobacco products (also referred to as “heat‐not‐burn” or HNB) are being promoted as another “less harmful” alternative to smoking. Instead of vaporizing a liquid, these products heat a consumable rolled tobacco plug that is inserted into a blade heater. Chemicals are released to the indoor environment not only upon puffing, but also in sidestream emissions generated during the few minutes in which the tobacco is being heated. Similar to e‐cigarettes, predicted indoor levels of those compounds are lower than for conventional smoking, but not negligible.14, 21

In summary, there is growing evidence describing the nature and magnitude of passive vaping. While secondhand and thirdhand exposures are likely lower than those associated with conventional smoking, e‐cigarettes can still be a significant source of indoor pollution. For that reason, smoke‐free measures that have proved effective to protect against cigarette smoking should also be implemented for vaping.