One of the biggest, and most uncertain, concerns with e-cigarettes is the wide variety of flavorings used in the liquids. A 2018 paper reported that over 15,500 distinct e-cigarette flavors were on the market and available to consumers. These flavorings are, in most cases, considered GRAS—generally recognized as safe—for ingestion. But little, if any, research exists on the potential health hazards from inhaling those flavoring chemicals. An example of this potential problem is the use of diacetyl as a butter flavoring in the food industry. While diacetyl is considered safe to ingest, inhalation of diacetyl has been associated with bronchiolitis obliterans, a rare and serious disease of the lungs. Acetyl propionyl (aka 2,3-pentandione) is another flavoring agent that is considered GRAS for ingestion but whose risks associated with inhalation may be as high as those of diacetyl, based on inhalation studies with rats. Both diacetyl and acetyl propionyl have been found in many e-cigarette flavorings. It is unknown what, if any, health effects are associated with the inhalation of other flavors that are currently affirmed as GRAS by the FDA. In addition, current literature reveals little about the potential synergistic effects of the main chemical components, the numerous flavoring additives, and potential pyrolysis byproducts. Given the wide variety of e-cigarette devices and users’ vaping styles, which can range from “stealth vaping” (where nearly all emissions are intentionally limited) to “cloud chasing” (where the intent is to generate as large and long of a “cloud” of vapor as possible), it is difficult to predict secondhand exposures to bystanders. However, research has shown that users do not absorb all of the nicotine and other chemicals they inhale. Therefore, secondhand exposures are clearly possible. The amount of scientific literature on potential surface deposition of nicotine from e-cigarette use (potentially resulting in thirdhand exposure) has been increasing over the past few years. Some recent research has shown that e- cigarette aerosols can spread through ventilation systems to adjacent parts of a building, where they can then be deposited onto surfaces, and that nicotine deposition can be retained on surfaces for up to three days or more. Such surface deposition can potentially contribute to thirdhand exposure to nicotine and other potentially hazardous contaminants. According to work published in the Proceedings of the National Academy of Sciences, surface deposition of nicotine from three days of exposure to traditional cigarette smoke followed by only three hours of exposure to nitrous acid or nitrogen oxides (which are naturally present in ambient air) was sufficient to form potentially carcinogenic tobacco-specific nitrosamines.  E-CIGARETTES AND INDOOR AIR QUALITY It is important for industrial hygienists and safety and health professionals to understand these issues in order to limit the impact of e-cigarettes on the indoor environment. In its 2015 document, “Current Intelligence Bulletin 67: Promoting Health and Preventing Disease and Injury Through Workplace Tobacco Policies,” NIOSH recommended that employers “establish and maintain smoke-free workplaces that protect those in workplaces from involuntary, secondhand exposures to tobacco smoke and airborne emissions from e-cigarettes and other electronic nicotine delivery systems.” The American Society of Heating, Refrigerating, and Air-Conditioning Engineers' (ASHRAE) Standard 62.1: Ventilation for Acceptable Air Quality contains requirements for ventilation of spaces that are free of environmental tobacco smoke, and provides requirements for separation of ETS-free areas from any areas containing ETS. Addendum C to the standard clarifies that ETS “includes smoke produced from the combustion of cannabis and controlled substances and the emissions produced by electronic smoking devices” and states that “provision of acceptable indoor air quality is incompatible with the presence of ETS, including cannabis smoke and e-cigarette emissions” (emphasis added). Because the magnitude of health and safety hazards that vaping may present to nonusers remains unclear, it is important to manage and control vaping in indoor locations where smoking is currently restricted. To better understand, and protect the public from, potential health and safety risks associated with vaping, the AIHA white paper offers four recommendations. First, e-cigarettes should be considered a source of aerosols, volatile organic compounds, and particulates in the indoor environment that have not been thoroughly characterized or evaluated for health risk or safety. Second, additional research should be conducted on the health effects from inhaling e-cigarette flavorings and other ingredients; the effects of secondhand emissions, thirdhand exposures, e-cigarettes, and nicotine addiction; and the lifecycle and end-of-use issues associated with e-cigarette manufacturing, use, and disposal. Third, the health risks and economic consequences of accidental exposure by children, adults, and pets should be addressed, including requirements for proper labeling and child-resistant packaging. Finally, as e-cigarettes are a potential source of pollutants, it is prudent to manage and control vaping in indoor environments consistent with current smoking policies until and unless research demonstrates that these devices will not significantly increase the risk of adverse health effects to occupants.   CHERYL L. (CHERI) MARCHAM, PhD, CSP, CIH, CHMM, FAIHA, is an assistant professor and program chair for the Master of Science in Occupational Safety Management degree program at Embry-Riddle Aeronautical University. She is a past member of the AIHA Board, a current director on the ABIH Board, and the 2018 recipient of AIHA’s Henry F. Smyth, Jr. Award. JOHN (JACK) SPRINGSTON, CIH, CSP, FAIHA, is the Industrial Hygiene Services manager for ATC Group Services’ New York operations. Send feedback to The Synergist.