in the range of 0.1-4.8 parts per billion (ppb) in rural air, 4–10.5 ppb in urban air, and 7–260 ppb in industrial air samples. In the United States and some European countries a threshold limit value of 500 parts per million (ppm) was established for occupational (8-hour time-weighted average) exposure. It is considered a volatile organic compound (VOC) and emissions are regulated by many governments, but it is not listed by the U.S. Environmental Protection Agency (EPA) as a hazardous air pollutant under the Clean Air Act. With a relatively short half-life, it is not expected to bioaccumulate. Propene has low acute toxicity from inhalation and is not considered to be carcinogenic. Chronic toxicity studies in mice did not yield significant evidence suggesting adverse effects. Humans briefly exposed to 4,000 ppm did not experience any noticeable effects. Propene is dangerous from its potential to displace oxygen as an asphyxiant gas, and from its high flammability/explosion risk. Bio-propylene is the bio-based propylene. It has been examined, motivated by diverse interests such a carbon footprint. Production from glucose has been considered. More advanced ways of addressing such issues focus on electrification alternatives to steam cracking. Storage and handling Propene is flammable. Propene is usually stored as liquid under pressure, although it is also possible to store it safely as gas at ambient temperature in approved containers. Occurrence in nature Propene is detected in the interstellar medium through microwave spectroscopy. On September 30, 2013, NASA also announced that the Cassini orbiter spacecraft, part of the Cassini-Huygens mission, had discovered small amounts of naturally occurring propene in the atmosphere of Titan using spectroscopy. See also Los Alfaques Disaster Inhalant abuse 2014 Kaohsiung gas explosions 2020 Houston explosion References Alkenes Monomers Commodity chemicals Petrochemicals Gases Allyl compounds