You are probably aware of the concept of second-hand smoke, which increases the risk of disease and death. You should probably be aware of another deadly scourge: second-hand stress.
Natural selection has wired us to sense the stress of others and make it our own. If you are a gazelle and you don’t freak out when others around you do, then you might be the one about to be consumed by a predator you haven’t seen yet. Animals detect the stress of others through various sensory signals such as alarm calls, olfactory cues, or visual behaviors. Plants detect distress signals of others in the form of ethylene gas that activates their own stress response (fittingly, second-hand smoke contains ethylene). The ability to detect second-hand stress is a survival instinct that can promote evolutionary fitness.
Ethylene is a gaseous hydrocarbon with the molecular structure C2H4. It is commonly produced when hydrocarbons are exposed to oxidative stress, such as that found during lightning, volcanic eruptions, forest fires, and photochemical reactions on the ocean surface. Plants coopted ethylene biosynthesis during evolution to manage their response to oxidative stress from biotic and abiotic sources. Further exaptations of ethylene include modulation of plant life history events such as development, transformation, senescence, and death.
Due to a number of factors described below, humans may be subject to increasing ethylene exposure. The potential health consequences of ethylene exposure are not part of the public consciousness and warrant further exploration.