Secondhand vape exposure (also called "passive vaping" or "secondhand aerosol exposure") refers to the inhalation of exhaled e-cigarette aerosol by non-users. Unlike secondhand smoke, which is produced by combustion, secondhand vape aerosol is created by heating e-liquid — typically propylene glycol (PG), vegetable glycerin (VG), nicotine, and flavorings — at 150-300C without burning.

The key distinction: cigarette smoke is combustion byproduct; vape aerosol is heated liquid. As we documented in our vaping vs smoking health comparison, combustion produces 70+ known carcinogens and thousands of chemical byproducts. Heating without combustion produces far fewer compounds, but the exhaled aerosol is not "just water vapor" — it contains measurable levels of nicotine, particulate matter, and volatile organic compounds (VOCs).