Naturally occurring organic compounds typically found in essential oils and citrus oils, used as cleaning agents in semi-aqueous systems. D-limonene, a hydrocarbon, is the most commonly used terpene in precision cleaning applications; some terpenes contain oxygen. Terpene compounds are found in motor fuels, dry cleaning processes; antifreeze; resins, perfumes, medicines, varnishes, lacquers and solvents. When evaporated, applied, spilled, or combusted they emit hydrocarbons into the atmosphere, which allows the creation of harmful ozone (O 3 ).