Kerosene

Kerosene (CAS 8008-20-6) is a refined petroleum mid-distillate — the same hydrocarbon family used as jet fuel and lamp oil. In engine degreasers it serves as a co-solvent, supplementing a heavier aliphatic distillate with lower-molecular-weight hydrocarbons that flash-evaporate and carry grease into solution more aggressively. The trade-off is a higher vapor pressure (and stronger "diesel" odor) than the heavier distillate alone, which raises the inhalation risk during application.

The ingredient is on California's Safer Consumer Products Candidate Chemicals List. EPA classifies the constituent hydrocarbons as VOCs under 40 CFR 59. ACGIH assigns a skin designation, meaning dermal absorption is a meaningful exposure route during prolonged contact. At 20–30% of a formulation, kerosene contributes meaningfully to the parent mixture's aspiration hazard, STOT RE classification, and overall VOC burden.

For users: the smell of kerosene in an engine-degreaser product is the chemistry telling you to use it outdoors. The vapor pressure is high enough that confined-space spraying produces measurable airborne concentrations within minutes.