The leakage of unburned liquefied petroleum gas (LPG) is a major source of urban nonmethane hydrocarbons (NMHCs) in the air of Santiago, Chile. Roughly 5% of the LPG that is sold in Santiago leaks in its unburned form to the atmosphere. Because of the leakage, propane is the most abundant NMHC in Santiago's air, even under heavy traffic conditions. NMHCs are an important precursor to the formation of ground-level ozone, and the LPG leakage may contribute as much as 15% to the excess ozone levels in Santiago. Improvement to the local air quality may be obtained by lowering the rates of LPG leakage, and by minimizing the use of alkene-rich LPG formulations.