Fugitive emission sources in petroleum refineries are generally defined as volatile organic contaminant (VOC) emission sources from leaking equipment (valves, flanges, drains, etc.) associated with refinery processes. They are attributed to the stripping or evaporation of leaked or spilled petroleum product. Fugitive emissions are difficult to measure because of the diffuse nature of the emitted air pollutants and the high variability in emission rates. In the case of fugitive emissions from refinery drains, a U.S. EPA sponsored study concluded that they represented only 2.2 per cent of fugitive emissions from a hypothetical 330,000 bpd refinery. The study examined 49 process drains at 13 refineries and assumed that the drains were not sealed. Based on that assessment, the U.S. EPA allocated an emission factor (often referred to as the AP-42 factor) of 0.070 total non-methane hydrocarbons (TNMHCs) lb/hr per drain, despite the associated high, ten-fold, variability (95 percent confidence limits of 0.023 to 0.20).