The dramatic increase in U.S. and Canadian crude oil production in recent years, coupled with the increase in crude oil transport by rail, has raised questions about whether properties (e.g., flammability) of these crude types—particularly Bakken crude oil from North Dakota and Canada's oil sands—differ sufficiently from other crude oils to warrant any additional handling considerations. The U.S. Pipeline and Hazardous Materials Safety Administration (PHMSA) recently fined several oil companies for improperly classifying their crude oil rail shipments. Potential safety concerns have similarly been raised over pipeline shipments of crude oil from Canada's oil sands projects. Crude oil is highly variable and can exhibit a wide range of physical and chemical properties. In fact, crude oil samples drawn from the same oil field can vary significantly. Lighter oils may be prone to ignite more readily than heavier crude oils depending on the range of light hydrocarbons they contain. In addition to flammability, other factors such as specific gravity (density) and entrained gases may also play important factors in rail car loading, and corrosivity and sulfur content may affect rail car structural integrity. Bakken crude oil (traded as North Dakota Light) is a light sweet crude oil high in light-end paraffinic range hydrocarbons, as well as heavy-end asphaltic range hydrocarbons. Light sweet crudes, like Bakken, are easier to process directly into gasoline and middle-distillate fuels (e.g., diesel) than heavier crude oils. PHMSA has issued a Safety Alert to notify emergency responders, shippers, carriers, and the public that recent derailments and resulting fires indicate that the type of crude oil transported from the Bakken region of North Dakota may be more flammable than traditional heavy crude oil. Under PHMSA's "Operation Classification," as it is officially known, tank car inspections will determine whether a tank car's contents are properly classified based on factors that include volatility , corrosivity , hydrogen sulfide content and the composition or concentration of entrained gases inside the contents. PHMSA is reinforcing the requirement to properly test, characterize, classify, and, where appropriate, sufficiently degasify hazardous materials prior to and during transportation. "Operation Classification" will be an ongoing effort, and PHMSA will continue to collect samples and measure the characteristics of Bakken crude as well as oil from other locations. This report discusses the properties that PHMSA addressed in its safety alert, provides background on the composition of various U.S. crude oils, and compares Bakken crude oil to other crude oil produced or transported in the United States. For further background on crude oil transport by rail, see CRS Report R43390, U.S. Rail Transportation of Crude Oil: Background and Issues for Congress , by [author name scrubbed] et al. There is some risk with mixing any type of crude oil with air in the proper proportion, in the presence of a source of ignition, which can cause rapid combustion or an explosion. This proportion is the "flammable range" and or alternatively the "explosive range." The flammable range includes all concentrations of flammable vapor or gas in air, in which a flash will occur or a flame will travel if the mixture is ignited at or above a certain temperature (flash point). The Lower Flammable Limit (LFL) is the minimum concentration of vapor or gas in air below which propagation of flame does not occur on contact with a source of ignition. The Upper Flammable Limit (UFL) defines the maximum proportion of vapor in air above which propagation of flame does not occur. The terms Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL) are used interchangeably with LFL and UFL. Liquids having a flash point at or above 100"F (37.8"C) are classed as "combustible" and below 100"F (37.8"C) as "flammable." Crude oils may differ in terms of combustibility or flammability depending upon their volatile components. The Occupational Safety and Health Administration (OSHA) requires manufacturers and shippers of hazardous materials to provide Material Safety Data Sheets (MSDS) that must include the material's fire and explosive properties, among other properties. PHMSA requires similar reporting information. According to a Material Safety Data Sheet prepared by Cenovus Energy, Bakken crude has a flash point of 95" F, making it a flammable liquid. Cenovus lists benzene in concentrations of 0.1% to 1% by volume, which is relatively high compared to other crudes. Benzene is a naturally occurring hydrocarbon in the gasoline range and suspected as a cause of Bakken crude's low flash point. The PHMSA alert reminds emergency responders that light sweet crude oil, such as that coming from the Bakken region, poses significant fire risk if released from the package (tank car) in an incident. Crude oil falls into Department of Transportation (DOT) packing group (PG) I—most serious hazard—or II—moderate hazard. In the recent Lac-Megantic (Quebec) train derailment involving crude shipped from North Dakota, the shippers had reportedly mislabeled the oil as PG III (low hazard). In November 2013, the Association of American Railroads (AAR) urged PHMSA to increase federal tank car safety by requiring higher standards for DOT-III tank non-pressure cars built to transport flammable liquids, and all existing cars to be retrofitted to this higher standard or phased out of flammable service. (See " Rail Car Capacity and Load Limit " regarding DOT-III tank cars.) AAR specifically recommended that PHMSA increase design standards for new cars, or require a retrofit of existing cars to eliminate the option for rail shippers to classify a flammable liquid with a flash point between 100 and 140 degrees Fahrenheit as a combustible liquid. Volatility refers to petroleum's evaporation characteristics. ASTM D323-08 S tandard Test Method for Vapor Pressure of Petroleum Products (Reid Method ) is used to determine the vapor pressure at 37.8°C (100°F) of petroleum products and crude oils with initial boiling point above 0°C (32°F). (ASTM refers to the American Society for Testing and Materials.) Vapor pressure is an important consideration for both crude oil producers and refiners in determining general handling and initial refinery treatment. Vapor pressure also serves as an indirect measure of the evaporation rate of volatile petroleum solvents; with higher vapor pressures indicating greater losses from evaporations. The New York Mercantile Exchange (NYMEX) contract specifications for crude oil futures contracts restrict Reid Vapor Pressure (RVP) to less than 9.5 psi at 100" F. Bakken crude oil has an RVP of at least 8.75. Corrosivity due to the presence of naphthenic acids in crude oil is a particular concern for refineries, and had been raised as an issue in permitting the Keystone XL pipeline to transport Canadian oil sand derived crude oil. It is measured as the number of milligrams of potassium hydroxide (mgKOH/g) needed to neutralize the acids in one gram of oil, and reported as Total Acid Number (TAN). As a rule-of-thumb, crude oils with a TAN greater than 0.5 are considered potentially corrosive. Bakken crude oil has a TAN of less than 0.1. The specifications for DOT-III non-pressure tank cars that haul crude oil require a lining with acid-resistant rubber or other approved rubber compound vulcanized or bonded directly to the metal tank. In general, pipelines transporting hazardous liquids must use inhibitors to mitigate internal corrosion. In the specific case of the proposed Keystone XL pipeline, the potential for corrosion is linked to the basic sediment and water entrained in the crude oil. A crude oil's free sulfur content is an indication of potential corrosiveness from the formation of acidic sulfur compounds. Sulfur oxides released into the air during combustion of refined petroleum products are also a major air pollutant. During the decomposition of organic matter that occurs with hydrocarbons in some geologic formations, sulfur may chemically combine with hydrogen to form hydrogen sulfide gas (H 2 S), a highly corrosive, flammable, and toxic gas. Oil and gas reservoirs with high concentrations of H 2 S can be particularly problematic to produce. H 2 S causes sulfide-stress-corrosion cracking in the standard steel casing and valves used to construct oil wells, and thus require a switch to costly stainless steel. PHMSA has a similar concern for sulfide-stress-corrosion in tank cars. During drilling, detection of H 2 S could result in abandoning the well due to concerns for worker safety. (Worker exposure to no more than 0.03 ppm for up to 8 hours is generally considered safe.) Sulfur content is measured as an overall percentage (by weight) of free sulfur and sulfur compounds in a crude oil. Total sulfur content in crude oils generally ranges from below 0.05% to 5%. Crude oils with less than 0.5% free sulfur or other sulfur-containing compounds are typically referred to as "sweet," and above 0.5% sulfur as "sour." Light-sweet crude, however, may contain H 2 S. Bakken crude oil is sweet (below 0.25% free sulfur); however, H 2 S may occur at problematic levels. In May 2013, Enbridge Energy Partners (a crude oil shipper) detected H 2 S at concentrations of 1,200 parts-per-million in a crude oil storage tank.