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In the United States

There is no "standard" composition or set of specifications for gasoline. In the United States, because of the complex national and individual state and local programs to improve air quality, as well as local refining and marketing decisions, petroleum refiners must supply fuels that meet many different standards. State and local air quality regulations involving gasoline overlap with national regulations and that leads to adjacent or nearby areas having significantly different gasoline specifications. According to a detailed study in 2006, there were at least 18 different gasoline formulations required across the United States in 2002. Since many petroleum refiners in the United States produce three grades of fuel and the specifications for fuel marketed in the summer season vary significantly from the specifications in the winter season, that number may have been greatly understated. In any event, the number of fuel formulations has probably increased quite a bit since 2002. In the United States, the various fuel formulations are often referred to as "boutique fuels". In general, most of the gasoline specifications meet the requirements of the so-called Reformulated Gasoline (RFG) mandated by federal law and implemented by the U.S. Environmental Protection Agency (U.S. EPA).

Some of the major properties and gasoline components focused upon by the various national and state or local regulatory programs are:

• Vapor pressure: The vapor pressure of a gasoline is of concern because evaporative emissions of the hydrocarbons in the gasoline lead to the formation of ozone in the atmosphere which reacts with vehicular and industrial emissions of gaseous nitrogen oxides (NOx) to form what is called photochemical smog. Smog is a combination of the words smoke and fog and traditionally referred to the mixture of smoke and sulfur dioxide that resulted from the burning of coal for heating buildings in places such as London, England during the 19th century and the first half of the 20th century. Modern photochemical smog does not come from coal burning but from vehicular and industrial emissions of hydrocarbons and nitrogen oxides. It appears as a brownish haze over large urban areas and is irritating to the eyes and lungs.

• Nitrogen oxides: Various nitrogen oxides (NOx) are formed during the combustion of gasoline in vehicles and the combustion of other fuels in industrial facilities. NOx is one of the ingredients involved in the atmospheric chemistry that produces photochemical smog and, as such, is a prominent air pollutant. In fact, it is one of the six so-called "criteria air pollutants" that are regulated by National Ambient Air Quality Standards (NAAQS) of the United States. The NOx emitted by vehicular engines using gasoline are largely controlled by the use of on-board devices, called catalytic converters, installed on most modern automobiles and other vehicles. They convert the NOx emissions into gaseous nitrogen and oxygen. They also convert any gaseous carbon monoxide emissions into gaseous carbon dioxide as well as converting any unburned gasoline hydrocarbons into gaseous carbon dioxide and water vapor.

• Toxic metals:
  • Tetra-ethyl lead (TEL)  —  In the 1920's, petroleum refining technology was rather primitive and produced gasolines with an octane rating of about 40 – 60. But automotive engines were rapidly being improved and required better gasolines, which led to a search for octane rating enhancers. That search culminated in 1924 in the development and widespread usage of tetra-ethyl lead (TEL), a colorless, viscous liquid with the chemical formula of (CH₃CH₂)₄Pb. TEL became commercially available as what was called TEL fluid, which contained 61.5 weight % TEL. The addition of as little as 0.8 ml of that TEL fluid per litre (equivalent to 0.5 gram of lead per litre) of gasoline resulted in significant octane rating increases. For about the next 50 years, TEL was used as the most cost effective way to raise the octane rating of gasolines. During that period, petroleum refining technology grew until high-octane gasolines could, in fact, be produced without using TEL. Also, in about the 1940's, it was discovered that the lead being emitted in the exhaust gases from vehicular internal combustion engines was a toxic air pollutant that seriously affected human health. Because of its toxicity and the fact that catalytic converters being installed in vehicles could not tolerate the presence of lead, the U.S. EPA launched an initiative in 1972 to phase out the use of TEL in the United States and it was completely banned for use in on-road vehicles as of January 1996. Using TEL in race cars, airplanes, marine engines and farm equipment is still permitted. TEL usage has also been phased out by most nations worldwide. As of 2008, the only nations still allowing extensive use of TEL are the Democratic People's Republic of Korea, Myanamar, and Yeman.
  • Methylcyclopentadienyl manganese tricarbonyl (MMT)  —  In Canada, MMT has been used as an octane enhancer in gasoline since 1976. It is also permitted for use as a gasoline octane enhancer in Argentina, Australia, Bulgaria, France, Russia, United States and conditionally in New Zealand. MMT is a yellow liquid with chemical formula of (CH₃C₅H₄)Mn(CO)₃. According to the U.S. EPA, ingested manganese is a required element of the diet at very low levels but it is also a neurotoxin and can cause irreversible neurological disease at high levels of inhalation. The U.S. EPA has a concern that the use of MMT in gasoline could increase inhalation manganese exposures. After completing a 1994 risk evaluation on the use of MMT in gasoline, the U.S. EPA was unable to determine if there is a risk to the public health from exposure to emissions of MMT gasoline. As of now (2009), gasoline in the United States is allowed to contain MMT at a level equivalent to 0.00826 g/L (1/32 g/gallon) of manganese. However, there are still many concerns about the possible adverse health effects from the usage of MMT and less than one percent of the gasoline marketed in the United States contains MMT.

• Other toxic compounds: Gasoline contains some benzene (C₆H₆) which is an aromatic compound that is a known human carcinogen. For that reason, the amount of benzene in gasoline is limited by environmental regulations. In general, the combustion of aromatics can lead to the formation of other compounds that have deleterious effects on human health, such as aldehydes, butadiene, and polycyclic aromatic hydrocarbons (PAHs). Therefore, the total amount of aromatics in gasoline is also limited by environmental regulations.

• Olefins: Photochemical smog is formed by various atmospheric chemistry reactions between nitrogen oxides and what are called reactive hydrocarbons in the presence of sunlight. In the context of photochemical smog formation, some hydrocarbons are more reactive than others. For example, olefins are very reactive and methane is not reactive to any extent. For that reason, the olefin content of gasolines is limited by environmental regulations.

• Sulfur: Any sulfur compounds in gasoline will result in combustion exhaust emissions of sulfur dioxide to the atmosphere. Such emissions contribute to the formation of so-called acid rain and they also interfere with the on-board catalytic converters and reduce their efficiency. Therefore, the sulfur content of gasoline is limited by environmental regulations.

• Oxygen: Oxygen-containing compounds called oxygenates such as ethanol (with a chemical formula of C₂H₅OH) or methyl tertiary-butyl ether (MTBE) (with a chemical formula of C₅H₁₂O) are added to gasolines for two reasons. The first reason is that the oxygen reduces the emissions of unburned hydrocarbons as well as the emissions of carbon monoxide. The second reason is that they significantly enhance the octane rating of gasolines which makes up for the octane rating loss resulting from the limiting of the high-octane aromatics and olefins as well as the banning of TEL usage. MTBE was widely used during the 1990s as an oxygenate in the United States until it was found to be polluting underground water supplies. In the United States, it has now been largely replaced as an oxygenate by ethanol. Gasolines containing ethanol are now sold in every state in the United States and nearly half of the gasoline sold in the United States now contains up to 10 volume % ethanol either as an octane enhancer or to meet air quality requirements.

As mentioned earlier above, there are a great many different sets of specifications or standards for gasolines marketed in the United States. The specifications tabulated below are those that have been mandated by law in the state of California. They are known as the California Reformulated Gasoline (CaRFG) Phase 3 Standards and are perhaps the most environmentally restrictive specifications in the United States:

California Reformulated Gasoline (CaRFG) Phase 3 Standards
Effective as of August 29, 2008

Property	Measurement unit	Flat Limit (a)	Average Limit (a)
Reid vapor pressure (b)	psi (c)	7.00 or 6.90 (d)	not applicable
Sulfur concentration	ppmw	20	15
Benzene concentration	ppmv	0.8	0.7
Aromatics concentration	ppmv	25.0	22.0
Olefins concentration	ppmv	6.0	4.0
Temperature at 50 volume % distilled (T50)	°F (e)	213	203
Temperature at 90 volume % distilled (T90)	°F	305	295
Oxygen concentration	weight % (f)	1.8 – 2.2	not applicable
Oxygenates other than ethanol	--	prohibited	not applicable
(a)"Flat" limits apply to every batch of finished gasoline. "Average" limits allow specific batches to exceed the "flat" limits as long as the gasoline produced over a 180-day period meets the "average" limits and never exceeds the specified "cap" limits. (b) Reid vapor pressure (RVP) is measured as per ASTM method D-323 and differs slightly from the true absolute vapor pressure. (c) 1 psi = 6.89 kPa (d) The Reid vapor pressure flat limit of 6.90 psi applies when a California gasoline producer or importer uses the CaRFG Phase 3 Predictive Model to certify a gasoline blend not containing ethanol. Otherwise, the 7.0 psi limit applies. (e) °C = (°F − 32)(5/9) (f) Volume % ethanol in gasoline = [(0.3529/weight % oxygen) − 0.0006] −1. Thus, 1.8 – 2.2 weight % oxygen in a gasoline equals 5.1 – 6.3 volume % ethanol in the gasoline.

Blendstock for Oxygenate Blending (BOB)

Some water usually exists in today's gasoline pipeline systems and in many gasoline storage facilities. Ethanol is very soluble in water and the resulting aqueous solutions of ethanol are very corrosive. For that reason, ethanol is not blended into gasoline at the producing petroleum refineries. Instead, ethanol is blended into gasoline at terminals near the end user markets.

In other words, to meet the current specification required of reformulated gasolines, petroleum refiners in the United States basically produce blending stocks to which ethanol is added at terminals or other points at or near the end-user markets. A blendstock to be used in producing a reformulated gasolines is known as a BOB (Blendstock for Oxygenated Blending). A BOB to be used in producing a reformulated gasoline meeting the specifications mandated by the U.S. EPA is known as an RBOB. A BOB to be used in producing reformulated gasolines meeting the California specifications is known as a CaRBOB or CARBOB.