In western Europe, the second largest market for petrol in the world, concentrations of MTBE in petrol vary from 0 to 15%, depending on petrol grade, oil company, and country. As examples, 98–99 RON petrol grade (high performance, super, super plus) may typically contain 5–13% MTBE, whereas 92–95 RON grade (premium) contains 0.5–8% MTBE.
The use of MTBE as an octane enhancer in the United States began in 1979. By 1990, the Clean Air Act Amendments in the United States required fuel oxygenates, such as MTBE at 15% and ethanol, to be added to petrol in some metropolitan areas heavily polluted by carbon monoxide to reduce carbon monoxide and ozone concentrations. Areas that exceeded the national ambient air-quality standard for carbon monoxide were required to use oxygenated fuels by November 1, 1992. Despite the clear benefits of using oxygenates, there have been restrictions placed on its use in many areas of the United States because of growing numbers of detections of MTBE in drinking water resulting from leaking underground petrol tanks.