The Mexican government is replacing methyl tertiery butyl ether (MTBE), an additive that increases the oxygen content of fuel, with ethanol, which is made from sugarcane. This is because MTBE has been known to contaminate groundwater and has been banned in the US.

The government announced this decision after traces of MTBE was discovered in aquifers, which are the city's main source of drinking water. Mexico's gasoline producer, the state-owned Pemex, has been keeping the contents of its gasoline a secret. Environmentalists allege that it has been using MTBE, which is leaking into the city's water supplies. Even the government has now started criticising the quality of Pemex's fuel. However, the company says that traces of MTBE in petrol pose no health risk. Pemex also asserts that its fuel meets international environmental standards and will help in reducing vehicular air pollution. Mexico faces one of the worst air pollution problems in the world, according to the World Health Organisation.