More than half a dozen U.S. senators are mounting increased opposition to the Environmental Protection Agency’s (EPA) decision to allow some vehicles to fuel up with higher blends of ethanol in their gasoline.

In two consecutive letters to the EPA, the senators criticized the agency’s decision to allow 15% ethanol blends (E15) in gasoline for model year 2007 and newer vehicles, and asked the agency to first analyze the effects of increased ethanol use on the vehicle fleet. The senators also requested that the EPA conduct an analysis of the effect of the increased use of ethanol on the availability of gasoline.

The letters also state concerns that the partial waiver on E15 could place consumers "in considerable danger" if they used E15 in unapproved engines. They warn of ethanol’s “corrosive properties” and its “tendency to clog motors not designed to accommodate biofuels.”

Oil companies, car makers, the National Petrochemical & Refiners Association and other organizations have filed lawsuits to stop the EPA from allowing the use of E15.