The U.S. EPA is proposing to approve HFO-1234yf (also known as R-1234yf, HFC-1234yf, and 2,3,3,3-tetrafluoroprop-1-ene) as a substitute for CFC-12 in the air-conditioning systems of passenger cars and light-duty trucks. The Clean Air Act requires the agency to review alternatives for ozone-depleting substances and to approve of substitutes that do not present a risk more significant than other alternatives that are available. Under that authority, the Significant New Alternatives Policy program of EPA proposes to expand the list to include the hydrofluoroolefin substance. The agency says HFO-1234yf does not contribute to stratospheric ozone depletion. The proposal includes several "use conditions" that would allow the substance's deployment for vehicle A/C only if the A/C system incorporates certain mechanical features to prevent leakage and if the automaker conducts failure mode and effects analysis on the system per SAE J1739. The proposal applies to "new" A/C systems and does not address retrofit, although the idea of allowing HFO-1234yf for retrofit is one of many topics on which EPA is seeking comment. Comments on the proposal are due to EPA by Dec. 18 unless EPA schedules a hearing, in which case the hearing would be held Nov. 3 and comments would be due Jan. 4.