Each year in the United States, 10,000 people die from pollution created by gasoline engines [source: Reilly]. Among other irritants and pollutants, gasoline releases nitrogen oxide and acetaldehyde. Acetaldehyde molecules react with sunlight to form smog. These emissions make thousands of people sick every year with respiratory ailments and cancers [source: Reilly]. Biofuels discharge fewer toxins into the air than fossil fuels. When compared to conventional diesel, biodiesel reduces smog-forming particulate matter, which reduces cases of asthma and other respiratory illnesses. In addition, biodiesel doesn't emit any sulfur oxides and sulfates, which contribute to acid rain.
For its part, ethanol generally burns better and more robustly than gasoline, generating less pollution. When compared with gasoline, an E85 fuel blend (15 percent ethanol, 85 percent gasoline) burned in an efficient engine produces fewer toxins, including 40 percent less carbon dioxide; 20 percent less particulate matter; and 80 percent fewer sulfates [source: Energy Future Coalition]. However, scientists at Stanford University in California say ethanol releases many of the same pollutants as gasoline. In addition, a large amount of unburned ethanol escapes into the air, forming acetaldehyde molecules and ultimately smog. But, engines fueled by ethanol emit fewer cancer-causing substances than gasoline [source: Reilly].