According to the U.S. Forest Service the use of old-style two-stroke engines, previously common in vehicles designed for off-road use, also causes concerns about pollution. This is because "two-stroke engines emit about 20 to 33 percent of the consumed fuel through the exhaust" (as the engine lubricant is a "total loss system" and is emitted by design) and "discharge from two-stroke snowmobile engines can lead to indirect pollutant deposition into the top layer of snow and subsequently into the associated surface and ground water".