but the set of rules governing pollution emissions for off-road vehicles was only released in January 2005. In another example of regulation, only four-stroke snowmobiles are allowed in Yellowstone National Park since a bylaw was recently passed to minimize CO2 emissions and noise. In Yellowstone, snowmobiles account for 80% of total hydrocarbon emissions and 50% of carbon monoxide emissions in the winter. This is just less than 2% and 1% respectively of the overall annual pollution within the park. Snowmobiles are only allowed to be ridden on the unplowed roads used in the summer, and riding off the roads is prohibited. This accounts for less than 1% (0.002%) of the park area. In 2005 the US Forest Service published a Travel Management Rule for off-highway vehicles, strengthening the implementation of Executive Orders issued in the 1970s. However, these rules were not applied to snowmobiles. In 2015, following a decision in a lawsuit brought by Winter Wildlands Alliance against the Forest Service, the rules were extended to snowmobiles. National Forests with sufficient snow for winter recreation are now required to designate where OSVs are allowed to travel and where they are prohibited. In doing so, the Forest Service must minimize 1) damage to soil, watershed, vegetation, and other forest resources; 2) harassment of wildlife and significant disruption of wildlife habitats; and 3) conflicts between motor vehicle use and existing or proposed recreational uses of National Forest System lands or neighboring Federal lands. Air Most snowmobiles are still powered by two-stroke engines, although Alpina and Yamaha have been using four-strokes since 2002 and 2003, respectively. However, in the last decade several manufacturers have been successful in designing less polluting motors, and putting most of them in production. Yamaha and Arctic-Cat were the first to mass-produce four-stroke models, which are significantly less polluting than the early two-stroke machines. Alpina offers only four-stroke EFI engines equipped with a catalytic converter and dual oxygen-probe. Bombardier's E-Tec two-stroke motors emit 85% less pollutants than previous carbureted two-strokes. Polaris has developed a fuel-injection technology called "Cleanfire Injection" on their two-strokes. The industry is also working on a direct-injected "clean two strokes" that is better in terms of NOX emissions. Independent researchers, undergraduates and graduate students participate in contests to lessen the impact of emissions from snowmobiles. The Clean Snow Mobile Challenge is held yearly at Michigan Technological University regrouping the entries from universities from across United States