Winter Air Quality
Under the Clean Air Act Amendments of 1977, Yellowstone is one of 156 national parks and wilderness areas that are designated Class I airsheds, requiring the most stringent protection. Although park visitation is far lower in winter than in summer, oversnow vehicles (OSVs) produce more emissions. Carbon monoxide and particulate matter are monitored at the West Entrance and at Old Faithful, where OSVs are most concentrated. The levels of these pollutants have declined since 2002 because of fewer snowmobiles in the park and because they are required to have “Best Available Technology” (BAT) and be accompanied by a licensed guide.