The best days for hiking in Rocky Mountain National Park occur when high barometric pressure occurs. However, those days have become the likeliest to generate an inversion - a smoggy haze of particulates and ozone. Personally, I need exercise but prefer to avoid aerobic breathing when the level of particulates and ozone is high. Some of the particulates come from vehicles in Estes Valley on the eastern side of Rock, but much comes from vehicles and development along the Front Range, from Greeley to Denver and farther south to Colorado Springs. Some of the particulates come from oil and gas mining easterly from Greeley and from similar activities in Western Slope locales. Circa 2009, the air in eastern RMNP is seldom clear.