On September 17, 1999, the U.S. Environmental Protection Agency (EPA) issued a letter (PDF 134K) in anticipation of their approval of a formal State Implementation Plan (SIP) revision submitted by the Air Resources Board regarding the wintertime oxygen requirement in the Lake Tahoe Air Basin. This letter states that the U.S. EPA will exercise enforcement discretion and not enforce a wintertime oxygen requirement in the Lake Tahoe Air Basin during the 1999/2000 winter season (October 1 through January 31).