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On December 22, 2017, the U.S. Environmental Protection Agency (EPA) released a response letter indicating that the agency intends to agree with the State of California's designation recommendations for the revised 2015 National Ambient Air Quality Standard (NAAQS) for ozone, which lowered the federal 8-hour ozone standard from 0.075 ppm to 0.070 ppm. California has until February 28, 2018, to submit any additional information for consideration, including 2015-2017 data, before EPA promulgates its final determinations in the spring of 2018. EPA subsequently posted notice of its response in the Federal Register on January 5, which opened a 30-day public comment period.

The proposed nonattainment designations will affect several counties across California, including areas of Indian country. The complete list of California recommendations, EPA's intended nonattainment areas, and further technical analysis can be found in EPA's Technical Support Document.