SACRAMENTO – State air quality regulators on Tuesday retreated from plans to win California more time to attain federal air-pollution control standards on soot particles and other fine matter that can cause lung disease. Critics had warned that a delay would affect millions of Californians who live in the smoggiest parts of the state. The reversal by the California Air Resources Board came a day after environmental groups and one lawmaker criticized the agency. On Monday, the groups released a letter from the ARB director saying that the board might ask for a federal waiver on levels of particulate matter 2.5, tiny particles of soot each 2 1/2 microns or less in width. (There are about 25,000 microns in an inch.) PM 2.5 comes primarily from diesel-engine exhaust and can lodge in the lungs. In a letter Tuesday, ARB Executive Director Catherine Witherspoon said her inquiry to federal regulators did not constitute an official board action. 160Want local news?Sign up for the Localist and stay informed Something went wrong. Please try again.subscribeCongratulations! You’re all set! Of concern to environmental groups was a letter Witherspoon had written March 12 in which she argued that the San Joaquin Valley and the South Coast Air Quality Management District couldn’t meet a 2015 deadline set by the federal government. She suggested that federal regulators give the state until 2020. “That letter was misinterpreted by several stakeholders in California as an official petition for a 5-year delay,” Witherspoon said Tuesday.