California’s San Joaquin Valley Air Pollution Control Board unanimously approved a plan yesterday intended to improve some of the worst air quality in the nation — but critics say the plan makes too many concessions to industry to be effective. Enviros, doctors, and residents who have suffered from respiratory ailments allege that efforts to reduce dust and diesel smoke will be fatally crippled by the plan’s failure to place stringent controls on massive dairy farms and farm equipment, and they dispute the board’s claim that the plan will reduce particulate matter by 5 percent per year, as required under the federal Clean Air Act. The critics also note that the plan does not contain any source of funding for enforcement. If the plan is not approved by the U.S. EPA — which rejected last year’s version — the San Joaquin Valley could eventually lose $2 billion in federal highway funds, as well as control over air-pollution regulation.