CHEBOYGAN, Michigan — People living near a northern Michigan paper mill that caught fire, filling the air with thick grey smoke, emerged from their homes Thursday after sheltering in place for more than a day, as environmental officials found the local air quality had improved to safe levels. Plumes of smoke could be seen for miles around on Wednesday morning after the fire broke out at the Tissue Depot property in downtown Cheboygan, about 290 miles (466 kilometers) northwest of Detroit. Fire crews contained the blaze to a storage building at the mill on Wednesday afternoon and they remained on the scene Thursday “putting out hot spots,” officials said. Cheboygan residents living with a 1-mile (1.6-kilometer) radius of the Tissue Depot were told to shelter in place until 1:30 p.m. Thursday, when officials lifted that order following air testing by the U.S. Environmental Protection Agency.