Man-made mineral fibers (MMMF) and other airborne dusts were measured in 105 rooms in a representative sample of public buildings, excluding rooms with physically damaged boards or buildings with notable indoor climate problems. There were no differences in the MMMF concentrations with respect to the type of binder. The average concentrations ranged from 17 to 210 respirable MMMF/m³. The average concentration of the reference group was intermediate and therefore indicated that sources other than ceiling boards contributed to the obtained values. No grouping by concentration of MMMF on cupboards was possible. Airborne concentrations of respirable MMMF were 2.7 times lower in rooms with mechanical ventilation than in rooms with natural ventilation. For nonrespirable MMMF, the most important factor was the quality of the cleaning. The concentration in poorly cleaned rooms was 5.5 times that of well-cleaned rooms. Ventilation, quality of cleaning, and number of persons affected the non-MMMF and total dust concentrations.