Smoking impairs long-term dust clearance from the lung.

The time for the long-term clearance of dust from human lungs was measured. Three heavy cigarette smokers and nine nonsmokers inhaled a harmless trace amount of magnetic dust, Fe3O4. From periodic measurements with a sensitive magnetic detector of the amount of this dust remaining in the lungs, a clearance curve was determined for each subject. This magnetic tracer method allows clearance to be safely followed for a much longer time than with radioactive tracer methods. The dust clearance in the smokers is considerably slower than in the nonsmokers. After about a year, 50 percent of the dust originally deposited remained in the lungs of the smokers whereas only 10 percent remained in the lungs of the nonsmokers. The smokers therefore retained five times more dust than the nonsmokers. This impaired clearance of Fe3O4 suggests impaired clearance in smokers of other dusts, such as toxic occupational and urban dusts. The higher retention of these dusts may contribute to the higher incidence of lung diseases in smokers.