Nearly all metal dusts contain iron, which is one of the most common constituents in steel. Iron has one common ferrimagnetic oxide, magnetite. Although it is considered nontoxic, magnetite can be used as a tracer for more active and pathogenic constituents of inhaled dusts. After the chest area has been magnetized, magnetite produces a weak remanent magnetic field around the body. The field is measured and the amount of dust in the lungs is calculated on the assumption that the composition of lung dust is the same as that of airborne dust. This method has been applied to shipyard and stainless steel welders, to iron and steel workers, to stainless grinders, and to foundry workers. A clear magnetic signal was found for each subject. The maximum amount of lung dust found thus far was 10 g in the lungs of a stainless steel electrode welder. Referents showed signals of 0.5 mg magnetite or less.