Fuming over stainless steel.

Welders value the bonds they forge when joining pieces of stainless steel; such joints tend to be stronger than welds in mild (ordinary) steel. Lungs also can discriminate between those bonds -- or at least the metals vaporized during their formation, a new study suggests. It shows that welding fumes from stainless steel are more irritating and linger longer.

It should probably come as no surprise that different metals behave differently in the lung. Yet until now, notes James M. Antonini of Harvard School of Public Health in Boston, welders and toxicologists generally have not differentiated between types of fumes when investigating why welders suffer more respiratory disease than the general population.

His team took fume particles collected by air filters during welding, added them to salt water, and then spritzed this mix into the airways of rats. At regular intervals, they killed the animals and looked for signs of the metal and any toxicity.

Overall, 35 days after exposure, some 20 percent of the mild steel particles remained in the animals' lungs, compared to fully 50 percent of the stainless steel debris. Lungs of animals that had inhaled stainless steel fume particles also remained more inflamed at 35 days, as evidenced by their hosting five times as many inflammation-provoking cells (neutrophils).

Finally, adapting a new type of computer-enhanced microscopy to their studies enabled the Harvard researchers to map in three dimensions where the various fume particles settled and to study patterns of their removal over time.

The good news, Antonini says, is that while the fumes are toxic, the lungs can clear them out.