Silica: freshest dust may be deadliest.

Sand blasters, rock drillers, and silica-flour millers all face a high risk of acute silicosis, a life-threatening inflammatory disease that disables its victims--sometimes in as little as 5 years--by killing lung cells and letting their liquids leak out. Many other workers who inhale the same substance, dust-size pieces of crystalline quartz, develop instead a slowly debilitating scarring of their lungs. Which of these respiratory diseases develops depends on how freshly hewn the inhaled mineral particles were, a new study suggests.

Previously, Val Vallyathan and his coworkers at the National Institute for Occupational Safety and Health in Morgantown, W. Va., showed that freshly cut surfaces of silica dust harbor copious quantities of free radicals (a class of biologically damaging molecules or molecular fragments that contain an unpaired electron). In the presence of moisture, such as that found in the lung, the freshly ground microscopic mineral shards generate large amounts of hydroxyl radicals, the most destructive free radical. In contrast, surfaces of several-months-old particles of silica dust hosted fewer free radicals.

Now, together with NIOSH colleagues in Cincinnati, Vallyathan's team has exposed young male rats to air laced with either freshly cut or 60-day-old silica dust. Animals inhaled the dust-laden air 5 hours a day for 10 days. Then the researchers flushed the materials from the animals' lungs and examined the lungs for signs of inflammation, free radical activity (as measured by the oxidation of lipids), and cell damage.

In the September American Journal of Respiratory and Critical Care Medicine, they report that animals breathing in fresh dust experienced somewhat more inflammation and 40 percent more lipid oxidation than did rats breathing in aged dust. Vallyathan's team also found significantly more signs of cellular breakdown in the materials washed from the lungs of animals exposed to fresh dust.

Taken together, Vallyathan concludes, these data argue that in situations where silica particles will be fractured, such as during rock tunneling or mining, workers should religiously use the respirators and other air-filtering devices provided them. He suggests that employers also consider substituting less toxic materials for silica in sand-blasting operations.