An Eruption of Silicosis.

Volcanoes may pose long-term health hazards, according to an article in the 19 February 1999 issue of Science by researchers studying the aftermath of the 1997 eruption of the Soufriere Hills volcano in Montserrat. "Ash is an often neglected hazard, particularly the longer-term effects," says coauthor Ray Dupree, a physicist at the University of Warwick in Coventry, United Kingdom. According to the researchers, certain types of eruptions--like that of Soufriere Hills--may be potentially more hazardous than others in terms of the ash produced.

Scientists have long known that volcanic ash creates minor health problems such as eye irritation and exacerbates existing problems such as asthma. But the study, led by community medicine specialist Peter Baxter of the University of Cambridge, reveals that certain types of volcanic ash may create long-term problems such as silicosis, an irreversible scarring disease of the lungs that occurs over long periods of exposure. The study is the first to link a technical mechanism of eruption to a specific health hazard, and is also one of the most detailed studies to date to quantify and define the ratio of silica minerals in volcanic ash versus glass and other particles.

In the ash generated at Soufriere Hills by pyroclastic flows--mixtures of volcanic ash and gases that hug the ground and flow down valleys--the team found high levels of a form of crystalline silica called cristobalite, which is known to cause silicosis. After assaying the ash around the island, the team found that the majority of particles under 10 micrometers in diameter contained 10-24% crystalline silica, mainly cristobalite, by weight. Moreover, large amounts of the particles were less than 3 micrometers in diameter--small enough to penetrate deep into the lungs.

The team also found that the hazard is increased during long-lived eruptions, which spew ash over years or decades. Dome eruptions, in which a mass of magma builds up over the volcano's vent, generating myriad pyroclastic flows and ash plumes, also increase the hazard because they produce more fine ash and more fine crystalline silica. "The amount of fine respirable ash generated in such eruptions and the content of crystalline silica in the ash are significantly increased by crushing processes and transport processes in the pyroclastic flows formed by dome [eruptions]," explains Bristol University geologist and volcanologist Steve Sparks, a study coauthor. In contrast, the 1980 eruption of Mount St. Helens in Washington State produced short-lived ashfalls with vastly fewer fine particles containing only 4% cristobalite by weight.

The team measured suspended particle concentrations in areas where there was human activity and found that such activity is an important factor in resuspending ash and increasing exposure of populations, Sparks says. Reoccupation of some of the devastated and evacuated areas was delayed because of the poor air quality conditions caused by the ash, which had to be removed before people were allowed back.

Laboratory experiments indicate that cristobalite can be more toxic than quartz, a known cause of silicosis in workers with long-term exposure to silica dusts, but the reason for cristobalite's increased toxicity is not known, says Sparks. Cristobalite toxicity varies greatly according to the age of the stone, heating, and other treatments, he adds.

Although dome eruptions are fairly common worldwide, the study findings may not apply to domes of other volcanoes, especially those of different chemical compositions, which may produce different minerals in varying percentages. Plus, other hazards associated with the creation of volcanic clouds, including hydrochloric and hydrofluoric acid gases and sulfuric acid particles, are largely unstudied.

Indeed, much work remains to determine the specific long-term health risks volcanoes pose to humans. Even though follow-up studies such as a chest X ray survey of islanders most exposed to ash are being undertaken, getting hard data on human exposure and health effects may take years. "Conditions like silicosis take very long times to develop," Sparks says.