Crystalline silica (mostly cristobalite) was produced by vapor-phase crystallization and devitrification in the andesite lava dome of the Soufriere Hills volcano, Montserrat. The sub-10-micrometer fraction of ash generated by pyroclastic flows formed by lava dome collapse contains 10 to 24 weight percent crystalline silica, an enrichment of 2 to 5 relative to the magma caused by selective crushing of the groundmass. The sub-10-micrometer fraction of ash generated by explosive eruptions has much lower contents (3 to 6 percent) of crystalline silica. High levels of cristobalite in respirable ash raise concerns about adverse health effects of long-term human exposure to ash from lava dome eruptions.

Background and Aims:The Soufriere Hills volcano, Montserrat, has been erupting since July 1995 and volcanic ash has fallen on the island throughout most of the eruption. The ash contains substantial quantities of respirable particles and unusually large amounts (15–20%) of the crystalline silica mineral, cristobalite. The purpose of the surveys described here, undertaken between December 1996 and April 2000, was to determine levels of personal exposure of islanders to volcanic ash and cristobalite in order to inform advice on the associated risks to health and the measures required to reduce exposure.Methods:Surveys of personal exposure to respirable dust and cristobalite were undertaken using cyclone samplers. In addition, direct reading instruments (DUSTTRAK) were used to monitor ambient air concentrations of PM10at fixed sites and also to provide information about exposures to airborne particles associated with selected activities.Results:Environmental concentrations of airborne ash have been greatest in the areas where the most ash has been deposited and during dry weather. Individual exposure to airborne ash was related to occupation, with the highest exposures among gardeners, cleaners, roadworkers, and police at roadside checkpoints. During 1997 many of these individuals were exposed to concentrations of cristobalite that exceeded the ACGIH recommended occupational exposure limit. Since the population became confined to the north of the island in October 1997, even those in relatively dusty occupations have received exposures to cristobalite well below this limit.Conclusions:Most of the 4500 people who have remained on island since the eruption began have not been exposed to sufficiently high concentrations of airborne dust for long enough to be at risk of developing silicosis. However, more than a dozen individuals continued to experience frequent high occupational exposures to volcanic ash, some of whom may have had sufficient exposure to crystalline silica to be at risk of developing mild silicosis. If volcanic activity were to deposit further ash over the occupied areas of the island during the coming years, the risks of silicosis will become more substantial.

Most authors consider that long-term inhalation studies represent the most reliable method of obtaining data on the pathogenicity of mineral fibers. However, because of the length of time required and the great cost of inhalation work, other methods of examining the biological effects of mineral fibers have been used. The program of work described here was designed to compare a wide range of techniques, including inhalation, for determining the biological effects of a selection of man-made mineral fibers. Silicon carbide whiskers were examined as a very durable material and also a less durable glass microfiber (code 100/475/ as a relatively soluble man-made vitreous fiber. Amosite asbestos was used as a positive control. long-term inhalation studies with full-life-span follow-up demonstrated that amosite and silicon carbide were both fibrogenic and carcinogenic in rats, while glass microfiber produced very little fibrosis and a few benign pulmonary tumors in numbers similar to those found in controls. Silicon carbide was unusual in that most of the tumors produced were pleural mesotheliomas. Early effects of the fibers were examined after a short period of inhalation, and the pathogenic dusts amosite and silicon carbide were found to produce rapid pulmonary inflammation as determined by the presence of significant numbers of neutrophils in pulmonary lavage fluid. Less pulmonary inflammation followed the inhalation of glass microfiber. Similarly, amosite and silicon carbide inhalation was found to cause a rapid increase in the rate of proliferation of bronchoalveolar lining cells, while the rate of cell division in animals treated with microfiber remained normal. When injected into the peritoneal cavities of rats, all three fiber types produced mesothe liomas, although the glass microfiber produced many fewer and these took much longer to develop. An examination of early inflammation in the peritoneal cavity of mice following injection showed little difference between the fiber types, all of which caused raised numbers of neutrophils in peritoneal fluid. fiber durability was examined both in vivo and in vitro. Following inhalation it was found that fewer very long glass microfibers (>20 pm in length) remained in rats lungs after 12 mo of inhalation than amosite or silicon carbide.Subsequently, however, amosite and microfiber were removed from the lung at similar rates for most fiber dimensions, in contrast to silicon carbide fibers, where clearance was much less. When dust was administered by intratracheal injection, these differences in fiber removal from lung tissue were much less marked. An examination of the in vitro solubility of fibers and their chemical composition after extraction from lung tissue demonstrated that glass microfiber showed significant leaching while amosite showed much less and silicon carbide showed almost no change in chemical composition. The possibility of using short-term in vivo and in vitro tests to provide a screening system to eliminate the need for some of the longer and more expe...

In this article the volumetric overload hypothesis, which predicts the impairment of clearance of particles deposited in the lung in terms of particle volume, is reevaluated. The degree to which simple expressions of retained lung burden explain pulmonary responses to overload was investigated using data from a series of chronic inhalation experiments on rats with two poorly soluble dusts, titanium dioxide and barium sulfate. The results indicated that the difference between the dusts in the level of inflammation and translocation to the lymph nodes could be explained most simply when the lung burden was expressed as total particle surface area. The shape of the statistical relationship for both lung responses indicated the presence of a threshold at approximately 200-300 cm(2) of lung burden. On the basis of this and other similar results, a hypothesis regarding a generic mechanism for the impairment of clearance and associated lung responses is proposed for such "low-toxicity" dusts.

Frequent ash fallout from long-lived eruptions (with active phases greater than 5 years) may lead to local populations experiencing unacceptably high cumulative exposures to respirable particulate matter. Ash from Montserrat has been shown to contain significant levels of cristobalite and other reactive agents that are associated with an increased risk of developing pneumoconiosis (including silicosis) and other long-term health problems. There are a number of difficulties associated with estimating risks in populations due to uncertain and wide ranging individual exposures, change in behaviour with time and the natural variation in individual response. Present estimates of risk in workers and other population groups are simplifications based on a limited number of exposure measurements taken on Montserrat (1996Montserrat ( -1999, and exposure−response Editorial responsibility: J Stix curves from epidemiological studies of coal workers exposed to siliceous dust. In this paper we present a method for calculating the long-term cumulative exposure to cristobalite from volcanic ash by Monte Carlo simulation. Code has been written to generate synthetic time series for volcanic activity, rainfall, ash deposition and erosion to give daily ash deposit values and cristobalite fraction at a range of locations. The daily mean personal exposure for PM 10 and cristobalite is obtained by sampling from a probability distribution, with distribution parameters dependent on occupation, ground deposit depth and daily weather conditions. Output from multiple runs is processed to calculate the exceedance probability for cumulative exposure over a range of occupation types, locations and exposure periods. Results are interpreted in terms of current occupational standards, and epidemiological exposure−response functions for silicosis are applied to quantify the long-term health risk. Assuming continuing volcanic activity, median risk of silicosis (profusion 1/0 or higher) for an average adult after 20 years continuous exposure is estimated to be approximately 0.5% in northern Montserrat to 1.6% in Cork Hill. The occupational group with the highest exposure to ash are gardeners, with a corresponding 2% to 4% risk of silicosis. In situations where opportunities for in-depth exposure studies are limited, computer simulations provide a good indication of risk based on current expert knowledge. By running the code for a range of input scenarios, the cost-benefit of mitigation measures (such as a programme of active ash clearance) can be estimated. Results also may be used to identify situations where full exposure studies or fieldwork would be beneficial.