Two mathematical models for predicting dispersion of particles in the human lung.

The dispersion of particles in the human lung is modeled as a series of virtual mixing tanks. Using the experimental results of Scherer et al. (1975, J. Appl. Physiol., 38(4), pp. 719-723) for a five-generation glass lung model, it is shown that each generation of the glass lung behaves like an independent virtual mixing tank. The corresponding resident time distribution is shown to have a variance approximately equal to the square of the average time a particle spends in the generation. By assuming that each generation of the human lung behaves as an independent virtual mixing tank, the realistic lung data provided by Weibel (1963, Morphometry of the Human Lung, Spinger-Verlag, New York) are used to validate this assumption in two ways. First, the half-width of the exhaled particle concentration profile is obtained. Second, a system of differential equations, with the concentration of particles in each mixing tank as its solution, is derived and solved numerically. This gives the exhaled concentration profile. Both techniques yield similar results to each other, and both give excellent agreement with the experimental data. The virtual mixing tank approach allows the complex mixing that occurs in the branching pathways of the lung to be more simply modeled. The model, thereby derived, is simple to change and could lead to enhancements in the understanding of the underlying processes contributing to the ventilation of the lung in health and disease.

Estimating historical respirable crystalline silica exposures for Chinese pottery workers and iron/copper, tin, and tungsten miners.

Collaborative studies of Chinese workers, using over four decades of dust monitoring data, are being conducted by the National Institute for Occupational Safety and Health (NIOSH) and Tongji Medical University in China. The goal of these projects is to establish exposure-response relationships for the development of diseases such as silicosis or lung cancer in cohorts of pottery and mine workers. It is necessary to convert Chinese dust measurements to respirable silica measurements in order to make results from the Chinese data comparable to other results in the literature. This article describes the development of conversion factors and estimates of historical respirable crystalline silica exposure for Chinese workers. Ambient total dust concentrations (n>17000) and crystalline silica concentrations (n=347) in bulk dust were first gathered from historical industrial hygiene records. Analysis of the silica content in historical bulk samples revealed no trend from 1950 up to the present. During 1988-1989, side-by-side airborne dust samples (n=143 pairs) were collected using nylon cyclones and traditional Chinese samplers in 20 metal mines and nine pottery factories in China. These data were used to establish conversion factors between respirable crystalline silica concentrations and Chinese total dust concentrations. Based on the analysis of the available evidence, conversion factors derived from the 1988-1989 sampling campaign are assumed to apply to other time periods in this paper. The conversion factors were estimated to be 0.0143 for iron/copper, 0.0355 for pottery factories, 0.0429 for tin mines, and 0.0861 for tungsten mines. Conversion factors for individual facilities within each industry were also calculated. Analysis of variance revealed that mean conversion factors are significantly different among facilities within the iron/copper industry and within the pottery industry. The relative merits of using facility-specific conversion factors, industry-wide conversion factors, or a weighted average of the two are discussed. The exposure matrix of the historical Chinese total dust concentrations was multiplied by these conversion factors to obtain an exposure matrix of historical respirable crystalline silica concentrations.

Ultrafine beryllium number concentration as a possible metric for chronic beryllium disease risk.

Beryllium is a lightweight metal which causes a chronic granulomatous lung disease among workers who become sensitized to it. Recent research has shown a persistence of the disease despite efforts at control with mean exposures below the Occupational Safety and Health Administration (OSHA) occupational exposure limit of 2 microg/m3. Results of our current research confirm a previous finding in certain plants that particle number concentrations are higher in areas where historical estimate of risk showed a high risk of disease despite relatively lower mass concentrations. By providing side-by-side measurements of both particle number and mass, this research adds support to the proposal that particle number rather than particle mass may be more reflective of target organ dose and subsequently a more appropriate measure of exposure for chronic beryllium disease. Our evidence also shows that particle mass exposure measurements and particle number exposure measurements were not correlated.

Lung cancer risk, silica exposure, and silicosis in Chinese mines and pottery factories: the modifying role of other workplace lung carcinogens.

BACKGROUND: Aims of our study were to explore whether and to what extent exposure to other lung carcinogens, or staging and clinical features of silicosis modify or confound the association between silica and lung cancer. METHODS: We used data from a nested case-control study, conducted in the late 1980s in 29 Chinese mines and potteries (10 tungsten mines, 6 copper and iron mines, 4 tin mines, 8 pottery factories, and 1 clay mine), that included 316 lung cancer cases and 1,356 controls, matched by decade of birth and facility type. The previous analysis of these data presented results by type of mine or factory. RESULTS: In our study, pooling all 29 Chinese work sites, lung cancer risk showed a modest association with silica exposure. Risk did not vary after excluding subjects with silicosis or adjusting the risk estimates by radiological staging of silicosis. Strong correlation among exposures prevented a detailed evaluation of the role of individual exposures. However, lung cancer risk was for the most part absent when concomitant exposure to other workplace lung carcinogens, such as polycyclic aromatic hydrocarbons (PAHs), nickel or radon-daughters, was considered. The cross classification of lung cancer risk by categories of exposure to respirable silica and total respirable dust did not show an independent effect of total respirable dust. Silicosis showed a modest association with lung cancer, which did not vary by severity of radiological staging, or by radiological evidence of disease progression, or by level of silica exposure. However, among silicotic subjects, lung cancer risk was significantly elevated only when exposure to cadmium and PAH had occurred. CONCLUSIONS: Our results suggest that, among silica-exposed Chinese workers, numerous occupational and non-occupational risk factors interact in a complex fashion to modify lung cancer risk. Future epidemiological studies on silica and lung cancer should incorporate detailed information on exposure to other workplace lung carcinogens, total respirable dust, and on surface size and age of silica particles to understand whether and to what extent they affect the carcinogenic potential of silica.

Estimating factors to convert Chinese 'Total Dust' measurements to ACGIH respirable concentrations in metal mines and pottery industries.

Historical data on the dust exposures of Chinese workers in metal mines (iron/copper, tin, tungsten) and pottery industries are being used in an ongoing joint Chinese/United States epidemiological study to investigate the exposure-response relationship for the development of silicosis, lung cancer, and other diseases. The historical data include 'total dust' concentrations determined by a Chinese method. Information about particle size distribution and the chemical and mineralogical content of airborne particles is generally not available. In addition, the historical Chinese sampling strategy is different from a typical American eight-hour time-weighted average (TWA) sampling strategy, because the Chinese samples were collected for approximately 15 minutes during production so the sample could be compared to their maximum allowable concentration (MAC) standard. Therefore, in order to assess American respirable dust exposure standards in light of the Chinese experience, factors are needed to convert historical Chinese total dust concentrations to respirable dust concentrations. As a part of the joint study to estimate the conversion factors, airborne dust samples were collected in 20 metal mines and 9 pottery factories in China during 1988 and 1989 using three different samplers: 10mm nylon cyclones, multi-stage 'cassette' impactors, and the traditional Chinese total dust samplers. More than 100 samples were collected and analysed for each of the three samplers. The study yielded two different estimates of the conversion factor from the Chinese total dust concentrations (measured during production processes) to respirable dust concentrations. The multivariate analysis of variance (MANOVA) reveals that, with a fixed sampling/analysis method, conversion factors were not statistically different among the different job titles within each industry. It also indicates that conversion factors among the industries were not statistically different. However, the two estimates consistently showed that conversion factors were the lowest in the pottery industry. Average conversion factors were then calculated for each of the estimates across the industries studied. A pooled mean conversion factor, 0.25+/-0.04, was then derived for all the job titles and industries. Respirable dust levels were estimated from the historical 'total dust' concentrations collected between 1952 and 1992 by adopting the American standard.

A nested case-control study of lung cancer among silica exposed workers in China.

In an attempt to assess whether silica induces lung cancer, a nested case-control study of 316 male lung cancer cases and 1352 controls was carried out among pottery workers and tungsten, copper-iron, and tin miners from five provinces in south central China. Exposure to dust and silica for each study subject was evaluated quantitatively by cumulative exposure measures based on historical industrial hygiene records. Measurements on confounders such as inorganic arsenic, polycyclic aromatic hydrocarbons (PAHs), and radon were also collected from the worksites. Information on cigarette smoking was obtained by interviews of the subjects or their next of kin. A significant trend of increasing risk of lung cancer with exposure to silica was found for tin miners, but not for miners working in tungsten or copper-iron mines. Concomitant and highly correlated exposures to arsenic and PAHs among tin miners were also found. Risk of lung cancer among pottery workers was related to exposure to silica, although the dose-response gradient was not significant. Risks of lung cancer were significantly increased among silicotic subjects in iron-copper and tin mines, but not in pottery factories or tungsten mines. The results of this study provide only limited support for an aetiological association between silica and lung cancer.

A personal cascade impactor: design, evaluation and calibration.

A cascade impactor has been developed that is suitable for personal sampling. The impactor can be used with four, six, or eight stages with cut points ranging from 0.5 micron to 21 micron aerodynamic diameter. Although the impactor can be operated over a fairly wide flow rate range, it is designed specifically to be operated at 2 L/min so that a personal sampler may be used as the air mover. The nozzles of the impactor are radial slots with six slots per stage for the first six stages. Circular nozzles arranged in a radial pattern are used for the last two stages. On each stage, the area between the nozzles is used as the impaction surface for the stage before it. This design allows for the impactor to be compact, rugged and lightweight (170 gm for the four-stage design). The impactor stages are contained in a mounting bracket that can be clipped to the lapel or pocket. Although the impactor is designed for personal use, it also can be used as a compact impactor for general sampling tasks. The particle cut-off characteristics of each stage and the interstage losses were determined using monodisperse aerosols. The particle losses were found to be low and the cut-off characteristics sharp. Thus, size distributions can be obtained as accurately as with larger cascade impactors.

Particle size distributions in underground coal mines.

This paper reports the results of a sampling program undertaken by NIOSH to determine the particle size distributions in underground coal mines. The program made use of newly developed cascade impactors capable of being operated with personal sampling pumps. Particle size distribution samples were collected at selected areas in 10 continuous mining sections in three states. Results plotted as mass frequency distributions indicate that a bimodal distribution may exist in areas away from the operations of the continuous miner. A primary size mode of about 17 micron and a secondary size mode of about 5 micron were measured consistently throughout the 10 mines studied.

Leukemia risk among U.S. white male coal miners. A case-control study.

The relevance of occupational exposure to electrical and magnetic fields (EMF) in the etiology of leukemia has been raised in several studies. Underground coal miners represent an occupational group with situationally determined EMF exposure, as high-voltage power distribution lines are strung overhead in the mines and converters and step-down transformers provide power to mining equipment. Risk in occupational exposure to EMF was examined in a case-control study of 40 leukemia decedents and 160 control subjects who died of causes other than cancer or accident and who were matched on age at death. The control subjects were selected from a group of 6,066 persons whose deaths were reported in four National Institute for Occupational Safety and Health cohort mortality follow-up studies. Based on these data, 25 or more years of underground mining, a surrogate of EMF exposure, was found to pose a statistically significant risk for leukemia (International Classification of Diseases [ICD] codes 204 through 207, eighth revision), myelogenous leukemia (ICD 205), and chronic lymphocytic leukemia (CLL) (ICD 204.1). Accumulative exposure to chemical agents probably poses a risk for acute myelogenous leukemia, although this relationship fell short of being statistically significant. Although CLL has not previously been attributed to environmental agents, these data suggest a possible CLL risk from prolonged exposure to EMF.