Health Effects of Occupational Exposure
to Respirable Crystalline Silica

The diameter of a sphere with a density of 1 g/cm³ and with the same velocity (due to gravity) as the particle of interest [EPA 1996]. Particles of a given aerodynamic diameter move within the air spaces of the respiratory system identically, regardless of density or shape [NIOSH 1995a].

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Chronic obstructive pulmonary disease (COPD)

Includes airways diseases such as asthma, chronic bronchitis, and emphysema and is characterized by airways dysfunction [Becklake 1992].

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Clearance

The translocation and removal of deposited particles from the respiratory tract.

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Concentration

The amount of a substance (e.g., dust particles) contained per unit volume of air.

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Confidence interval (CI), confidence limits

A range of values (determined by the degree of presumed random variability in the data) within which the value of a parameter (e.g., a mean or relative risk) is believed to lie with the specified level of confidence. The boundaries of a confidence interval are the confidence limits [Last 1988]. These include the lower confidence limit and the upper confidence limit.

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Crystalline silica (or free silica)

Silicon dioxide (SiO₂). Crystalline refers to the orientation of SiO₂ molecules in a fixed pattern as opposed to a nonperiodic, random molecular arrangement defined as amorphous. The three most common crystalline forms of silica encountered in the workplace environment are quartz, tridymite, and cristobalite [NIOSH 1974].

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ILO category

The determination of profusion of small opacities observed by reading chest radiographs according to classification of pneumoconioses guidelines developed by the International Labour Organization (ILO). The latest classification guidelines were published by the International Labour Office in 1980 [ILO 1980].

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Incidence

The frequency with which new cases of a disease occur in a given time period.

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Incidence rate

The rate at which new events occur in a population. The number of new events (e.g., new cases of a disease diagnosed or reported during a defined period) is divided by the number of persons in the population in which the cases occurred [Last 1988].

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Inhalable dust

The particulate mass fraction of dust in the work environment that can be inhaled and deposited anywhere in the respiratory tract.

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Nontuberculous mycobacteria

Mycobacteria species other than the Mycobacterium tuberculosis complex (e.g., Mycobacterium avium complex).

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Prevalence

The number of disease cases in a specific population at a particular time [Last 1988].

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Prevalence rate (ratio)

The total number of all individuals with an attribute or disease at a given time or during a given period divided by the population at risk of having the attribute or disease at this point in time or midway through the period [Last 1988].

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Proportionate mortality ratio (PMR)

Ratio of the proportion of deaths from a specific cause in an exposed population compared with the corresponding ratio in the nonexposed population. For example, the proportion of deaths from disease X in the exposed population could be compared with the proportion of deaths from disease X in the nonexposed population [NIOSH 2000].

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Quartz

Crystalline silicon dioxide (SiO₂) not chemically combined with other substances and having a distinctive physical structure.

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Respirable crystalline silica

That portion of airborne crystalline silica that is capable of entering the gas-exchange regions of the lungs if inhaled; by convention, a particle-size-selective fraction of the total airborne dust; includes particles with aerodynamic diameters less than approximately 10 µm and has a 50% deposition efficiency for particles with an aerodynamic diameter of approximately 4 µm.

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Sarcoidosis

A rare multisystem granulomatous disease characterized by alterations in the immune system [Fanburg 1992].

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Scleroderma (progressive systemic sclerosis)

A rare multisystem disorder characterized by inflammatory, vascular, and fibrotic changes usually involving the skin, blood vessels, joints, and skeletal muscle [Archer and Gordon 1996].

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Standardized mortality ratio

The ratio of the number of deaths observed in the study population to the number of deaths expected if the study population had the same rate structure as the standard population [Last 1988].

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Standardized rate ratio

A rate ratio in which the numerator and denominator rates have been standardized to the same (standard) population distribution [Last 1988].

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