Page:NIOSH Manual of Analytical Methods - Chapter R.pdf/1

 R. DETERMINATION OF AIRBORNE CRYSTALLINE SILICA

by Rosa J. Key-Schwartz, Ph.D., NIOSH/DART; Paul A. Baron, Ph.D., NIOSH/DART; David L. Bartley, Ph.D., NIOSH/DART; Faye L. Rice, NIOSH/EID, and Paul C. Schlecht, NIOSH/DART. Portions of this chapter were adapted from: NIOSH [2002]. Hazard Review: Health effects of occupational exposure to respirable crystalline silica.

1. INTRODUCTION

A variety of air sampling and analysis methods for determining workers' exposures to crystalline silica have been published by the National Institute for Occupational Safety and Health (NIOSH), the Occupational Safety and Health Administration (OSHA), the Mine Safety and Health Administration (MSHA), the United Kingdom Health and Safety Executive (UKHSE) and others. The following chapter provides information on the structure of silica, health effects, airborne exposure standards, sampling considerations and analytical considerations. The purpose is to provide information to the industrial hygienist, chemist, laboratory manager and the client of laboratory services to make an informed decision on which crystalline silica method should be used, and the quality assurance procedures that should be employed to obtain accurate results.

2. STRUCTURE, PROPERTIES AND OCCURRENCE OF SILICA

Silica is commonly found in the earth's crust and refers to the chemical compound silicon dioxide (SiO$2$). Physical properties (such as specific gravity, refractive index and X-ray diffraction pattern) and toxicological properties depend on the chemical composition and the molecular structure. Silica occurs in both crystalline and non-crystalline forms. Crystalline forms are physical states in which the silicon dioxide molecules are arranged in a repetitive pattern that has unique spacing, lattice structure and angular relationship of the atoms. Crystalline silica forms (often referred to as polymorphs) include quartz, cristobalite, tridymite, keatite, coesite, stishovite and moganite [2,3,4]. Figure 1 shows the lattice arrangements of quartz, cristobalite and tridymite. Table 1 lists some physical properties of these polymorphs. 3/15/03