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ACGIH® - American Conference of Governmental Industrial Hygienists is an organization of professional personnel in governmental agencies or educational institutions engaged in occupational safety and health programs. ACGIH established recommended occupational exposure limits for chemical substances and physical agents. See TLV.

ACTION LEVEL - An Action Level, calculated as an eight (8)-hour time-weighted average (and generally lower than the PEL), is a concentration designated for specific substances listed in 29 CFR part 1910 that initiates certain required activities such as exposure monitoring and medical surveillance.

ACUTE EFFECT - Adverse effect on a human or animal that has severe symptoms developing rapidly and coming quickly to a crisis.

ACUTE TOXICITY - Acute effects resulting from a single dose of, or exposure to, a substance.

AEROSOL - A fine aerial suspension of particles sufficiently small in size to confer some degree of stability from sedimentation (e.g., smoke or fog).

AGATE - Cryptocrystalline form of silica. Composed of extremely fine (submicroscopic) crystals of silica.

AGGREGATE - Can mean either a group of materials or any of several hard, inert substances (such as sand, gravel or crushed stone) used for mixing with cement.

AIR-LINE RESPIRATOR - A respirator that is connected to a compressed breathable air source by a hose of small inside diameter. The air is delivered continuously or intermittently in a sufficient volume to meet the wearer's breathing requirements.

AIR-PURIFYING RESPIRATOR - A respirator that uses chemicals to remove specific gases and vapors from the air or that uses a mechanical filter to remove particulate matter. An air-purifying respirator must only be used when there is sufficient oxygen to sustain life and the air contaminant level is below the concentration limits of the device.

AMORPHOUS - see NONCRYSTALLINE.

ANSI - American National Standards Institute is a privately funded, voluntary membership organization that identifies industrial and public needs for national consensus standards and coordinates development of such standards.

ASTM - American Society for Testing and Materials is the world's largest source of voluntary consensus standards for materials, products, systems and services. ASTM is a resource for sampling and testing methods, health and safety aspects of materials, safe performance guidelines, and effects of physical and biological agents and chemicals.

ASYMPTOMATIC - Showing no symptoms.

ATM - Atmosphere, a unit of pressure equal to 760 mmHg (mercury) at sea level.

ATMOSPHERE - SUPPLYING RESPIRATOR - A respirator that provides breathable air from a source independent of the surrounding atmosphere. There are two types: air-line and self-contained breathing apparatus.

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BEDROCK - The rock underlying the soil or other surface material.

BENIGN - Not recurrent or not tending to progress. Not cancerous.

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CAA - Clean Air Act was enacted to regulate/reduce air pollution. CAA is administered by the U.S. Environmental Protection Agency.

CARCINOGEN - Causing cancer. A substance or agent capable of causing or producing cancer in mammals, including humans.

In scientific literature, the terms tumorigen, oncogen and blastomogen all have been used synonymously with carcinogen although occasionally, tumorigen has been used specifically to connote a substance that induces benign tumors. The Federal Register (Vol. 52, No. 163, p. 31884) reports the following definition of carcinogen under Federal regulation 29CFR1201: "A chemical is considered to be a carcinogen if:
(a) It has been evaluated by the International Agency for Research on Cancer (IARC), and found to be a carcinogen or potential carcinogen; or

(b) It is listed as a carcinogen or potential carcinogen in the Annual Report on Carcinogens published by the National Toxicology Program (NTP) (latest edition); or

(c) It is regulated by OSHA as a carcinogen."

CARCINOGENICITY - The ability to produce cancer.

CAS - Chemical Abstracts Service is an organization under the American Chemical Society. CAS abstracts and indexes chemical literature from all over the world in "Chemical Abstracts". "CAS Numbers" are used to identify specific chemicals or mixtures.

cc - Cubic centimeter is a volume measurement in the metric system that is equal in capacity to one milliliter (ml). One quart is about 946 cubic centimeters.

CEILING LIMIT (PEL or TLV) - The maximum allowable human exposure limit for an airborne substance which is not to be exceeded even momentarily. Also see PEL and TLV.

CFR - Code of Federal Regulations. A collection of the regulations that have been promulgated under United States law.

CHALCEDONY - Cryptocrystalline silica. Composed of extremely fine (submicroscopic) silica crystals.

CHEMICAL - An element (e.g., chlorine) or a compound (e.g., sodium bicarbonate) produced by chemical reaction.

CHEMICAL CARCINOGENESIS - An IARC term, although one that is widely used elsewhere. It refers to the "induction by chemicals (or complex mixtures of chemicals) of neoplasms that are usually observed, the earlier induction of neoplasms that are commonly observed, and/or the induction of more neoplasms than are usually found" (IARC Monograph 42). Neoplasms are tumors.

CHEMICAL CARTRIDGE RESPIRATOR - A respirator that uses various chemical substances to purify inhaled air of certain gases and vapors.

CHEMICAL NAME - The name given to a chemical in the nomenclature system developed by the International Union of Pure and Applied Chemistry (IUPAC) or the Chemical Abstracts Service (CAS). The scientific designation of a chemical or a name that will clearly identify the chemical for hazard evaluation purposes.

CHEMTREC - Chemical Transportation Emergency Center is a national center established by the Chemical Manufacturers Association (CMA) to relay pertinent emergency information concerning specific chemicals on requests from individuals. CHEMTREC has a 24-hour toll-free telephone number (800-424-9300) to help respond to chemical transportation emergencies.

CHERT - Cryptocrystalline silica. Composed of extremely fine (submicroscopic) silica crystals.

CHRONIC EFFECT - An adverse effect on a human or animal body, with symptoms that develop slowly over a long period of time or that recur frequently. Also see ACUTE.

CHRONIC EXPOSURE - Long-term contact with a substance.

CHRONIC TOXICITY - Adverse (chronic) effects resulting from repeated doses of or exposures to a substance over a relatively prolonged period of time. Ordinarily used to denote effects in experimental animals.

CLEAN AIR ACT - See CAA.

CMA - Chemical Manufacturers Association. See CHEMTREC.

COESITE - A rare form of crystalline silica. Believed at first to exist only as a synthetic form of crystalline silica, it was formed in the laboratory using two different methods. Subsequently, it has been found in nature in the Meteor Crater in Arizona. It was also found to occur as a result of shock-wave experiments and nuclear explosions.

COLLOIDAL SILICA - Extremely fine amorphous silica particles dispersed in water. Colloids do not settle out of suspension over time. Colloidal silica is used commercially as binders and stiffeners and as polishing agents.

COMMON NAME - Any means used to identify a chemical other than its chemical name (e.g., code name, code number, trade name, brand name or generic name). See GENERIC.

CONC - See Concentration.

CONCENTRATION - The relative amount of a substance when combined or mixed with other substances. Examples: 2 ppm hydrogen sulfide in air, or a 50 percent caustic solution.

CONFINED SPACES - Any area that has limited openings for entry and exit that would make escape difficult in an emergency, has a lack of ventilation, contains known and potential hazards and is not intended nor designated for continuous human occupancy.

CONSOLIDATION - In geological terms, any process by which loose, soft, or liquid earth materials harden into rock.

CPSC - Consumer Products Safety Commission has responsibility for regulating hazardous materials when they appear in consumer goods. For CPSC purposes, hazards are defined in the Hazardous Substances Act and the Poison Prevention Packaging Act of 1970.

CRISTOBALITE - The form of crystalline silica that is stable at the highest temperature. It occurs naturally in volcanic rock.

CRYPTOCRYSTALLINE SILICA - Silica with submicrometer crystals formed from amorphous, often biogenic, silica that undergoes compaction over geologic time. Examples are flint and chert. Also called microcrystalline silica.

CRYSTALLINE - Having a highly structured molecular arrangement. The atoms and molecules form a three-dimensional, repeating pattern or lattice.

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DENSITY - The mass (weight) per unit volume of a substance. For example, lead is much more dense than aluminum.

DERMAL - Relating to the skin.

DERMAL TOXICITY - Adverse effects resulting from skin exposure to a substance. Ordinarily used to denote effects in experimental animals.

DEVITRIFY - To change from glassy form to the crystalline state.

DHHS - U.S. Department of Health and Human Services (replaced U.S. Department of Health, Education and Welfare). NIOSH and the Public Health Service (PHS) are part of DHHS.

DIATOMACEOUS EARTH - see DIATOMITE.

DIATOMITE - A rock, high in amorphous silica content, formed from the structures of tiny fresh- and salt-water organisms called diatoms. Diatomite has several commercial uses.

DILUTION VENTILATION - Air flow designed to dilute contaminants to acceptable levels. Also see GENERAL EXHAUST.

DIMENSION STONE - Building stone quarried and prepared in regularly shaped blocks to fit a particular design.

DOL - U.S. Department of Labor. OSHA and MSHA are part of DOL.

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EMPLOYEE EXPOSURE - The exposure to airborne respirable crystalline silica that would occur if the employee were not using a respirator.

EPA - U.S. Environmental Protection Agency.

EPIDEMIOLOGICAL STUDIES - Studies of illness involving human subjects over the long term. They generally involve analysis of real-world incidence of the illness with little or no attempt to control factors that potentially could contribute to the onset or severity of the illness. This contrasts with laboratory studies, which are generally performed on animals, are short term and have variables that are controllable.

ETHYL SILICATE - A colorless, flammable liquid with a faint odor. It is an OSHA-regulated substance (Si(OC2H5)4).

EXPOSURE OR EXPOSED - State of being open and vulnerable to a hazardous chemical by inhalation, ingestion, skin contact, absorption or any other course; includes potential (accidental or possible) exposure.

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FORMULA - The scientific expression of the chemical composition of a material (e.g., water is H2O, sulfuric acid is H2SO4, sulfur dioxide is SO2).

FREE SILICA - Informal name for a pure crystalline silica, which is chemically uncombined.

FUSED QUARTZ - The material formed by the rapid melting of quartz crystals. A meteor strike or a lightning bolt striking sand can form fused quartz. The term quartz glass is often erroneously used to mean fused quartz, but quartz glass is a misnomer because quartz is crystalline and glass is noncrystalline.

FUSED SILICA - The material formed by heating cristobalite to the melting point (1710 degrees C) and cooling it rapidly.

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g - Gram is a metric unit of weight. One ounce is about 28.4 grams.

g/kg - Grams per kilogram is an expression of dose used in oral and dermal toxicology testing to denote grams of a substance dosed per kilogram of animal body weight. Also see "kg" (kilogram).

GENERAL EXHAUST - A system for exhausting air containing contaminants from a general work area. Also see LOCAL EXHAUST.

GENERIC NAME - A designation or identification used to identify a chemical by other than its chemical name (e.g., code name, code number, trade name, and brand name).

GLASSY - Having a semistructural molecular arrangement. Atoms and molecules may form a pattern but it has only short-term or partial order and does not repeat predictably in three dimensions.

GRAVIMETRIC SAMPLING - Quantitative chemical sampling in which the substances in a compound are measured by weight.

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H

HAZARD WARNING - Words, pictures, symbols or a combination thereof presented on a label or other appropriate form to inform of the presence of various materials.

HAZARDOUS CHEMICAL - Any chemical whose presence or use is a physical hazard or a health hazard.

HCS - Hazard Communication Standard is an OSHA regulation issued under 29 CFR Part 1910.1200 and 1926.59.

HEALTH HAZARD - A chemical for which there is significant evidence, based on at least one study conducted in accordance with established scientific principles, that acute or chronic health effects may occur in exposed employees. The term "health hazard" includes chemicals that are carcinogens, toxic or highly toxic agents, reproductive toxins, irritants, corrosives, sensitizers, hepatotoxins, nephrotoxins, neurotoxins, agents that act on the hematopoietic system, and agents that damage the lungs, skin, eyes or mucous membranes.

HIGH-EFFICIENCY PARTICULATE AIR [HEPA] FILTER - A filter that is at least 99.97 percent efficient in removing mono-dispersed particles of 0.3 micrometers in diameter.

HOST ROCK - A rock that contains ores or minerals of value.

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I

IARC - The International Agency for Research on Cancer, an agency of the United Nations' World Health Organization.

IGNEOUS ROCK - A rock that has solidified from a molten state.

INSOL - See INSOLUBLE.

INSOLUBLE - Incapable of being dissolved in a liquid.

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J

JASPER - Cryptocrystalline silica. Composed of extremely fine (submicroscopic) silica crystals.

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K

kg - Kilogram is a metric unit of weight, about 2.2 U.S. pounds. Also see "g/kg," "g" and "mg."

KEATITE - A synthetic and rare form of crystalline silica, formed by the crystallization of amorphous silica. It is transformed to cristobalite at a temperature of 1620° C.

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L

L - Liter is a metric unit of capacity. A U.S. quart is about 9/10 of a liter.

LC - Lethal concentration is the concentration of a substance being tested that will kill.

LCL - Lethal concentration, low, lowest concentration of a gas or vapor capable of killing a specified species over a specified time.

LC₅₀ - The concentration of a material in air that will kill 50 percent of a group of test animals with a single exposure (usually 1 to 4 hours). The LC50 is expressed as parts of material per million parts of air, by volume (ppm) for gases and vapors, or as micrograms of material per liter of air (mg/l) or milligrams of material per cubic meter of air (mg/m3) for dusts and mists, as well as for gases and vapors.

LD - Lethal dose is the quantity of a substance being tested that will kill.

LDL - Lethal dose low, lowest administered dose of a material capable of killing a specified test species.

LD₅₀ - A single dose of a material expected to kill 50 percent of a group of test animals. The LD50 dose is usually expressed as milligrams or grams of material per kilogram of animal body weight (mg/kg or g/kg). The material may be administered by mouth or applied to the skin.

LESION - Any damage to a tissue.

Lfm - Linear feet per minute, a unit of air velocity.

LITHIFY - To harden into rock.

LOCAL EXHAUST - A system for capturing and exhausting contaminants from the air at the point where the contaminants are produced (welding, grinding, sanding, other processes or operations). Also see GENERAL EXHAUST.

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M

M - Meter is a unit of length in the metric system. One meter is about 39 inches.

M³ - Cubic meter is a metric measure of volume, approximately 35.3 cubic feet or 1.3 cubic yards.

MACROPHAGE - A large phagocyte, a type of cell in the body that engulfs foreign materials and consumes debris and foreign bodies.

MAGMA - A natural hot melt of rock-forming materials (primarily silicates) and steam. Magma is in motion. Geologists speculate that it originates just below the Earth's crust and is fed by new material from greater depths.

MALIGNANT - Tending to become progressively worse and to result in death.

MATERIAL IMPAIRMENT - Cited in the Federal Register (1/19/89), regulation 29 CFR 1910, as " ...life-threatening effects; disabling effects; various diseases; irritation to different organs or tissues; and changes in organ functions indicative of future health decrements" (p. 2361).

MECHANICAL EXHAUST - A powered device, such as a motor-driven fan or air steam venturi tube, for exhausting contaminants from a workplace, vessel or enclosure.

MECHANICAL FILTER RESPIRATOR - A respirator used to protect against airborne particulate matter-like dusts, mists, metal fume and smoke. Mechanical filter respirators do not provide protection against gases, vapors or oxygen-deficient atmospheres.

METAL - A type of element. Metals are usually hard and lustrous, malleable (they can be pounded into sheets), ductile (they can be drawn into wires), and can conduct electricity and heat.

METALLOID - A group of elements, eight in all, that form the boundary (on the periodic table) between the metal elements (such as copper, iron, tin, gold) and the nonmetal elements (such as carbon, nitrogen, hydrogen). Metalloids possess some of the properties of the metals and some of the properties of the nonmetals.

METAMORPHIC ROCK - Rocks that have undergone changes from the pressure and temperatures in the Earth's interior.

METASTABLE - Possessing an energy state that is not stable, yet will not change spontaneously. An outside force is required to change.

METER - A unit of length; equivalent to 39.37 inches.

METHYL SILICATE - An OSHA-regulated substance, (CH3O)4,Si; it exists in the form of colorless needles.

mg - Milligram is a metric unit of weight that is one-thousandth of a gram. mg/kg - Milligrams of substance per kilogram of body weight is an expression of toxicological dose.

mg/m³ - Milligrams per cubic meter is a unit for expressing concentrations of dusts, gases, or mists in air.

MICROCRYSTALLINE SILICA - see CRYPTOCRYSTALLINE SILICA.

MICRON - (Micrometer) A unit of length equal to one-millionth of a meter; approximately 0.000039 of an inch.

MICROGRAM ( µg) - one-millionth of a gram.

MINERAL - Naturally occurring crystalline solids, most of which are made from oxygen, silicon, sulfur and any of six common metals or metal compounds.

MIST - Suspended liquid droplets generated by condensation from the gaseous to the liquid state, or by breaking up a liquid into a dispersed state, such as splashing, foaming or atomizing. Mist is formed when a finely divided liquid is suspended in air.

MIXTURE - Any combination of two or more chemicals if the combination is not, in whole or part, the result of a chemical reaction.

ml - Milliliter is a metric unit of capacity, equal in volume to 1 cubic centimeter (cc), or approximately one-sixteenth of a cubic inch. One-thousandth of a liter.

MOLECULAR WEIGHT - Weight (mass) of a molecule based on the sum of the atomic weights of the atoms that make up the molecule.

mppcf - Million particles per cubic foot is a unit for expressing concentration of particles of a substance suspended in air. Exposure limits for mineral dusts (silica, graphite, Portland cement, nuisance dusts and others), formerly expressed as mppcf, are now more commonly expressed in mg/m3.

MSDS - Material Safety Data Sheets. Older terminology, now referred to as Safety Data Sheets (SDSs)

MSHA - Mine Safety and Health Administration, U.S. Department of Labor.

MW - See MOLECULAR WEIGHT.

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N

NCI - National Cancer Institute is that part of the National Institutes of Health that studies cancer causes and prevention as well as diagnosis, treatment and rehabilitation of cancer patients.

NEOPLASM - A tumor.

NIOSH - National Institute for Occupational Safety and Health, is part of the U.S. Centers for Disease Control in the U.S. Department of Health and Human Services (DHHS). Among other activities, NIOSH tests and certifies respiratory protective devices and air-sampling detector tubes, recommends occupational exposure limits for various substances and assists OSHA and MSHA in occupational safety and health investigations and research.

NONCRYSTALLINE - Having an unstructured molecular arrangement. The atoms and molecules are randomly linked, forming no pattern.

NONMETAL - Elements that do not exhibit the properties of metals. Usually poor conductors of electricity and heat.

NRC - National Response Center is a notification center that must be called when significant oil or chemical spills or other environment-related accidents occur. The toll-free telephone number is 800-424-8802.

NTP - National Toxicology Program. The NTP publishes an Annual Report on Carcinogens.

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O

OBJECTIVE DATA - Information, such as air monitoring data from industry-wide surveys or calculations based on the composition of a substance, demonstrating employee exposure to respirable crystalline silica associated with a particular product or material or a specific process, task or activity. The data must reflect workplace conditions closely resembling or with a higher exposure potential that the processes, types of material, control methods, work practices, and environmental conditions in the employer's current operations.

OPAL - An amorphous form of silica.

ORAL - Used in or taken into the body through the mouth.

ORAL TOXICITY - Adverse effects resulting from taking a substance into the body by mouth. Ordinarily used to denote effects in experimental animals.

ORGANIC COMPOUND - A chemical compound containing carbon.

OSHA - The Occupational Safety and Health Administration, an agency of the U.S. Department of Labor.

OVERBURDEN - Material overlying the ore in a deposit.

OVEREXPOSURE - Exposure to a hazardous material beyond the allowable exposure limits.

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P

PEL - See PERMISSIBLE EXPOSURE LIMIT.

PERMISSIBLE EXPOSURE LIMIT (PEL) - An occupational exposure limit established by OSHA's regulatory authority. It may be a time-weighted average (TWA) limit or a maximum concentration exposure limit.

PHAGOCYTIZED - To be removed from the body by the action of phagocytes, cells in the body that engulf foreign materials and consume debris and foreign bodies. It is believed that upon exposure to airborne crystalline silica particles, 80 percent of the particles are phagocytized and eliminated within a short time.

PHYSICIAN OR OTHER LICENSED HEALTH CARE PROFESSIONAL [PLHCP] - An individual whose legally permitted scope of practice (i.e., license, registration, or certification) allows him or her to independently provide or be delegated the responsibility to provide some or all of the particular health care services required by the OSHA Silica Standards.

PIEZOELECTRICITY - The ability of some crystals to convert mechanical pressure to electricity and to convert electricity to vibration. A quartz crystal in a watch is an example of applied piezoelectricity.

POLYMERIZATION - A chemical reaction in which small organic molecules combine to form larger molecules that contain repeating structural units of the original molecules. The product of polymerization is called a polymer.

POLYMORPH - Literally "many forms." To be polymorphic means to have or assume several forms. In reference to crystals, it is the characteristic of crystallizing in more than one form. For example, crystalline silica can be in the form of quartz, cristobalite, tridymite or others.

PNEUMOCONIOSIS - A condition of the lung in which there is permanent deposition of particulate matter and the tissue reaction to its presence. It may range from relatively harmless forms of iron oxide deposition to destructive forms of silicosis.

PRECIPITATED SILICA - Amorphous silica that is precipitated from either a vapor or solution.

PULMONARY - Relating to, or associated with, the lungs.

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Q

QUARTZ - The most common type of crystalline silica. Some publications will use quartz and crystalline silica interchangeably, but the term crystalline silica actually encompasses several forms: quartz, cristobalite, tridymite and several rarer forms.

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R

RADIOLARIAN EARTH - Soil, high in amorphous silica content, composed predominantly from the remains of radiolaria. Radiolarian earth that has been consolidated (hardened) into rock is called radiolarite.

RADIOLARITE - A rock, high in amorphous silica content, formed from the shells of tiny fresh- and salt-water organisms called radiolaria.

RCRA - Resource Conservation and Recovery Act is environmental legislation aimed at controlling the generation, treating, storage, transportation and disposal of hazardous wastes. It is administered by EPA.

REACTIVITY - Chemical reaction with the release of energy. Undesirable effects - such as pressure buildup, temperature increase, formation of noxious, toxic or corrosive byproducts - may occur because of the reactivity of a substance to heating, burning, direct contact with other materials, or other conditions in use or in storage.

RECOMMENDED EXPOSURE LIMIT (REL) - The REL, established by the National Institute for Occupational Safety and Health, is the highest allowable airborne concentration which is not expected to injure the workers. It may be expressed as a ceiling limit or as a time-weighted average (TWA).

REL - See Recommended Exposure Limit

REGULATED AREA - an area, demarcated by the employer, where an employee's exposure to airborne concentrations of respirable crystalline silica exceeds, or can be reasonably expected to exceed, the OSHA PEL.

RESPIRABLE CRYSTALLINE SILICA - Quartz, cristobalite, and/or tridymite contained in airborne particles that are determined to be respirable by a sampling device designed to meet the characteristics for respirable-particle-size-selective samplers specified in the International Organization for Standardization (ISO) 7708:1995: Air Quality - Particle Size Fraction Definitions for Heath-Related Sampling

May be defined as dust that contains particles small enough to enter the gas-exchange region of the human lung (about 3.5 microns).

One of the studies to which IARC refers in its monograph (Vol. 42, 1987) found that particle size for crystalline silica (in the form of an alpha quartz sandstone with the trademark Min-U-Sil 15) was distributed as follows: particles larger than 5 microns constituted about 0.1 percent of the sample, particles between 2 and 4.9 microns, about 7 percent; and particles less than 1.9 micron, 92.8 percent.

The Silicosis and Silicate Disease Committee (National Institute for Occupational Safety and Health) states that particles less than 1 micron in size are the most troublesome and that particles in the range of 0.5 to 0.7 microns are retained in the lung. (See Archives of Pathology and Laboratory Medicine, Vol. 112, July 1988).
As early as 1943, however, the Department of Labor established a limit of no more than 5 million particles of free silica under 10 microns in size per cubic foot of air. (See Silicosis, Industrial Health Series No. 9, U.S. Department of Labor, Division of Labor Standards, 1943).

RESPIRATORY PROTECTION - Devices that will protect the wearer's respiratory system from overexposure by inhalation to airborne contaminants. Respiratory protection is used when a worker must work in an area where he/she might be exposed to concentration in excess of the allowable exposure limit.

RESPIRATORY SYSTEM - The breathing system that includes the lungs and the air passages (trachea or "windpipe," larynx, mouth and nose) to the air outside the body, plus the associated nervous and circulatory supply.

ROCK CYCLE - A cycle taking place over geologic time in which the three types of rock are related. Sedimentary rocks are changed into metamorphic rocks or melted to create igneous rocks. Sedimentary, metamorphic and igneous rock may be eroded to make sediments that then harden into sedimentary rock.

ROUTES OF ENTRY - The means by which material may gain access to the body, for example, inhalation, ingestion and skin contact.

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SDS - Safety Data Sheets

SEDIMENTARY ROCK - A rock formed by the accumulation and consolidation of minerals that have been either transported to a particular site by wind, water or ice, or precipitated by a chemical reaction at the site.

SELF-CONTAINED BREATHING APPARATUS - A respiratory protection device that consists of a supply or a means of respirable air, oxygen or oxygen-generating material, carried by the wearer.

SEMICONDUCTOR - Materials that act as conductors within certain temperature ranges; at other temperatures they act as insulators. The elements silicon and germanium are examples of semi-conductors of electricity. This unusual electrical capability has led to silicon's use in transistors, integrated circuits and computer chips.

SILICA - A compound formed from silicon and oxygen. Silica is a polymorph, that is, it exists in more than one state. The states of silica are crystalline and noncrystalline (also called amorphous). Crystalline silica can take several forms: quartz (most common), cristobalite, tridymite and four more rare forms.

SILICA BRICK - Brick composed of silica that is used as lining in furnaces.

SILICA GEL - Amorphous silica, prepared in formation with water. Removal of the liquid creates xerogels and further treatment with alcohol creates aerogels. Silica gels are used as drying agents and to alter viscosity of liquids.

SILICA FLOUR - Finely ground quartz, typically 98 percent of the particles are below 5 microns in diameter.

SILICA SAND - A common term in industry. It generally is used to mean a sand that has a very high percentage of silica, usually in the form of quartz. Silica sand is used as a source of pure silicon and as a raw material for glass and other products. Also called quartz sand.

SILICATES - Compounds formed from silicon, oxygen, and other elements. See SILICATE MINERALS.

SILICATE MINERALS - Minerals containing silicon, oxygen and a metal or metal compound. Silica tetrahedra form the framework of silicate minerals. Examples are olivine, pyroxene, amphibole, feldspar and mica.

SILICA W - A synthetic form of crystalline silica. It reacts rapidly with water transforming into amorphous silica.

SILICEOUS - A term used to describe a rock with a high silica content, especially one containing free silica rather than silicates.

SILICIC ROCK - An igneous rock containing more than two-thirds SiO4 by weight, usually as quartz or feldspar. Granite is an example of silicic rock.

SILICON - The second most common element in the Earth's crust. (Oxygen is the most common.) Silicon's chemical symbol is Si. Silicon is a metalloid, possessing some of the properties of a metal and some of the properties of a nonmetal. Pure silicon does not exist in nature. Silicon derived in the laboratory exists as black to gray, lustrous, needle like crystals and is an OSHA-regulated substance.

SILICON CARBIDE (SiC) - A green to blue-black iridescent crystal. It is an OSHA-regulated substance.

SILICON DIOXIDE - Silica (SiO₂). SILICONES - Synthetic compounds formed from two or more silicon atoms linked with carbon compounds. Most silicones contain oxygen as well. Silicones are formed by a process called polymerization; the molecular structure is a chain, not the tetrahedral shape of the molecules of silica or the silicates. See also POLYMERIZATION.

SILICON TETRAHYDRIDE (SiH₄) - A colorless gas used in the manufacture of semiconductors. Also called silane. It is an OSHA-regulated substance.

SILICOSIS - A pneumoconiosis characterized by scarring of lung tissue and fibrosis, which is contracted by prolonged exposure to high levels of respirable silica dust or acute levels of respirable silica dust.

SPECIFIC CHEMICAL IDENTITY - The chemical name, Chemical Abstracts Service (CAS) Registry Number or any precise chemical designation of a substance.

SPECIFIC GRAVITY - The weight of a material compared to the weight of an equal volume of water is an expression of the density (or heaviness) of a material. Insoluble materials with specific gravity of less than 1.0 will float in (or on) water. Insoluble materials with specific gravity greater than 1.0 will sink in water. Most (but not all) flammable liquids have specific gravity less than 1.0 and, if not soluble, will float on water - an important consideration for fire suppression.

STABILITY - The ability of a material to remain unchanged. For SDS purposes, a material is stable if it remains in the same form under expected and reasonable conditions of storage or use. Conditions that may cause instability (dangerous change) are stated; for example, temperatures above 150° F.; shock from dropping.

STABLE - Possessing an energy state that is balanced and will not change spontaneously, resistant to energy change.

STATES OF MATTER - A substance can be in a solid, liquid or gas state. These three are called states of matter.

STEL - Short-Term Exposure Limit (ACGIH terminology). See TLV.

STISHOVITE - The most dense form of crystalline silica. It is rare and at first was believed to exist only as a synthetic. It was initially found to occur as a result of shock-wave experiments and of nuclear explosions. Subsequently, it was found in nature in the Meteor Crater in Arizona.

SUPPLIED-AIR RESPIRATORS - Air line respirators or self-contained breathing apparatus.

SYNONYM - Another name or names by which a material is known. Methyl alcohol, for example, is known as methanol or wood alcohol.

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THRESHOLD LIMIT VALUE (TLV®) - Threshold Limit Value is a term used by ACGIH to express the airborne concentration of material to which nearly all persons can be exposed day after day without adverse effects. ACGIH expresses TLVs in three ways:

TLV-TWA: The allowable Time-Weighted Average concentration for a normal eight hour workday or 40-hour workweek.
TLV-STEL: The Short-Term Exposure Limit, or maximum concentration for a continuous 15-minute exposure period (maximum of four such periods per day, with at least 60 minutes between exposure periods, and provided the daily TLV-TWA is not exceeded.
TLV-C: The ceiling exposure limit - the concentration that should not be exceeded even instantaneously.

TLV® - See Threshold Limit Value.

TOXIC - A chemical falling within any of the following categories:

A chemical that has a median lethal dose (LD50) of more than 50 milligrams per kilogram but not more than 500 milligrams per kilogram of body weight when administered orally to albino rats weighing between 200 and 300 grams each.
A chemical that has a median lethal dose (LD50) of more than 200 milligrams per kilogram but not more than 1,000 milligrams per kilogram of body weight when administered by continuous contact for 24 hours (or less if death occurs within 24 hours) with the bare skin of albino rabbits weighing between two and three kilograms each.
A chemical that has a median lethal concentration (LC50) in air of more than 200 parts per million but not more than 2,000 parts per million by volume of gas or vapor, or more than two milligrams per liter but not more than 20 milligrams per liter of mist, fume, or dust, when administered by continuous inhalation for one hour (or less if death occurs within one hour) to albino rats weighing between 200 and 300 grams each.

TOXIC SUBSTANCE - Any substance that can cause acute or chronic injury to the human body, or which is suspected of being able to cause diseases or injury under some conditions.

TOXICITY - The sum of adverse effects resulting from exposure to a material, generally, by the mouth, skin or respiratory tract.

TRADE NAME - The trademark name or commercial trade name for a material or product.

TRIDYMITE - A form of crystalline silica. It is found in nature in volcanic rocks and stony meteorites. It is also found in fired silica bricks.

TSCA - Toxic Substances Control Act (federal environmental legislation administered by EPA) regulates the manufacture, handling and use of materials classified as "toxic substances."

TWA - Time-Weighted Average exposure is the airborne concentration of a material to which a person is exposed, averaged over the total exposure time - generally the total workday (8 to 12 hours). Also see TLV.

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VAPOR PRESSURE - The pressure exerted by a saturated vapor above its own liquid in a closed container. When quality control tests are performed on products, the test temperature is usually 100° F, and the vapor pressure is expressed as pounds per square inch (psig or psia), but vapor pressures reported as MSDSs are in millimeters of mercury (mmHg) at 68° F (20° C), unless stated otherwise. Three facts are important to remember:

Vapor pressure of a substance at 100¡ F will always be higher than the vapor pressure of the substance at 68° F (20° C).
Vapor pressures reported on MSDSs in mmHg are usually very low pressures; 760 mmHg is equivalent to 14.7 pounds per square inch.
The lower the boiling point of a substance, the higher its vapor pressure.

VENTILATION - See GENERAL EXHAUST, LOCAL EXHAUST, and MECHANICAL EXHAUST.

VITREOUS SILICA - Glassy silica. The term is sometimes used to refer to any noncrystalline substance.

VITRIFY - To form as a glass.

VOLCANIC TUFFS - Deposits of volcanic ash that have hardened into rock.

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WORK AREA - A room or defined space in a workplace where hazardous chemicals are produced or used, and where employees are present.