Glossary of Terms

This page was last revised on October 22, 1997

AB 1807 (Tanner): A California State law (Health and Safety Code Section 39650 et seq.) which became effective January 1984 and established the framework for California's toxic air contaminant identification and control program.

Acceptable daily intake (ADI): The highest daily amount of a substance that may be consumed over a lifetime without adverse effects.

Acute exposure: One or a series of short-term exposures generally lasting less than 24 hours.

Aerosol: A small (less than one micron) particle of solid or liquid matter that can remain suspended in the air.

Air basin: An area that has similar meteorology and geography that has an impact on air in consideration of political boundary lines whenever practicable. California has been divided into 14 air basins.

Air monitoring: The periodic or continuous sampling and analysis of air pollutants in ambient air or from individual pollutant sources.

Air pollutants: Foreign and/or natural substances occurring in the atmosphere that may result in adverse effects on humans, animals, vegetation, and/or materials.

Air Resources Board (ARB): California Air Resources Board. The State's lead air quality agency consisting of an eleven-member Governor-appointed board. It is responsible for attainment and maintenance of the state and federal air quality standards, and is fully responsible for motor vehicle pollution control and oversees county and regional air pollution management programs.

Air toxics "Hot Spots" Information and Assessment Program (AB 2588): A California program (Health and Safety Code Section 44300 et seq.) which requires certain stationary sources to report the type and quantity of specific toxic substances they routinely release into the air. The goals of the law are to inventory air toxics emissions, assess potential health risks from these emissions, notify the public of potentially significant health risks, and require high risk facilities to reduce emissions below significant levels.

Allotropic: The existence, especially in the solid state, of two or more crystalline or molecular structural forms of an element.

Ambient air: Outdoor air.

Anhydrous: An inorganic compound that does not contain water either adsorbed on its surface or combined as water of crystallization.

Atmosphere: The gaseous mass or envelope of air surrounding the earth.

Atmospheric half-life: The time required for the concentration of a pollutant or reactant to fall to one-half of its initial value.

Atmospheric lifetime: The time required for a pollutant or reactant concentration to fall to 1/e of its initial value (e is the base of natural logarithms, 2.718).

Bioaccumulation: Long-term progressive increases in amount of a chemical in an organism resulting in the organism's capability to remove the substance from the body faster than it's being absorbed.

Boiling point: The temperature at which a specific liquid boils; temperature at which the vapor pressure of a specific liquid equals or is slightly greater than the external atmospheric pressure; e.g. water boils at 212 ^oF or 100 ^oC. When a value is not given it should be assumed that the pressure is at 1 atmosphere or 760 mm Hg.

Cal/EPA: In July 1991, the California Environmental Protection Agency (Cal/EPA) was created to coordinate the State's environmental quality programs and assure that there is a cabinet level voice for environmental protection.

Cancer: Uncontrolled invasive growth of cells.

CAPCOA: California Air Pollution Control Officers Association.

CAS registry number: The Chemical Abstracts Service Registry Number (CAS) is a numeric designation assigned by the American Chemical Society's Chemical Abstracts Service and uniquely identifies a specific chemical compound. This entry allows one to conclusively identify a material regardless of the name or naming system used.

Central nervous system: Brain and spinal cord.

Chronic exposure: Long-term, exposure usually lasting one year to a lifetime.

Consumer products: A chemically formulated product used by household and institutional consumers including, but not limited to, detergents; cleaning compounds; polishes; floor finishes; cosmetics; personal care products; home, lawn, and garden products; disinfectants; sanitizers; and automotive specialty products but does not include paint, furniture coatings, or architectural coatings.

Conversion factor: To convert from ppm to mg/m³. mg/m³ = (ppm) x (molecular weight of the substance)/(24.45). For example, formaldehyde: 1.23 mg/m³ = (1 ppm) x (30.03)/(24.45).

Deciliter: One tenth (10^-1) of a liter.

Density: Ratio of mass per unit volume expressed as grams per liter (g/L) for gases and grams per cubic centimeter (g/cm³) for liquids and solids. Examples: air = 1.29 g/L; water = 1 g/cm³.

Dispersion model: A mathematical model or computer simulation used to predict the movement of airborne pollution. Models take into account photochemistry and meteorology which dilute effluents and transport them away from the point of emission.

Dose: The amount of a pollutant that is absorbed. A level of exposure which is a function of a pollutant's concentration, the length of time a subject is exposed, and the amount of the pollutant that is absorbed. The concentration of the pollutant and the length of time that the subject is exposed to that pollutant determine dose.

Emission factor: The rate of emission of a particular substance from its source.

Emission inventory: A record of the estimated amount of pollutants (commonly in lbs or tons) emitted from mobile and stationary sources into the atmosphere over a specific period such as a day or a year.

Emission rate: The weight of a pollutant emitted per unit of time (e.g. tons/year).

Environmental tobacco smoke: Primarily a combination of sidestream smoke from the burning end of a cigarette, pipe or cigar, and exhaled mainstream smoke from the smoker. Other components include smoke emitted at the mouthpiece during puff drawing, and for cigarettes and cigars; compounds diffusing through the wrapper.

Excess carcinogenic risk: Typically, the number of estimated potential excess lifetime cancer cases occurring per million persons continuously exposed for 70 years to a given concentration of a toxic air contaminant. For example, the excess carcinogenic risk from acetaldehyde exposure is 7 to 75 potential lifetime cancer cases per million persons continuously exposed to 1 ppb acetaldehyde (7 - 75 potential lifetime cancer cases/10⁶ ppb persons). In this case, 7 is the risk based on the lower bound potency and 75 is the risk based on the upper bound potency.

Exothermic: Reactions which take place with the evolution of energy in any form - heat, light, electrical, mechanical, etc.

Exposure: The concentration of the pollutant in the air multiplied by the population exposed to that concentration over a specified time period.

Exposure assessment: Measurement or estimation of the magnitude, frequency, duration and route of exposure to a substance for the populations of interest.

Flash point: The temperature at which a liquid or volatile solid gives off vapor sufficient to form an ignitable mixture with the air near the surface of the liquid or within the test vessel. There are two methods used for testing flash points; open cup and closed cup. The open cup method more nearly approximates actual conditions.

Henry's Law constant (H): The air to water partition coefficient, and therefore, relates the chemical concentration in the gas phase to its concentration in the water phase. The amount of gas that dissolves in a given quantity of liquid is proportional to the pressure of the gas above the liquid. The higher the pressure of a gas above the liquid, the greater the amount of dissolved gas in the liquid. H can be determined by dividing the vapor pressure in atmospheres (atm) by the water solubility in mole/m³ to give H in atm-m³/mole. H provides an indication of the partition between air and water at equilibrium and also is used to calculate the rate of evaporation from water.

International Agency for Cancer (IARC): An expert international agency of the World Health Organization which publishes evaluations of evidence on the carcinogenicity of a wide range of chemicals.

Individual cancer risk: The probability, expressed as chances in a million, that a person experiencing 70 years of continuous area-wide outdoor exposure to a toxic air contaminant will develop cancer.

Indoor air pollution: Air pollutants that occur within buildings or other enclosed spaces. Some examples of indoor air pollutants are tobacco smoke, asbestos, formaldehyde, pesticides, solvents from pesticides and cleaners, and radon.

Limit of detection (LOD): The lowest concentration of a substance that can be measured by an instrument.

LC₅₀ (mean lethal concentration): The concentration of a chemical that killed up to 50 percent of the exposed population.

Log octanol water partition coefficient (K_ow): The ratio of the chemical concentration in octanol divided by the concentration in water.

Mean: Average.

Median: The middle value in a population distribution, above and below which lie an equal number of individual values; midpoint.

Melting point: The temperature at which a solid becomes a liquid. At this temperature, the solid and liquid have the same vapor pressure.

Milligram: One one thousandth (10^-3) of a gram.

Miscible: The ability of a liquid or gas, to dissolve uniformly in another liquid or gas.

Molecular formula: The formula which identifies the atoms and the number of each kind in the molecules of a compound. Elements in the molecular formula are listed according to the Hill convention (C,H, then other elements in alphabetical order).

Molecular weight: The sum of the atomic weights of the atoms in a molecule. For example, methane (CH₄) is 16.043, the atomic weights being carbon = 12.011, hydrogen = 1.008.

Mutagenic: The ability of a chemical or physical agent to produce heritable changes in the DNA of living cells.

Nanogram: One billionth (10^-9) of a gram.

Non-carcinogenic effects: Threshold, below which adverse effects other than cancer will not occur. Non-cancer effects include birth defects, organ damage, death and many others.

Oxidation: The chemical reaction of a substance with oxygen or a reaction in which the atoms in an element lose electrons and its valence is correspondingly increased.

Photochemical: A term referring to chemical reactions brought about by the sunlight. The reaction of nitrogen oxides with oxygen in the presence of sunlight to form ozone is an example of a photochemical reaction.

Photolysis: Chemical decomposition induced by light or other radiant energy.

Photo-oxidation: Oxidation under the influence of radiant energy (as photochemical oxidation).

Picogram: One trillionth (10^-12) of a gram.

Polycyclic aromatic hydrocarbons (PAHs): Organic compounds which include only carbon and hydrogen with a fused ring structure containing at least two benzene (six-sided) rings. Polycyclic aromatic hydrocarbons may also contain additional fused rings that are not six-sided.

Polymer: A macromolecule formed by the chemical union of five or more identical combining units called monomers.

Polycyclic organic matter (POM): Organic compounds with more than one benzene ring, with a boiling point greater than or equal to 100^oC.

Polymorphism: A single substance which crystallizes in two or more different forms (under different conditions) is said to be polymorphous (many forms).

Potency value: A function of the estimated potential excess cancer risk associated with a given dose of a substance. Potencies are expressed either in units of inverse dose as a potency slope (i.e. (mg/kg/day)^-1) or, for inhalation exposures, as a unit risk factor [i.e. (g/m³)^-1]. Potency values represent the theoretical probability of extra cancer cases occurring in the exposed population assuming a 70 year lifetime exposure. The derivation of the carcinogenic potency values takes into account the available information on pharmacokinetics, mechanism of carcinogenic action, and the effect of different models on low dose extrapolation. ^.

Proposition 65: Safe Drinking Water and Toxic Enforcement Act of 1986, also known as Proposition 65. This Act is codified in California Health and Safety Code Section 25249.5, et seq. No person in the course of doing business shall knowingly discharge or release a chemical known to the state to cause cancer or reproductive toxicity into water or into land where such chemical passes or probably will pass into any source of drinking water, without first giving clear and reasonable warning to such individual.

Reference Exposure Concentration (RfC): An estimate, derived by the U.S. EPA (with an uncertainty spanning perhaps an order of magnitude) of a daily exposure to the human population, (including sensitive subgroups) that is likely to be without appreciable risk of deleterious effects during a lifetime of exposure. The RfC is derived from a no or lowest observed adverse effect level from human or animal exposures, to which uncertainty or "safety" factors are applied.

Reference Dose (RfD): An estimate delivered by the U.S. EPA (with uncertainty spanning perhaps an order of magnitude) of the daily exposure to the human population (including sensitive subpopulations) that is likely to be without deleterious effects during a lifetime. The RfD is reported in units of mg of substance/kg body weight/day for oral exposures.

Reference Exposure Level (REL): RELs are used by the California Environmental Protection Agency (Cal/EPA) as indicators of potential adverse health effects. A REL is a concentration level (g/m³) or dose (mg/kg/day) at (or below) which no adverse health effects are anticipated for a specified time period. RELs are generally based on the most sensitive adverse health effect reported in the medical and toxicological literature. RELs are designed to protect the most sensitive individuals in the population by the inclusion of margins of safety.

Risk assessment: An evaluation of risk which estimates the relationship between exposure to a harmful substance and the likelihood that harm will result from that exposure. Risk assessments are generally expressed as the estimated chance per million that a person, exposed over some period of time (e.g. a 70 year lifetime) and some specified concentration of exposure, will experience a certain effect.

Specific gravity: The ratio of the density of a substance to the density of a reference substance. For solids and liquids, specific gravity is numerically equal to density. For gases, specific gravity is different because of the differences in reference substances, which are usually water (1g/cm³) for solids and liquids and air (0.00129 g/cm³, or 1.29 g/L at 0 ^oC and 760 mm Hg). The specific gravity of a gas is the ratio of its density to that of air. For example, the density of hydrogen is 0.089 g/L but its specific gravity is 0.069 (i.e., 0.089/1.29). The specific gravity of solids and liquids is the ratio of their density to that of water at 4 ^oC (i.e., 1), as 1 cm³ weighs 1 gram.

Stationary source: A non mobile source of air pollutants which can be either a point or area source.

Topography: The geographic surface map of a place or region.

Toxic air contaminant: As defined by California Health and Safety Code, Section 39655 (a): an air pollutant which may cause or contribute to an increase in mortality or in serious illness, or which may pose a present or potential hazard to human health. Substances which have been identified by the United States Environmental Protection Agency as hazardous air pollutants (e.g. benzene, asbestos) shall be identified by the Board as toxic air contaminants.

Toxic Hot Spot: An area where the concentration of air toxics is significantly higher than background levels, and where exposed individuals may have an elevated risk of adverse health effects.

Troposphere: The lowest region of the atmosphere extending from the earths' surface to the tropopause (about 15 kilometers) characterized by decreasing temperature with increasing altitude.

Unit risk factor: The estimated probability of a person contracting cancer as a result of constant exposure to an ambient concentration of one microgram per cubic meter of a substance over a 70 year lifetime.

United States Environmental Protection Agency (U.S. EPA): The Federal agency charged with setting policy and guidelines, carrying out legal mandates, for the protection, and national interests in environmental resources.

Unit cancer risk: The number of potential excess cancer cases from a lifetime exposure to 1 microgram per cubic meter (g/m³) of a given substance; e.g. a unit risk value of

5.5 x 10^-6 (g/m³) would indicate an estimated 5.5 cancer cases per million people exposed to an average concentration of 1g/m³ of a specific carcinogen for 70 years.

Volatile: Evaporating readily at normal temperatures and pressures.

Volatile organic compounds (VOCs): Any compound containing at least one atom of carbon, excluding: carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, ammonium carbonate, and excluding the following: methane, methylene chloride, 1,1,1-trichloroethane, trichlorofluoromethane (CFC-11), dichlorodifluoromethane (CFC-12), chlorodifluoromethane (HCFC-22), trifluoromethane (HFC-23), 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113), 1-chloro-1,1-difluoro-2-chloro-2,2-difluoroethane (CFC-114), chloropentafluoroethane (CFC-115), 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123), 1,1,1,2-tetrafluoroethane (HFC-134a), 1,1-dichloro-1-fluoroethane (HCFC-141b), 1-chloro-1,1-difluoroethane (HCFC-142b), 2-chloro-1,1,1,2-tetrafluoroethane (HCFC-124), pentafluoroethane (HFC-125), 1,1,2,2-tetrafluoroethane (HFC-134), 1,1,1-trifluoroethane (HFC-143a), and 1,1-difluoroethane (HFC-152a), cyclic, branched, or linear completely methylated siloxanes, the following classes of perfluorocarbons: (A) cyclic, branched, or linear, completely fluorinated alkanes; (B) cyclic, branched, or linear, completely fluorinated ethers with no unsaturations; (C) cyclic, branched, or linear, completely fluorinated tertiary amines with no unsaturations; and (D) sulfur-containing perfluorocarbons with no unsaturations and with the sulfur bonds to carbon and fluorine, and the following low-reactive organic compounds which have been exempted by the U.S. EPA: parachlorobenzotrifluoride (1-chloro-4-trifluoromethylbenzene), ethane, and acetone.

Vapor: The gaseous phase of liquids or solids at atmospheric temperature and pressure.

Vapor density: The vapor density is expressed in grams per liter (g/L) and is compared to the density of air (Air = 1).

Vapor pressure: The pressure, often expressed in millimeters of mercury (mm Hg), that is characteristic at any given temperatures of a vapor in equilibrium with its liquid or solid form.

Water solubility: The solubility of a substance in water provides information on the fate and transport in the environment. The higher the water solubility, the greater the tendency to remain dissolved and are less likely to volatilize from the water. Low water soluble substances will volatilize more readily in water, and will partition to soil or bioconcentrate in aquatic organisms.

Weight of evidence: The extent to which the available information support the hypothesis that a substance causes an effect in humans. For example, factors which determine the

weight-of-evidence that a chemical poses a hazard to humans include the number of tissue sites affected by the agent; the number of animal species, strains, sexes, and number of experiments and doses showing a response; the dose-response relationship, statistical significance in the occurrence of the adverse effect in treated subjects compared with untreated controls; and the timing of the occurrence of the adverse effect.