EPA-Expo-Box (A Toolbox for Exposure Assessors)
Inorganics and Fibers
Inorganic substances are a diverse chemical class based on the classical definition of substances that do not have carbon-based structures (with some exceptions, as noted below). Within this class, groups are defined by structural similarities. See below for more information on the major groups of inorganic substances.
- Metals in their "free" state, without any other atoms, are defined by a set of physical properties in the solid state: high reflectivity, high electrical conductivity, high thermal conductivity, and mechanical ductility and strength (Casarett et al., 2008). With the exceptions of gold, silver, platinum, and copper, most other metals are found in the environment as compounds. Some metals are known to be toxic. For example, childhood lead exposure has been linked to decreases in IQ and changes in behavior. On the other hand, some metals are essential nutrients at lower doses for humans and other organisms (U.S. EPA, 2007).
- Carbon-containing inorganics are carbon-based compounds that are grouped with inorganic substances because their behavior and physicochemical properties are more similar to inorganic substances than to organic compounds. One example is cyanides, such as hydrogen cyanide, which is used as an insecticide and is also produced as a chemical reaction byproduct. Cyanides are often found in the air as a result of car exhaust. Other carbon-containing inorganic substances include carbonates, carbonic acid, and oxides of carbon such as carbon dioxide and carbon monoxide.
- Fibers are elongated, thread-like strands of molecules of variable length that are often interwoven and entwined; natural and textile fibers are not often a concern for exposure assessors, but inhalation exposure of fiberglass or mineral fibers is often assessed. Asbestos is a fibrous mineral that occurs naturally in certain rock formations and can also be found in the ambient air and in some drinking water. Because of its high tensile strength and resistance to heat and most chemicals, in the past, asbestos was added to a variety of products to strengthen them and to provide heat insulation and fire resistance. When asbestos-containing materials are damaged or disturbed, microscopic fibers become airborne and can be inhaled into the lungs. Today, many uses of asbestos are banned in the United States.
- Nutrients are elements and compounds that provide essential nitrogen, phosphorous, potassium, and micro-nutrients applied to plants or crops by direct application to the soil, incorporation in the soil, or via irrigation. For example, nitrogen and phosphorus are elements that occur naturally in aquatic ecosystems and support the growth of algae and aquatic plants. On the other hand, eutrophication (nutrient over-enrichment) in aquatic systems, particularly due to excess nitrogen and phosphorous contamination, has been investigated as a major water quality issue.