Inorganics and Fibers
Importance of Physicochemical Characteristics
After contaminants are released to the environment, their transport, dispersion, and transformation are governed by certain physicochemical properties. Physicochemical properties data can help determine whether a contaminant is likely to remain in place, partition to other media, or transform physically, chemically, or biologically after release. Physicochemical properties data may be determined by measurements or estimated based on chemical structure. Sources of physicochemical values include:
Inorganic substances have distinguishing physicochemical properties, many of which are unique to inorganics. These properties lead to different considerations for exposure and risk assessment compared with those important for organic compounds.
Persistent, bioaccumulative, and toxic (PBT) contaminants are chemicals (organic or inorganic) that are toxic, persistent in the environment, and bioaccumulate in food chains. Like organic compounds, inorganic compounds can be persistent (i.e., long lasting) and bioaccumulative (i.e., able to be taken up by biota). EPA’s priority PBTs include mercury and alkyl-lead (the predominant type of organic lead compound). Profiles for these and other priority PBTs are available here: http://www.epa.gov/pbt/pubs/cheminfo.htm.
Some of the characteristics of inorganics most relevant to exposure assessment are discussed below for each of the major groups of inorganic substances.
|Metals (U.S. EPA, 2007)
- A metal (or, in a metal compound, the metallic atom) does not degrade through biological or chemical processes. As such, inorganic metals might be considered to persist for a long time (or indefinitely) in the environment.
- Metals can change forms or be part of mixtures. Speciation of metals can affect solubility. Speciation is an important property in aquatic environments. Environmental conditions (especially pH and the presence of certain competing ions) can strongly influence whether a metal is present in the dissolved phase or as a solid. Free-state metals and some metal compounds are not readily soluble, and this property can limit their bioavailability and toxicity.
- A metal compound’s bioavailability is an important factor that must be considered along with persistence. For example, over time, metals in surface soil tend to become less bioavailable. This tendency, however, can be reversible if environmental conditions change, such as an increase in soil acidity.
- Many inorganic metals have low bioavailability and thus do not bioaccumulate. However, some inorganic metals can bioaccumulate; bioaccumulation of cadmium, for example, has been observed in marine bivalves. Bioaccumulation of some inorganic metals has also been observed in humans.
- The toxicokinetics and toxicodynamics of metals differ according to the type of metal. Some metals—including copper, selenium, and zinc—are essential elements at low levels but toxic at higher levels. Other metals—including lead, arsenic, and mercury—have no known biological purpose. Metal exposures in the environment can occur in the context of metal mixtures (i.e., with other metal elements or organics) or methylated forms, which behave different than pure metal. An organism’s ability to regulate and/or store the metal will depend on the form of the metal or metal compound.
- Transformation of inorganic metals to carbon-containing inorganic compounds (i.e., organometallics) can occur through biotic processes (e.g., methylation of mercury or lead) or abiotic processes (e.g., chemical methylation of tin). Gaseous elemental mercury eventually oxidizes and deposits to soil and water surfaces, where it can be transformed into methylmercury (MeHg) via microbially-mediated processes. Organometallic compounds such as MeHg can behave more like organic compounds, especially in their tendency to more readily bioaccumulate in organisms.
- Organometallic compounds tend to be more bioaccumulative in fish, humans, and other animals than inorganic metals. The chemical properties associated with the presence of the carbon components of the compound can lead to greater absorption and interaction at biological surfaces (e.g., within the human digestive system) and longer persistence within bodily tissues.
- Organometallic compounds such as certain compounds containing lead, arsenic, and mercury are stable and persistent in the environment. MeHg is a highly persistent and bioaccumulative compound with known toxic effects on humans at relatively low levels.
- Fibers are defined by their distinguishing physical form. The characteristics of fibers are influential on exposure and are very relevant to how an exposure assessment is conducted. Their thin, strand-like properties can result in fibers becoming or remaining airborne; thus, inhalation exposures are often of concern.
- Fibers such as asbestos can accumulate in the lower airways and alveolar regions of the lungs.
- Nutrients are substances essential to health for humans, animals, plants, or microorganisms. In excess, any nutrient (including some metals) can be harmful. This occurs when the concentration of the metal becomes too high for the body’s natural clearance mechanisms.
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