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EPA ExpoBox

Exposure Assessment Tools by Chemical Classes - Inorganics and Fibers

Overview

Inorganics and Fibers

Inorganics are generally defined as substances that do not contain carbon or have structures that are not carbon-based. Examples include ammonia, hydrogen sulfide, all metals in their "free" state (e.g., silver, lead, aluminum, chromium, mercury, iron), and most elements, including nutrients (e.g., calcium, nitrogen, phosphorous). Some compounds that contain carbon are considered inorganic and referred to as carbon-containing inorganics because their behavior and characteristics are more similar to those of inorganic compounds (e.g., cyanides, carbonates, carbonic acid, and oxides of carbon such as carbon dioxide and carbon monoxide).

Inorganic substances might be naturally occurring, produced intentionally and incorporated in products, or produced unintentionally and released as byproducts. Inorganic is a highly general classification that refers to large and varied group of substances, and so generalizations regarding use and common routes of exposure are difficult to make for this broad class.

Potential routes of exposure include ingestion via drinking water (e.g., nutrients, metals), dermal via contact with contaminated soil (e.g., carbon-containing inorganics and metals), and inhalation (e.g., carbon monoxide, fibers including asbestos).

Fibers, while not always inorganic, are often discussed with inorganic chemicals because exposure to inorganic fibers such as asbestos is often of more concern than exposure to organic fibers. Inorganic fibers like fiberglass, mineral wool, refractory ceramic fibers, and asbestos share a similar structure: they are elongated, thread-like strands of molecules of variable length that are often interwoven and entwined.

The use of synthetic, inorganic fibers is widespread in consumer and industrial applications, such as textiles; plastics; construction and automotive materials (e.g., insulation materials, cements, steel fiber composites, sealants, rubbers); and electronics (e.g., cables). Inhalation of fibers, especially for workers, is often a concern because the size and shape of fibers lead to their tendency to penetrate deep into the lungs after inhaled.

It is not the intent of this tool set to provide information relevant to all inorganics and fibers. As mentioned above, this chemical class encompasses a large and varied group of substances. Instead, this tool set focuses on tools for assessing exposures to higher priority metals - lead and mercury - and metals identified as drinking water contaminants of concern; asbestos fibers; and general nutrient pollution.

Lead and mercury resources can be found here:
Lead
Mercury

Links to information about specific inorganic contaminants in drinking water are provided here:
Water

Resources related to asbestos and nutrient pollution are also available.
Asbestos
Nutrient Pollution
Nutrient Pollution Policy and Data

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Types

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.

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Physicochemical Characteristics

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: Priority PBT Profiles.

Some of the characteristics of inorganics most relevant to exposure assessment are discussed below for each of the major groups of inorganic substances.

Group Characteristics/Exposure Considerations
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.
Carbon-containing inorganics
  • 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
  • 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
  • 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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Routes

The relevant pathways of exposure to inorganics vary based the type of inorganic. Exposure to inorganics occurs most commonly from ingestion (food, soil [incidental], drinking water) or inhalation (dust). Exposure from dermal contact (e.g., from contaminated soil) can also occur. Exposure to fibers such as asbestos occurs mainly through inhalation. Lead and mercury have been studied at length due to the health effects related to exposure, so examples related to those metals are provided below. 

Route Example(s) for Inorganics and Fibers
Ingestion
  • Divalent mercury can be transformed to MeHg by anaerobic microbes. MeHg readily bioaccumulates in fish (unlike divalent mercury) and these fish may be ingested by humans. The most common way people are exposed to any form of mercury is by eating fish containing MeHg.
     
  • In general, lead exposure can occur by ingesting dusts, drinking water, or eating foods that contain lead.
     
  • For children, exposure can result from swallowing paint chips or dust from deteriorated lead-based paints.
Inhalation
  • Exposure to inorganics might occur by inhalation of soil particulates.
     
  • When asbestos-containing materials are damaged or disturbed, microscopic fibers become airborne and can be inhaled.
Dermal contact
  • Indirect exposure to inorganics in dust could occur by contacting dust that has settled on carpets, floors, clothing, counter tops, or other surfaces.

Note: The Routes Tool Set of EPA ExpoBox provides additional information and resources organized by route.

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Media

Inorganics occur in multiple environmental media, including air, water, and soil. Some also accumulate in living organisms, so can be found in biota (e.g., fish) and other food. Metals such as lead and mercury have been studied extensively due to their toxicity, and sources related to those metals are provided below. 

Media Sources of Inorganics and Fibers
Air
  • Burning coal leads to emissions of elemental mercury and divalent mercury. Divalent mercury that deposits in surface water can be transformed to MeHg by anaerobic microbes.
     
  • Lead in air can travel long distances before depositing onto soil or water.
Water
  • In surface water, lead adsorbs strongly to sediment particles, where it can persist for many years.
     
  • Arsenic, hexavalent chromium, and cyanide are a subject of concern in EPA reports and other resources, particularly as contaminants in drinking water.
     
  • Lead contamination of drinking water could occur after treatment (e.g., lead contamination from corrosion of plumbing materials).
     
  • Nutrient pollution in ground water used as a drinking water source can be harmful even at low levels.
Soil
  • Lead adsorbs strongly to soil particles and is unlikely to migrate to groundwater from soil.
Food
  • Lead and mercury can bioconcentrate in plants and animals used as food sources.

Note: The Media Tool Set of EPA ExpoBox provides additional information and resources organized by media.

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Exposed Populations

The general population might be exposed to inorganic contaminants through ingestion of contaminated food or water, incidental ingestion of soil, inhalation of soil particulates, and/or dermal contact with dust. Specific populations might experience increased exposures to the inorganic contaminants due to lifestage, behavior, or occupation.

These populations include developing fetuses, breast-feeding infants, children, and certain occupational workers. Specific examples are provided below related to methylmercury (MeHg) and lead.

  • Breast-fed infants may be exposed to MeHg if their mothers have been exposed.
     
  • Toddlers and young children may be susceptible to lead dust exposure because of certain behaviors (e.g. tendency to mouth objects or hands) and activities (e.g., crawling or playing on the floor indoors where dust contaminated with lead has settled, playing on the lawn where the soil is contaminated with lead) that increase their chances of exposure.
     
  • Certain occupations may result in higher exposure levels of inorganic contaminants to workers. For example, construction workers or utility workers could be exposed to lead in soil through inhalation of dust.

Note: The Lifestages and Populations Tool Set of EPA ExpoBox has addition information about particular population groups and lifestages.

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Tools

Resources for evaluating exposure to asbestos and metals can be found in the tables below.

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References

Casarett, LJ; Doull, J; Klaassen, CD. (2008). Casarett and Doull's Toxicology: The basic science of poisons. In CD Klaassen (Ed.), (7th ed.). New York, NY: McGraw-Hill.

U.S. EPA (U.S. Environmental Protection Agency). (2007). Framework for Metals Risk Assessment. (EPA 120/R-07/001). Washington, DC. 

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