Exposure Assessment Tools by Chemical Classes - Inorganics and Fibers

Overview

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.

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.

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). 

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

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. 

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

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. 

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

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.

Tools

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

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.