EPA-Expo-Box (A Toolbox for Exposure Assessors)
Nanoscale materials can be grouped by their source: they can be naturally occurring, unintentionally produced, or specifically engineered.
- Naturally occurring nanomaterials are formed and released into the environment as a result of natural processes such as weathering/erosion and volcanic eruptions. These include compounds like amorphous silica, carbon black, and ultrafine particulate matter. Naturally occurring particulate matter includes mineral aerosols from wind erosion of soil, sea salt aerosols from wave sprays and evaporation, and ash and smoke particulates from forest fires.
- Unintentional, or incidental, nanomaterials include materials that are produced as a byproduct of an intentional anthropogenic process such as industrial processing, laboratory procedures, and combustion. An example of an incidental nanomaterial is ultrafine particulate matter from automobile exhaust.
- Engineered nanomaterials include a wide range of compounds that are intentionally manufactured in a research or commercial setting. They are designed to exhibit specific properties and maintain uniform structures, so that they can be used in a wide range of advanced applications. There are many different chemical compositions of engineered nanoscale materials, each with unique physical-chemical properties. Nanomaterials have existed for millennia—silver, gold, and carbon nanomaterials were unknowingly manipulated to create lustrous pottery and jewelry, vibrant stained glass, and resilient swords as far back as the 4th Century. However, modern nanotechnology, which involves understanding and controlling nanoscale materials through imaging, measuring, modeling, and manipulating matter, is a relatively new and rapidly growing field. Many modern nanomaterials and novel applications are still being developed and research into their associated effects, exposure, and risks is ongoing.
The focus of this module is on exposure assessment of engineered nanomaterials because their small size and unique physical and chemical properties might affect how people are exposed. Exposure to other incidental or naturally occurring nanomaterials are discussed in other modules focused on general exposure assessment.
While there are no set categories or classification methods currently used to group engineered nanomaterials, there are distinct groups that exist based on the chemical composition and physical arrangement of the material—such as carbon-based nanotubes and nanofibers, metal or metal-oxide nanoparticles, and selenide-based quantum dots. The International Organization for Standardization (ISO) has published a standard (ISO/TS 27687:2008) that lists terms and definitions related to particles in the field of nanotechnologies. Major groups of engineered nanomaterials are described in more detail below.
|Major Groups of Engineered Nanomaterials Based on Chemical Composition and Physical Structure|
|Carbon-based Nanomaterials are molecules composed primarily of carbon; intentionally organized in a specific shape or form such as a hollow sphere, ellipsoid, or tube; and possess unique strength, conductivity, and thermal properties. While carbon-based nanomaterials contain only carbon, modified forms of carbon-based nanomaterials may contain surface groups with non-carbon atoms, or could be engineered to surround or encapsulate a non-carbon atom, such as a metal.
|Dendrimers are repetitively branched molecules—typically symmetrical and spherical—which provide internal cavities for other molecules and possess unique surface functionality.
|Metal-/Metal Oxide-Based Nanomaterials are generally spherical nanoscale particles composed entirely or partly of one or more metals; they possess unique optical, thermal, magnetic, conductivity, and oxidative/reductive properties.
|Quantum Dots are nanocrystalline semiconductors (usually metal complexes, selenides, or sulfides) with unique optical and electrical properties. For example, manipulating the size of quantum dot particles in solution will result in different fluorescent colors.
Exposure and Hazard Considerations for Nanoscale Materials
- Unique absorption and distribution patterns due to small size (e.g., can cross cell membranes more readily through use of transport structures meant for biological molecules, such as ion channels; are similar size to proteins so can be confused by the body).
- Surface chemistry—many nanomaterials have highly reactive OR highly non-reactive surface chemistries; meaning they will interact uniquely with other molecules in the body compared with materials larger than nanoscale. Surface chemistry is often complex and is a driving factor of how the molecule behaves (e.g., transport/fate).
- Surface-to-volume ratio—nanomaterials can have a much higher surface area to volume ratio compared to the same mass of a non-nanoscale-sized material, so they can be more reactive on a mass-to-mass basis. As a result, many nanomaterials may be more toxic than non-nanoscale-sized materials on a mass-to-mass basis.