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EPA-Expo-Box (A Toolbox for Exposure Assessors)

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Nanomaterials

Routes

Exposure to nanomaterials can occur through inhalation, ingestion, or dermal absorption. Further, biomedical uses can result in exposure via injections. The most likely routes differ greatly depending on the individual nanomaterial’s physical-chemical properties (and how these properties impact fate between environmental media) as well as the application in which the material is used. The National Nanotechnology Initiative Workshop on Human and Environmental Exposure Assessment (2009) came to several broad conclusions on exposure routes, including:

  • Exposure routes and scenarios will depend on the use of the nanomaterial
  • All routes should be considered, due to the wide variety of nanomaterial uses
  • "Dermal exposure data should be included in any survey," as more research is needed regarding possible skin absorption of nanomaterials
  • Both intended use and misuse of consumer products can lead to exposure, and many key aspects of exposure scenarios are not well understood

Researchers must consider how the physical-chemical properties of a specific nanomaterial impact fate between environment media, and how the application of a nanomaterial results in different exposure scenarios. Different nanomaterial applications can result in distinct primary exposure routes. For example, the use of nano-Ag in disinfectant spray could result in a primary exposure route of inhalation, whereas nano-Ag in a coating on a keyboard could result in primarily dermal exposure.

Material and product manufacturing processes as well as research and development efforts can result in unique occupational exposure scenarios following inhalation, ingestion, or dermal absorption routes. Furthermore, material and product manufacturing and product use, re-use, and recycling can result in numerous secondary exposure pathways that end with ingestion, inhalation, or dermal absorption. For example, nanomaterials may not be entirely removed during wastewater treatment processes and could settle in biosolids which are applied to agricultural fields. Because of their small size, nanomaterials in water may not be properly removed through drinking water treatment processes and could therefore end up in the drinking water supply, although, to date, data do not show release of nanomaterials into drinking water.

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Dermal Absorption
Dermal absorption of nanomaterials might be greater than for other products due to their small size: many nanoparticles are small enough that they are suspected to easily cross the skin barrier—though, further research is needed on this topic. Initial evidence suggests that nanomaterials will not pass through the dermal layer, but surface coatings or other modifications could facilitate absorption from this exposure route (Crosera et al., 2009).
Examples of applications of nanomaterials which may lead to exposure via dermal routes include:
  • Cosmetics that are directly applied to skin (creams, lotions, or powders)
  • Sporting equipment (e.g., baseball bats, tennis rackets, motorcycle helmets)
  • Plastic composites used in consumer goods (e.g., power tool casings, automobile bumpers, handheld electronic casings)
  • Fabrics/textiles (e.g., flame-retardant upholstery; clothing that is wrinkle/stain resistant, UV-protectant, antimicrobial, or sweat-wicking)
  • Antimicrobial soaps and antimicrobial coatings on a variety of consumer products

Inhalation
Some nanomaterials are intentionally aerosolized, as they are used in spray products or applied to other products during manufacturing using spray-dispersion techniques. Other nanomaterials are unintentionally aerosolized during different manufacturing or production activities, disposal, or recycling.

Depending on their physical-chemical properties, aerosolized nanomaterials may remain dispersed in ambient or indoor air where they can be inhaled, or the nanomaterials may aggregate/agglomerate with each other or sorb to dust. Nanomaterials that aggregate/agglomerate or sorb to dust are likely to settle out of the air, making inhalation a less relevant route of exposure.

Conversely, aerosolized nanomaterials that are inhaled could penetrate deeper into the lung than their non-nanoscale counterparts, resulting in differences in exposure along the respiratory tract.
Examples of applications of engineered nanomaterials which may lead to exposure through inhalation include:
  • Spray-on cosmetic products (e.g., spray-on sunscreens, hairsprays)
  • Household cleaners in spray containers (e.g., degreasers, stain-removers, and antibacterial/antimicrobial products)
  • Air purifiers and air filters
  • Pesticides (specifically ones that are applied using spray-dispersion methods)
  • Cement (dust released during cement mixing or construction)

Ingestion
Many pharmaceutical and biomedical applications of nanomaterials exist, and some of these applications result in direct ingestion of a nanomaterial. Additionally, nanomaterials released into the air or released from numerous consumer products or food packaging may sorb to dust particles or settle onto food. Similar to dermal and inhalation routes, the small size of nanomaterials may result in different absorption patterns following ingestion compared to their non-nanoscale counterparts.
Examples of applications of engineered nanomaterials which may lead exposure through ingestion include:
  • Pesticides (residues on produce, or released to drinking water supplies)
  • Food packaging
  • Food supplements
  • Biomedical applications (i.e., quantum dots are used for biomedical imaging; gold nanoparticles are used to detect biological events related to early-stage Alzheimer’s Disease; multiple nanomaterials are used in multifunctional therapeutics to deliver drugs to specific target tissues, thereby reducing risk of the delivered drug affecting non-target tissues)
  • Applications listed under dermal exposure could also result in ingestion, through hand-to-mouth contact; some inhaled nanomaterials could be subsequently swallowed

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