Nanoparticles – The Known and Unknown

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Written by Henry P. Shotwell, Ph.D., CIH, Vice-President and Robert E. Sheriff, MS, CIH, CSP, President

November 14, 2018


“Nanoparticle” is a catch-all term. There is no consensus on definitions or nomenclature for nanoparticles or nano aerosols. Other terms for this material include “Ultrafine Particle” (less than 100 nm in diameter). Current terminology  for measuring worker exposure to particulates does not take particle size ranges into account.

A nanometer is one-one thousandth of a micron, which is one-one thousandth of a milligram, which is one-one thousandth of a gram, which in turn, is the thousandth part of a kilogram which is the reference unit of mass in the metric system. Thus, 1 kg = 1,000g = 1,000,000 mg = 1,000,000,000 mcg = 1,000,000,000,000 ng. One nanogram is 1 x 10 E – 12 kg.  An example of nanoparticles are the materials used in 3D printers.

There are four main processes by which nano particles are produced: gas-phase, vapor deposition, colloidal and attrition. All four production methods carry with them the potential for occupational exposure by the inhalation (most prevalent), dermal and ingestion routes.

Exposures are likely to occur from process leakage, material transfer and storage, from bag out, from repair and maintenance activities, and from end-users, including other processes.

When released into the air, nanoparticles may agglomerate (individual particles held together in clumps by weak forces) or aggregate (clumps of nanoparticles held together by strong forces).

Adverse health effects from inhaling ultrafine particles are not associated with mass. The toxicity of inhaled, insoluble materials increases as particle size decreases and surface area increases. Nanoparticles are capable of penetrating alveolar cells and passing into the bloodstream. Similarly, nanoparticles can penetrate the skin. Nanoparticles can pass through the cells of the digestive system too, either after direct ingestion or by swallowing particles cleared from the respiratory tract by the mucociliary elevator, following inhalation.

Little is known about current industrial exposure levels or of the toxicological consequences from exposure to nanoparticles. But, Carbon Nano Tubes (CNT) can be considered to be fibers, similar to asbestos or fiberglass. They seem to have similar physical properties.

Currently, there are instruments available for industrial hygiene field use that can measure particle numbers, particle size ranges or surface area in ways that would be useful in assessing risk. Measuring particle mass is not enough. Particle number counts would be better; particle surface area would be best.  There are particle counters whose size range include nano-sized-particles.

Particle size governs aerosol behavior. At the nanoparticle level, diffusional forces dominate and the nanoaerosol behaves more like a gas than an airborne particulate. Nanoparticle diffusion is the same as Brownian Motion. Nanoaerosols have no inertia: they diffuse rapidly through air, and remain in the air for a long time. Therefore, engineering controls must treat nanoaerosols like gases in order to be efficient.


  1. Inhalation exposure is the most prevalent route.
  2. Little is known of the toxicology of nano-sized particulates.
  3. Surface area and numbers of particles seem to be more important indices of exposure than weight.
  4. Currently, there are effective means to measure and evaluate exposures to nano particulate surface area.

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