Chemicals

Nanomaterials: The Case for Occupational Exposure Limits

More and more workplaces are affected by the increasing prevalence of nanomaterials. The National Institute for Occupational Safety and Health (NIOSH) recently identified nanotechnology as one of the occupational safety and health issues identified as needing attention by the industry sector groups of the National Occupational Research Agenda (NORA). NIOSH also published a blog making the case for occupational exposure limits (OELs) for nanomaterials.

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Let’s Review: What Are Nanoscale Materials?

Nanomaterials are chemical substances that have structures with dimensions at the nanoscale—approximately 1–100 nanometers. To get an idea of their size, a human hair is approximately 80,000–100,000 nanometers wide.

The thinking is that nanomaterials may have properties different from the same chemical substances with structures at a larger scale, such as greater strength, lighter weight, and greater chemical reactivity. These different properties give nanoscale materials a range of potentially beneficial public and commercial applications. For example, they could improve products such as anticancer therapies, TVs, vehicles, batteries, and solar panels.

However, the special properties that make nanoscale materials of potentially great benefit can also present new challenges for risk assessment and decision making. Their small size may allow them to pass through cell membranes or the blood-brain barrier, possibly resulting in unintended effects.

Note: Engineered nanomaterials are materials created by manipulation of matter at the nanoscale to produce new materials, structures, and devices. Unbound engineered nanoparticles are loose particles, particularly, that are easily dispersible in the air. They can also be in liquid suspensions that can get into the air as mists or dried suspensions that leave unbound particles on surfaces.

Existing Exposure Limits

To date, NIOSH has published recommended exposure limits (RELs) for titanium dioxide (TiO2), carbon nanotubes (CNTs), and carbon nanofibers (CNFs). The REL for fine TiO2 is 2.4 micrograms per cubic meter of air (μg/m3) and 0.3 μg/m3 10-hour time weighted average (TWA) airborne concentration for ultrafine TiO2. The REL for CNTs and CNFs is no more than 1 μg/m3 8-hour TWA. There are currently no other nanomaterial exposure limits.

Developing OELs for Nanomaterials

NIOSH is involved in a research project to identify OELs for new nanomaterials as part of the development of a WHO guideline for working safely with nanomaterials.

As pointed out by Finnish researchers in a recent NIOSH blog entry, many studies have shown that nanoscale particles can be more biologically active such as eliciting greater pulmonary inflammation at a given mass dose. Therefore, the mass-based OEL for the bulk material is not automatically a safe level for the nanomaterial.

Bridging, Grouping, Measuring

According to the researchers, the most pragmatic approach for developing OELs is to compare the properties of the new material to an existing one such as comparing specific nanofibers to asbestos and assuming that the toxicity will be similar. This procedure is variously called ‘bridging’ or ‘read across.’

Another proposal outlined by the researchers is called “grouping,” simply stated, make groups of nanomaterials with similar toxicity. For example, for the nanomaterials of chemicals that already have established toxic effects such as causing cancer, British researchers have proposed to use an extra safety factor of 10 because of the nanosize. This means that the OEL for the bulk material is divided by ten to derive the OEL for the same but nanosized material. The assumption is that the toxic properties will be enhanced by the nanosize. Another proposed group is a group of insoluble or biopersistent nanomaterials, materials that, after inhalation, are not easily cleared away from the human body. For biopersistant nanomaterials, a factor 15 lower than the bulk OEL has been suggested. For soluble nanomaterials, a safety factor of two for the nanosize has been suggested.

Most OELs are expressed as a mass concentration (mg/m3). But this may not work for nanomaterials. One suggested alternative is the number concentration: the number of particles in a cubic meter of air. Another one is the surface concentration: the total surface of the particles in a cubic meter of air. The determinant could be what causes an increase of toxicity.

Key Takeaway for EHS Managers

Nanomaterials are found in an increasing number of workplaces. Many American workers are either involved in the manufacture of nanomaterials or handling products made with nanomaterials. Nanomaterial researchers are calling for OELs that are substantially lower than those for related bulk materials and incentives to reduce workplace exposure to nanomaterials.

Check tomorrow’s Advisor for tips on assessing exposure to nanomaterials in your workplace.

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