NANOMATERIAL SAFETY GUIDELINES

Thomas J. Bruno and Beverly L. Smith

The classification of nanomaterials includes nano-objects and nanoparticles; nano-objects are materials with at least one dimension (length, width, height, and/or diameter) that is between 1 nm (1 × 10^-9 m) and 100 nm, and nanoparticles are materials in which all three dimensions are on this scale. [Note that the ASTM definition allows for two dimensions between 1 nm and 100 nm.] Beyond scale, nanomaterials can be classified as natural, incidental, and engineered, depending on origin. Natural nanomaterials include volcanic products, viruses, sea spray, and mineral aerosols, and are ubiquitous in nature at appreciable concentrations. Incidental nanomaterials include metal vapors produced during welding, sandblasting dust and other industrial effluents, cooking smoke, and diesel engine particulates. The environmental health and safety aspects of natural nanomaterials have received some study, and among the incidental nanomaterials, welding vapors and diesel fuel particulates have received extensive study. In recent years, however, there has been a great emphasis on engineered nanomaterials, and it is this class, which includes metal nanoparticles, nanorods, nanowires, nanotubes, Buckyballs, nanocapsules, and quantum dots that are the main concern here. Study of the environmental health and safety risks of engineered nanomaterials remains an active area of research that is receiving increasing attention due to the widespread use of these materials in numerous applications ranging from medicine to energy storage. While much is still unknown regarding the fate and toxicity of this class of materials, here, we provide some general guidelines for the safe handling of nanomaterials. We begin with some simplified definitions or terms used in nanotechnology, needed for understanding of these safety guidelines as well as those provided elsewhere.

Aerodynamic diameter: An indirect measure of particle diameter defined as the diameter of a sphere with a density of 1000 kg/m³, having the same settling velocity of a particle of interest.

Agglomerate: A group of particles (which may include nanoparticles) held together in a loose cluster by weak forces that may include van der Waals forces, surface tension, and electrostatic forces. Agglomerates are often resuspendable.

Aggregate: A heterogeneous particle held together with relatively strong forces such that the particle is not easily disassembled. Aggregates are typically not resuspendable.

Buckyballs: Spherical carbon (C60) fullerenes.

Fullerenes: Molecules composed entirely of carbon, usually in the form of a hollow sphere, ellipsoid, or tubes.

Graphene: A one atom thick sheet of carbon.

Multi-walled carbon nanotube: Multiple sheets of sheet graphene wrapped into a tube of nanoscale dimensions.

Nanoaerosol: A collection of nanomaterials suspended in a gas.

Nanocolloid: A nanomaterial suspended in a gel or other semi-solid substance.

Nanocomposite: A solid material composed of two or more nanomaterials having different physical characteristics.

Nanohydrosol: A nanomaterial suspended in a solution.

Nanotube: A seamless tube with a diameter on the order of nanometers.

Nanowire: A wire of dimensions on the order of nanometers.

Quantum dot: A nanomaterial or nanoparticle that confines the motion of conduction band electrons, valence band holes, or excitons (pairs of conduction band electrons and valence band holes) in all three spatial directions.

Single-walled carbon nanotube: A single-sheet graphene wrapped into a tube of nanoscale dimensions.

Ultrafine particle: A (usually) airborne particle with a diameter less than 100 nm.

Safety Issues and Exposure Routes

The unique safety issues posed by nanomaterials result from the potential of deep penetration into tissue, the potential of passing through the blood-brain barrier, and the possible ability to translocate between organs. Biological effects result from the size, shape, polarity/charge, adsorptive capacity, and surface composition, and the ability to bind biological proteins and receptors. Nanomaterials have a higher reactivity than the parent compounds, often having catalytic effects and often presenting greater flammability or explosion risks. For example, bulk elemental gold is considered inert, but gold nanoparticles below 5 nm are catalytic toward a number of oxidation reactions.

The most obvious exposure route of nanomaterials is respiratory; particles depositable in the air exchange region of the lungs are considered respirable. Ingestion can occur from unintentional hand-to-mouth transfer. Finally, nanoparticles can be absorbed through skin or cuts/abrasions to the skin.

Guidelines for Safe Handling of Nanomaterials

The safe handling of nanomaterials will generally follow the usual laboratory safety grid:

Elimination – A change in the experimental design to avoid the hazard
Substitution – The use of a surrogate of lower hazard
Engineering Controls – The use of enclosures, fume hoods, etc.
Administrative Controls – Adherence to standard procedures and protocols
Personal Protective Equipment (PPE) – The last line of safety, including gloves, clothing, respirators, etc.

Clearly, elimination and substitution are most useful for nanoparticles of incidental origin. Research with engineered nanoparticles must make use of engineering controls, administrative controls, and PPE.

The use of non-regenerating general ventilation systems, such as fume hoods and high-efficiency particulate air (HEPA) dust collection systems, are critical to safe handling. Where possible, installation of ultralow particulate air (ULPA) filters should be used, since they are widely viewed as being more effective for engineered nanomaterials. The lab in which nanomaterials are handled should be under negative pressure relative to the surroundings (corridor, service galley, etc.). The entry on "Chemical Fume Hoods and Biological Safety Cabinets" in this section provides additional information on fume hood selection and operation.

Where possible, manipulations should be conducted in solution (or in a liquid phase) to minimize the potential of aerosol formation.

Solution phase nanomaterials should be handled wearing gloves. Gloves should be compatible with the solvent used to disperse the nanomaterials in solution. In general, nitrile gloves are recommended, and double gloving is advisable for heavy usage or prolonged usage. Gloves with cuffs, clothing with full sleeves to protect wrists, or a laboratory coat are recommended. Some glove materials may have reactivity with certain nanomaterials, and this must be considered before selecting the glove material. Liquid or solution phase nanomaterial manipulations are best conducted in a fume hood or biological safety cabinet, especially when employing nanomaterials dispersed in open containers, in solvents with known health risks, or when higher-risk activities such as sonication, agitation, and vortex mixing are involved. Manipulations conducted in closed containers need not be performed in a fume hood or biological safety cabinet; however, a closed container should be opened inside such an enclosure because aerosols could be released upon opening. Disposable bench covers should be used where spillage is possible. Any spillage should be cleaned up immediately. Contaminated gloves should be removed and replaced immediately.

Manipulation of dry nanomaterials must be performed in a fume hood or biological safety cabinet. Transport of dry nanomaterials from place to place in the lab must be done in closed containers.

For manipulations of air-sensitive nanomaterials, a glove box or glove bag is required.

Hand washing must be done after manipulations of nanomaterials. Work areas should be cleaned after completion of tasks. Adequate consideration should be given to tasks involved with the maintenance of equipment or instrumentation used in work on nanomaterials. Such maintenance should be done with the assumption of the presence of nanomaterials.

Waste Disposal

Though the fate and toxicity of nanomaterials remains largely unknown and is still an area of active investigation, nanomaterials and any by-products from their synthesis should be treated as potentially hazardous waste. Nanomaterials should be properly disposed of based on their nanomaterial and solvent compositions. Nanomaterials containing heavy metals should be treated accordingly and separately from other waste streams. When possible, it is advisable to collect or fully dissolve nanomaterials that are present in solution to limit the volume of waste generated. Often nanomaterials can be aggregated or precipitated from solution with an anti-solvent and filtered off to be recycled or collected as solid waste. This is called “crashing out” in laboratory vernacular. Alternatively, adsorbents such as activated charcoal can often be used to remove certain types of nanomaterials from solution upon filtration and collection of the adsorbent following exposure to the nanomaterial-containing solution. Other types of nanomaterials such as metal oxides can often be dissolved completely with strong acids.