Nanomaterials: Potential Risks and Safe Handling Methods
What are nanomaterials
Nanomaterials are generally in the 1-100 nm range and can be composed of many different base materials (carbon, silicon, and metals such as gold, cadmium, and selenium). Nanomaterials also have different shapes: referred to by terms such as nanotubes, nanowires, crystalline structures such as quantum dots, and fullerenes. Nanomaterials often exhibit very different properties from their respective bulk materials. About 40-50% of the atoms in nanoparticles are on the surface, which can result in greater reactivity than bulk materials.
Particles in the nanometer size range do occur in nature and as a result of existing industrial processes. Nanosized particles are part of the range of atmospheric particles generated by natural events such as volcanic eruptions and forest fires. They also form part of the fumes generated during welding, metal smelting, automobile exhaust, and other industrial processes. One concern about small particles that are less than 10 um is that they are respirable and reach the lungs.
The current nanotechnology revolution differs from past industrial processes because nanomaterials are being engineered and fabricated, rather than occurring as a byproduct of other activities. The nanomaterials being engineered have different properties compared to those of the parent compounds. Since their properties are different when they are small, it is expected that they will have different effects on the body and will need to be evaluated separately from the parent compounds for toxicity.
Currently nanomaterials have a limited commercial market. Some nanomaterials are used as catalyst supports in catalytic converters; nanosized titanium dioxide particles are used as a component of sunscreens; carbon nanotubes have been used to strengthen tennis rackets; components in silicon chips are reaching the 100 nm range. Research and industrial labs are working at the intersection of engineering and biology to extend uses to medicine as well as all areas of engineering. Government agencies in the US and Europe are beginning to fund toxicology research to understand the hazards of these materials before they become widely available.
What are the toxic effects of nanomaterials tested to date?
Any toxic effects of nanomaterials will be very specific to the type of base material, size, ligands, and coatings. One of the earliest observations was that nanomaterials, also called ultrafine particles (<100 nm), showed greater toxicity than fine particulates (<2.5 um) of the same material on a mass basis. This has been observed with different types of nanomaterials, including titanium dioxide, aluminum trioxide, carbon black, cobalt, and nickel. For example, nano size titanium dioxide particles produced 43 fold more inflammation than 250 nm particles in test animals. The increase in inflammation is believed to be due to the much greater surface area of the small particles for the same mass of material.
Nanoparticles are generally similar in size to proteins in the body. They are considerably smaller than many cells in the body. Cells growing in tissue culture will pick up most nanoparticles. The ability to be taken up by cells is being used to develop nanosized drug delivery systems and does not inherently indicate toxicity. Gold nanoparticles are being investigated in biomedical applications. Research has demonstrated that positively charged gold particles with quaternary ammonium substituted side chains were toxic to two types of mammalian cells while negatively charged gold particles with carboxylate substituted side chains did not show cellular toxicity even when tested at much higher concentrations.
Once in the body, some types of nanoparticles may have the ability to be distributed to other organs, including the central nervous system. Silver, albumin, and carbon nanoparticles all showed systemic availability after inhalation exposure. Significant amounts of carbon particles (22-30 nm in diameter) were found in the livers of rats after 6 hours of inhalation exposure to 80 or 180 ug/m3. In contrast, only very small amounts of 192Ir particles (15 nm) were found systemically. Research also found that inhaled carbon particles reached the olfactory bulb and also the cerebrum and cerebellum, suggesting that migration to the brain occurred through the nasal mucosa along the olfactory nerve to the brain. The ability of nanomaterials to move about the body may depend on their chemical reactivity, surface characteristics, and ability to bind to body proteins.
There is currently no consensus about the ability of nanoparticles to penetrate through the skin. Particles in the micrometer range are generally thought to be unable to penetrate through the skin. The outer skin consists of a 10 um thick, tough layer of dead keratinized cells that is resistant to penetration by particles, ionic compounds, and water soluble compounds. It had been demonstrated that 0.5 and 1 um dextran spheres penetrated "flexed" human skin in an in vitro experiment. Particles penetrated into the epidermis and a few entered the dermis only during flexing of the skin. Particles 2 and 4 um in diameter did not penetrate.
Nano sized titanium dioxide (40 nm) is currently being used in sunscreens and cosmetics as sun protection. The nm particles are transparent and do not give the cosmetics the white, chalky appearance that coarser preparations did. The nano particles have been found to penetrate into the outermost layer of the skin and more deeply into hair follicles and sweat glands than micrometer sized particles though they did not reach the epidermis layer and dermis layers. There is also a concern that nano sized titanium dioxide particles have higher photo-reactivity than coarser particles and may generate free radicals that can cause cell damage. Some manufacturers have addressed this issue by coating the particles to prevent free radical formation. The FDA has reviewed available information and determined that titanium dioxide particles are not a new ingredient but a specific grade of the original product.
Quantum dots (QD) are nanocrystals containing 1000 to 100,000 atoms and exhibiting unusual "quantum effects" such as prolonged fluorescence. They are being investigated for use in immunostaining as alternatives to fluorescent dyes. The most commonly used material for the core crystal is cadmium-selenium, which exhibits bright fluorescence and high photostability. Both bulk cadmium and selenium are toxic to cells. One of the primary sites of cadmium toxicity is the liver.
Early studies found that Cd-Se quantum dots were not toxic to test cell lines used for these studies. It was demonstrated that coating the Cd-Se QDs with ZnS, polyethylene glycol, or other coatings prevented toxicity during a two week incubation with liver cells. It was concluded that Cd-Se QDs can be made nontoxic with appropriate surface coatings but future use must be carefully evaluated to rule out release of Cd+2 over time.
Carbon nanotubes (CNT) can have either single or multiple layers of carbon atoms arranged in a cylinder. The dimensions of single wall carbon nanotubes (SWCNT) are about 1-2 nm in diameter by 0.1 um in length. Multiple wall carbon nanotubes (MWCNT) are 20 nm in diameter and 1 mm long. CNT behave like fibers in the lung. They have properties very different from bulk carbon or graphite. They have great tensile strength and are potentially the strongest, smallest fibers known. CNT have been tested in short term animal tests of pulmonary toxicity and the results suggest the potential for lung toxicity though there are questions about the nature of the toxicity observed and the doses used.
In an unpublished study, physiologically relevant doses, found granulomas (granulomas are small nodules that are seen in a variety of diseases), fibrosis (excess fibrous connective tissue), and increased indicators of inflammation from both SWCNT and quartz. More extensive inhalation studies are being conducted. One mitigating factor concerning lung toxicity is that CNT have a tendency to clump together to form nanoropes, which are large, non respirable clumps, and may prevent inhalation exposure in many instances. A cellular study found that one type of SWCNT inhibited cell growth and decreased cell adhesion ability of human kidney cells. Another study found that MWCNT were taken up by human skin cells, decreasing cell viability and causing release of inflammatory compounds.
Fullerenes are another category of carbon based nanoparticles. The most common type has a molecular structure of C60 which take the shape of a ball shaped cage of carbon particles arranged in pentagons or hexagons. Fullerenes have many potential medical applications as well as applications in industrial coatings and fuel cells, so a number of preliminary toxicology studies have been done with them. In cell culture, different types of fullerenes produced cell death at concentrations of 1 to 15 ppm in different mammalian cells when activated by light. Another found that toxicity could be eliminated when carboxyl groups were attached to the fullerene surface to increase water solubility. Cell death in this study appeared to be a function of damage to the cell membranes. An animal study found that water soluble polyalkylsulfonated C60 produced no deaths in rats when given orally but was moderately toxic when administered by injection. Doses of 100 to 600 mg/kg also produced an unusual form of kidney toxicity. Finally, the first study investigating aquatic toxicology, found that 48 hours of exposure to 0.5 and 1.0 ppm of uncoated pure C60 produced cell damage in the brains of fish. The changes in the brain as a result of the short exposure did not appear to affect the behavior of the fish but were an indication of stress. An additional concern generated by this study is the effects of release of durable carbon nanomaterials into the environment.
How to work safely with nanomaterials
The preliminary conclusions to be drawn from the toxicology studies is that some types of nanomaterials can be toxic, if they are not bound up in a substrate and they are available to the body. Researchers must use procedures that prevent inhalation and dermal exposures because at this time toxicological information is limited.
Based on particle physics and studies of fine atmospheric pollutants, the nanoparticle size range is the range of minimum settling. This means that once released into air, nanoparticles will remain airborne for considerable periods of time. Nanoparticles can be inhaled and will be collected in all regions of the respiratory tract; about 35% will deposit in the deep region of the lungs.
Because they are so small, nanoparticles follow airstreams more easily than larger particles, so they will be easily collected and retained in standard ventilated enclosures such as fume hoods. In addition, nanoparticles are readily collected by HEPA filters. Respirators with HEPA filters will be adequate protection for nanoparticles in case of spills of large amounts of material.
Working safely with nanomaterials involves following standard procedures that should be followed for any particulate material with known or uncertain toxicity: preventing inhalation, skin contact, and ingestion. Many nanomaterials are synthesized in enclosed reactors or glove boxes. The enclosures are under vacuum or exhaust ventilation, which prevent exposure during the actual synthesis. Inhalation exposure can occur during additional processing of materials removed from reactors, this processing should be done in fume hoods. In addition, maintenance on reactor parts that may release residual particles in the air should be done in fume hoods. Another process, the synthesis of particles using sol-gel chemistry, should be carried out in ventilated fume hoods or glove boxes.
The type of surface coating on nanoparticles often causes them to clump together so that few particles are actually released when particles are removed from reactors. In one of the few workplace industrial hygiene studies of nanoparticles, almost no release of fibers was found when carbon nanotubes were removed from a reactor and transferred into a secondary container. The SWCNT clumped together into nanoropes and remained attached to the substrate as it was removed from the reactor. The type of SWCNT investigated in this study were uncoated with about 30% Fe catalyst remaining as part of the nanoropes. Researchers are attempting to coat CNT and other nanoparticles with materials that make them less sticky and more easily dispersed; if successful, this would make them more easily aerosolized and require additional care when handling.
Since the ability of nanoparticles to penetrate the skin is uncertain at this point, gloves should be worn when handling particulates and solutions containing particles. A glove having good chemical resistance to any solution the particles are suspended in should be used. If working with dry particulate, a sturdy glove with good integrity should be used. Disposable nitrile gloves commonly used in many labs would provide good protection from nanoparticles for most procedures that don’t involve extensive skin contact. Two pairs of gloves can be worn if extensive skin contact is anticipated.
One potential safety concern with nanoparticles is fires and explosions if large quantities of dust are generated during reactions or production. This is expected to become more of a concern when reactions are scaled up to pilot plant or production levels. Both carbonaceous and metal dusts can burn and explode if an oxidant such as air and an ignition source are present. Nanodusts can be anticipated to have a greater potential for explosivity than larger particles.
There are currently no government occupational exposure standards for nanomaterials. One should also be aware that Material Safety Data Sheets (MSDS) may not have accurate information at this point in time. For example, the MSDSs that are accompanying some commercially available carbon nanotubes are referring to the graphite Permissible Exposure Limit (PEL) as a relevant exposure standard. Both graphite and carbon nanotubes are composed of carbon arranged in a honeycomb pattern. However, graphite is composed of coarse particles while CNTs are shaped like fibers and have much different tensile and conductive properties than graphite. CNTs are much more toxic in the short-term animal tests that have been performed to date. Consequently, the graphite PEL and toxicity information is not appropriate for MSDSs of CNTs. CNTs should be treated as potentially toxic fibers, if capable of being released into the air and not bound up in a substrate, and should be handled with appropriate controls.
How to dispose of nanomaterials
Before disposal of any nanomaterial, please call the EHS Office for a hazardous waste determination. There are no specific guidelines from the EPA for disposal of waste nanomaterials. Currently the disposal requirements for bulk materials should be followed. Bulk carbon is considered a flammable solid, so carbon based nanomaterials should be collected as hazardous waste. Many nanoparticles contain toxic metals such as cadmium and must also be picked up for hazardous waste disposal.
Summary
There have been a number of research articles on the toxicity of different types of nanomaterials. These studies have suggested effects at the cellular level and in short-term animal tests. The effects seen depend on the base material of the nanoparticle, its size and structure, and its constituents and coatings. Additional toxicology testing is being funded or planned by the National Science Foundation (NSF), the National Toxicology Program, and other research organizations in the US and in Europe. Exposure to nanomaterials can be controlled using the same precautions currently used to handle toxic materials: use of exhaust ventilation (such as fume hoods and vented enclosures) to prevent inhalation exposure during procedures that may release aerosols or fibers and use of gloves and Personal protective equipment to prevent dermal exposure. The EHS Office will continue to review health and safety information about nanomaterials as it becomes available and distribute it.
Nanomaterial handling process
Purpose
At this time there are no published regulations for safe work practices with nanomaterials. Man made nanoparticles may pose risk to human health due to their composition, size and ability to cross cell membranes. Engineered nanomaterials may exhibit higher toxicity due to their size compared to larger particles of similar composition. Current information about risk associated with nanoparticle exposure is limited. Safe work practices are generally based on understanding the hazards of the materials. Until more definitive information is available on the risks associated with nanomaterials precautionary work practices will be established and followed. Risk assessments and control strategies for nanotechnology research will be based on the most current toxicological data, exposure assessments, and exposure control information available.
Definitions
- Nanomaterials
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Nanomaterials are defined as having at least one dimension less than 100 nanometers (nm - 10-9 meters). From an exposure standpoint, only insoluble nanomaterials are considered.
Procedures
Engineering Controls
Control of airborne exposure to nanoparticles will primarily be accomplished using engineering control similar to those used for general aerosols and vapors. Conventional capture exhaust ventilation such as a properly operating chemical fume hood is effective in preventing exposure. Glove box containment is also effective. Passing capture exhaust through a HEPA filter will provide protection against release of nanoparticles into the environment.
Activities which would require engineering controls include:
- Working with nanomaterial in a liquid media during pouring or agitation which could release aerosols
- Fabricating nanoparticles
- Handling nanomaterials powder
- Maintenance or cleaning of equipment used to produce nanomaterials
Work Practices
Good work practices will help minimize exposure to nanomaterials: These work practices are consistent with general good laboratory practice.
- Storage or consumption of food or drink in areas where nanomaterials is handled is prohibited
- Application of cosmetics, etc... in areas where nanomaterials is handled is prohibited
- Personnel must wash hands before leaving area and after removing protective gloves.
- Lab coats can become contaminated and shall not be worn outside of the lab.
- Personnel should avoid touching the face or other exposed skin after working with nanomaterials before hand washing.
- All containers containing nanomaterials must be labeled consistent with existing laboratory requirements.
- Cleaning of areas must be done with wet wiping or HEPA vacuuming. Dry sweeping or using compressed air is prohibited. Disposal of contaminated cleaning materials must comply with hazardous waste disposal policies.
Personal Protective Equipment
There are currently no universally accepted guidelines on the selection of personal protective equipment (PPE) while working with nanomaterials. Some nanoparticles have shown the ability to penetrate the epidermis and possibly enter the bloodstream. If the particles are suspended in a liquid, penetration of the liquid can be enhanced if the glove is permeable to the liquid. Glove selection recommendations can be made by EHS based on the task and solvent used to suspend the nanoparticles. A glove selection chart based on solvent resistant properties of glove material is available on the web.
Outer clothing should consist of at least a lab coat while working with nanoparticles. Lab coats should not be worn outside the laboratory to avoid transferring any contamination to other areas of the facility. Clothing requirements can be upgraded to higher levels of protection as circumstances require.
If worker exposure to nanoparticles remains a concern after all feasible engineering controls are instituted, respiratory protection may be deemed necessary. The work site must be evaluated by EHS and appropriate respiratory protection recommended. The worker must comply with the respiratory protection program. This includes a physical evaluation and respirator fit testing on an annual basis. EHS will assist in the selection of the specific respirator requirements.
Clean up of spills
Cleanup of spills will comply with current prudent practices modified by any information on the reactive characteristics of the particles being used. Cleaning will be performed in a manner that minimizes production of aerosols. Clean up of spilled material will be approached by preferentially employing wet wipe methods if possible. Cleaning larger spills may involve collection of the bulk material first followed by wet wiping. Use of HEPA filtered vacuums may be allowable, dry sweeping is prohibited.
Considerations for animal studies
Dosing and necropsy of exposed animals must be conducted in a capture exhaust hood. All handling of animals and disposal of necropsy waste must be detailed in the research protocol and will be reviewed by EHS.
Preparation before beginning work
Prior to working with nanomaterials all available information on the specific material must be reviewed. Particle size is a significant component of nanoparticle toxicity potential and should be available for evaluation. If the research involves producing nanomaterial, information on particle size distribution, particle composition and particle configuration should be developed and made available to EHS as part of the research protocol.
Experimental Protocol
Experimental protocol developed for production or use of nanomaterial must include specific requirements addressing health and safety protection. The protocol must be reviewed by EHS.
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