The health and safety hazards of nanomaterials include the potential toxicity of various types of nanomaterials, as well as fire and dust explosion hazards. Because nanotechnology is a recent development, the health and safety effects of exposures to nanomaterials, and what levels of exposure may be acceptable, are subjects of ongoing research. Of the possible hazards, inhalation exposure appears to present the most concern, with animal studies showing pulmonary effects such as inflammation, fibrosis, and carcinogenicity for some nanomaterials. Skin contact and ingestion exposure, and dust explosion hazards, are also a concern.
Guidance has been developed for hazard controls that are effective in reducing exposures to safe levels, including substitution with safer forms of a nanomaterial, engineering controls such as proper ventilation, and personal protective equipment as a last resort. For some materials, occupational exposure limits have been developed to determine a maximum safe airborne concentration of nanomaterials, and exposure assessment is possible using standard industrial hygiene sampling methods. An ongoing occupational health surveillance program can also help to protect workers. Microplastics and nanoparticles from plastic containers are an increasing concern.
Background
Nanotechnology is the manipulation of matter at the atomic scale to create materials, devices, or systems with new properties or functions, with potential applications in energy, healthcare, industry, communications, agriculture, consumer products, and other sectors. Nanomaterials have at least one primary dimension of less than 100 nanometers, and often have properties different from those of their bulk components that are technologically useful. The classes of materials of which nanoparticles are typically composed include elemental carbon, metals or metal oxides, and ceramics. According to the Woodrow Wilson Center, the number of consumer products or product lines that incorporate nanomaterials increased from 212 to 1317 from 2006 to 2011. Worldwide investment in nanotechnology increased from $432 million in 1997 to about $4.1 billion in 2005.
Because nanotechnology is a recent development, the health and safety effects of exposures to nanomaterials, and what levels of exposure may be acceptable, is not yet fully understood. Research concerning the handling of nanomaterials is underway, and guidance for some nanomaterials has been developed. As with any new technology, the earliest exposures are expected to occur among workers conducting research in laboratories and pilot plants, making it important that they work in a manner that is protective of their safety and health.
A risk management system is composed of three parts. Hazard identification involves determining what health and safety concerns are present for both the nanomaterial and its corresponding bulk material, based on a review of safety data sheets, peer-reviewed literature, and guidance documents on the material. For nanomaterials, toxicity hazards are the most important, but dust explosion hazards may also be relevant. Exposure assessment involves determining actual routes of exposure in a specific workplace, including a review of which areas and tasks are most likely to cause exposure. Exposure control involves putting procedures in places to minimize or eliminate exposures according to the hierarchy of hazard controls. Ongoing verification of hazard controls can occur through monitoring of airborne nanomaterial concentrations using standard industrial hygiene sampling methods, and an occupational health surveillance program may be instituted.
A recently adopted risk management method is the Safe by design (SbD) approach. It aims to eliminate or reduce risks of new technologies including nanotechnology, at the design stage of a product or production process. Anticipation of risks is challenging because some risks could emerge only after a technology is implemented (at later stages in the innovation process). In the later cases, the application of other risk management strategies based on non-design principles need to be applied. It considers the purposes and constrains for implementation of SbD approaches in the industrial innovation process and on the basis of those, establish optimal workflows to identify risks and propose solutions to reduce or mitigate them as early as possible in the innovation process called Safe by Design strategies.
Hazards
Toxicity
Respiratory
Inhalation exposure is the most common route of exposure to airborne particles in the workplace. The deposition of nanoparticles in the respiratory tract is determined by the shape and size of particles or their agglomerates, and they are deposited in the alveolar compartment to a greater extent than larger respirable particles. Based on animal studies, nanoparticles may enter the bloodstream from the lungs and translocate to other organs, including the brain. The inhalation risk is affected by the dustiness of the material, the tendency of particles to become airborne in response to a stimulus. Dust generation is affected by the particle shape, size, bulk density, and inherent electrostatic forces, and whether the nanomaterial is a dry powder or incorporated into a slurry or liquid suspension.
Animal studies indicate that carbon nanotubes and carbon nanofibers can cause pulmonary effects including inflammation, granulomas, and pulmonary fibrosis, which were of similar or greater potency when compared with other known fibrogenic materials such as silica, asbestos, and ultrafine carbon black. Some studies in cells or animals have shown genotoxic or carcinogenic effects, or systemic cardiovascular effects from pulmonary exposure. Although the extent to which animal data may predict clinically significant lung effects in workers is not known, the toxicity seen in the short-term animal studies indicate a need for protective action for workers exposed to these nanomaterials. As of 2013, further research was needed in long-term animal studies and epidemiologic studies in workers. No reports of actual adverse health effects in workers using or producing these nanomaterials were known as of 2013. Titanium dioxide (TiO2) dust is considered a lung tumor risk, with ultrafine (nanoscale) particles having an increased mass-based potency relative to fine TiO2, through a secondary genotoxicity mechanism that is not specific to TiO2 but primarily related to particle size and surface area.
Dermal Some studies suggest that nanomaterials could potentially enter the body through intact skin during occupational exposure. Studies have shown that particles smaller than 1 μm in diameter may penetrate into mechanically flexed skin samples, and that nanoparticles with varying physicochemical properties were able to penetrate the intact skin of pigs. Factors such as size, shape, water solubility, and surface coating directly affect a nanoparticle's potential to penetrate the skin. At this time, it is not fully known whether skin penetration of nanoparticles would result in adverse effects in animal models, although topical application of raw SWCNT to nude mice has been shown to cause dermal irritation, and in vitro studies using primary or cultured human skin cells have shown that carbon nanotubes can enter cells and cause release of pro-inflammatory cytokines, oxidative stress, and decreased viability. It remains unclear, however, how these findings may be extrapolated to a potential occupational risk. In addition, nanoparticles may enter the body through wounds, with particles migrating into the blood and lymph nodes.
Gastrointestinal Ingestion can occur from unintentional hand-to-mouth transfer of materials; this has been found to happen with traditional materials, and it is scientifically reasonable to assume that it also could happen during handling of nanomaterials. Ingestion may also accompany inhalation exposure because particles that are cleared from the respiratory tract via the mucociliary escalator may be swallowed.
Fire and explosion
There is concern that engineered carbon nanoparticles, when manufactured on an industrial scale, could pose a dust explosion hazard, especially for processes such as mixing, grinding, drilling, sanding, and cleaning. Knowledge remains limited about the potential explosivity of materials when subdivided down to the nanoscale. The explosion characteristics of nanoparticles are highly dependent on the manufacturer and the humidity.
For microscale particles, as particle size decreases and the specific surface area increases, the explosion severity increases. However, for dusts of organic materials such as coal, flour, methylcellulose, and polyethylene, severity ceases to increase as the particle size is reduced below ~50 μm. This is because decreasing particle size primarily increases the volatilization rate, which becomes rapid enough that that gas phase combustion becomes the rate limiting step, and further decrease in particle size will not increase the overall combustion rate. While the minimum explosion concentration does not vary significantly with nanoparticle size, the minimum ignition energy and temperature have been found to decrease with particle size.
Metal-based nanoparticles exhibit more severe explosions than do carbon nanomaterials, and their chemical reaction pathway is qualitatively different. Studies on aluminum nanoparticles and titanium nanoparticles indicate that they are explosion hazards. One study found that the likelihood of an explosion but not its severity increases significantly for nanoscale metal particles, and they can spontaneously ignite under certain conditions during laboratory testing and handling.
High-resistivity powders can accumulate electric charge causing a spark hazard, and low-resistivity powders can build up in electronics causing a short circuit hazard, both of which can provide an ignition source. In general, powders of nanomaterials have higher resistivity than the equivalent micron-scale powders, and humidity decreases their resistivity. One study found powders of metal-based nanoparticles to be mid- to high-resistivity depending on humidity, while carbon-based nanoparticles were found to be low-resistivity regardless of humidity. Powders of nanomaterials are unlikely to present an unusual fire hazard as compared to their cardboard or plastic packaging, as they are usually produced in small quantities, with the exception of carbon black. However, the catalytic properties of nanoparticles and nanostructured porous materials may cause untended catalytic reactions that, based on their chemical composition, would not otherwise be anticipated.
Radioactivity Engineered radioactive nanoparticles have applications in medical diagnostics, medical imaging, toxicokinetics, and environmental health, and are being investigated for applications in nuclear medicine. Radioactive nanoparticles present special challenges in operational health physics and internal dosimetry that are not present for vapors or larger particles, as the nanoparticles' toxicokinetics depend on their physical and chemical properties including size, shape, and surface chemistry. In some cases, the inherent physicochemical toxicity of the nanoparticle itself may lead to lower exposure limits than those associated with the radioactivity alone, which is not the case with most radioactive materials. In general, however, most elements of a standard radiation protection program are applicable to radioactive nanomaterials, and many hazard controls for nanomaterials will be effective with the radioactive versions.
Hazard controls