Pollution from nanomaterials

Products containing nanoparticles such as cosmetics, coatings, paints, and catalytic additives can release nanoparticles into the environment in different ways. There are three main ways that nanoparticles enter the environment. The first is emission during the production of raw materials such as mining and refining operations. The second is emission during use, like cosmetics or sunblock getting washed into the environment. The third is emission after disposal of nanoparticle products or use during waste treatment, like nanoparticles in sewage and wastewater streams.[3]

The first emission scenario, causing 2% of emissions, results from the production of materials. Studies of a precious metals refinery found that the mining and refining of metals releases a significant amount of nanoparticles into the air. Further analysis showed concentration levels of silver nanoparticles far higher than OSHA standards in the air despite operational ventilation.[4] Wind speed can also cause nanoparticles generated in mining or related activities to spread further and have increased penetration power. A high wind speed can cause aerosolized particles to penetrate enclosures at a much higher rate than particles not exposed to wind.[5]

Construction also generates nanoparticles during the manufacture and use of materials. The release of nanoscale materials can occur during the evacuation of waste from cleanout operations, losses during spray drying, filter residuals, and emissions from filters.[6] Pump sprays and propellants on average can emit 1.1 x 10^8 and 8.6 x 10^9 particles/g.[7]

A significant amount of nanoparticles are also released during the handling of dry powders, even when contained in fume hoods. Particles on construction sites can have prolonged exposure to the atmosphere and thus are more likely to enter the environment. Nanoparticles in concrete construction and recycling introduce a new hazard during the demolition process, which can pose even higher environmental exposure risks. Concrete modified with nanoparticles is almost impossible to separate from conventional concrete, so the release may be uncontrollable if demolished using conventional means. Even normal abrasion and deterioration of buildings can release nanoparticles into the environment on a long-term basis.[6] Normal weathering can release 10 to 10^5 mg/m^2 fragments containing nanomaterials.[7]

Another emission scenario is release during use. Sunscreens can release a significant amount of TiO₂ nanoparticles into surface waters. Testing of the Old Danube Lake indicated that there were significant concentrations of nanoparticles from cosmetics in the water. Conservative estimates calculate that there were approximately 27.2 micrograms/L of TiO₂, if TiO₂ was distributed throughout the entire 3.5*10^6 M^3 volume of the lake.[8]

Although TiO₂ is generally considered as weakly soluble, these nanoparticles undergo weathering and transformation under conditions in acidic soils with high proportions of organic and inorganic acids. There are observable differences in particle morphology between manufactured and natural TIO₂ nanoparticles, though differences may attenuate over time due to weathering. However, these processes are likely to take decades.[9]

Copper and zinc oxide nanoparticles that get into the water can additionally act as chemosensitizers in sea urchin embryos.[10] It is predicted that exposure through sunscreen is probably the most important route for animals in aquatic systems to get exposure to harmful metal particles.[11] ZnOs from sunblock and other applications like paints, optoelectronics, and pharmaceuticals are entering the environment at an increasing rate. Their effects can be genotoxic, mutagenic, and cytotoxic.[12]

Nanoparticles can be transported through different mediums depending on their type. Emissions patterns have found that TiO₂ NPs accumulate in sludge-treated soils. This means that the dominating emission pathway is through wastewater. ZnO generally collects in natural and urban soil as well as landfills. Silver nanoparticles from production and mining operations generally enter landfills and wastewater. Comparing different reservoirs by how readily Nanoparticles pollute them, ~63-91% of NPs accumulate in landfills, 8-28% in soils, aquatic environments receive ~7%, and air around 1.5%.[3]

Knowledge of the effects of industrial nanoparticles (NPs) released into the environment remains limited. Effects vary widely over aquatic and terrestrial environments as well as types of organisms.[13] The characteristics of the nanoparticle itself plays a wide variety of roles including size, charge, composition, surface chemistry, etc.[14] Nanoparticles released into the environment can potentially interact with pre-existing contaminants, leading to cascading biological effects that are currently poorly understood.[15]

Several scientific studies have indicated that nanoparticles can cause a series of adverse physiological and cellular effects on plants including root length inhibition, biomass reduction, altered transpiration rate, developmental delay, chlorophyll synthesis disruption, cell membrane damage, and chromosomal aberration.[16] Though genetic damage induced by metal nanoparticles in plants has been documented, the mechanism of that damage, its severity, and whether the damage is reversible remain active areas of study.[17] Studies of CeO2 nanoparticles were shown to greatly diminish nitrogen fixation in the root nodules of soybean plants, leading to stunted growth. Positive charges on nanoparticles were shown to destroy the membrane lipid bilayers in animal cells and interfere with overall cellular structure. For animals, it has been shown that nanoparticles can provoke inflammation, oxidative stress, and modification of mitochondrial distribution.[18] These effects were dose-dependent and varied by nanoparticle type.[14]

Present research indicates that biomagnification of nanoparticles through trophic levels is highly dependent upon the type of nanoparticles and biota in question. While some instances of bioaccumulation of nanoparticles exist, there is no general consensus.[14][19]

There is no clear consensus on potential human and ecological impacts stemming from exposure to ENMs.[20] As a result, developing reliable methods for testing ENM toxicity assessment has been a high priority for commercial usage. However, ENMs are found in a variety of conditions making a universal testing method non-viable. Currently, both in-vitro and in-vivo assessments are used, where the effects of NPs on events such as apoptosis, or conditions like cell viability, are observed.[21]

In measuring ENMs, addressing and accounting for uncertainties such as impurities and biological variability is crucial. In the case of ENMs, some concerns include changes that occur during testing such as agglomeration and interaction with substances in the testing media, as well as how ENMS disperse in the environment.[20] For example, one investigation into how the presence of fullerenes impacted largemouth bass in 2004[22] concluded that fullerenes were responsible for neurological damage done to the fish, whereas subsequent studies revealed this was actually a result of byproducts resulting from the dispersal of fullerenes into tetrahydrofuran (THF) and minimal toxicity was observed when water was used in its place.[23] Fortunately, greater thoroughness in the process of testing could help to resolve these issues. One method that has proven useful in avoiding artifacts is the thorough characterization of ENMS in the laboratory conducting the testing rather than just relying on the information provided by manufacturers.[24]

In addition to problems that can arise due to testing, there is contention on how to ensure testing is done for environmentally relevant conditions, partly due to the difficulty of detecting and quantifying ENMs in complex environmental matrices.[25] Currently, straightforward analytical methods are not available for the detection of NPs in the environment, although computer modeling is thought to be a potential pathway moving forward.[26] A push to focus on the development of internationally agreed upon unbiased toxicological models holds promise to provide greater consensus within the field as well as enable more accurate determinations of ENMs in the environment.[27]