Silver nanoparticles (AgNPs) are gaining attention from the academic and regulatory communities, not only because of their antimicrobial effects and subsequent product applications, but also because of their potential health and environmental risks. Whereas AgNPs in the aqueous phase are under intensive study, those in the atmosphere have been largely overlooked, although it is well established that inhalation of nanoparticles is associated with adverse health effects. This review summarizes the present state of knowledge concerning airborne AgNPs to shed light on the possible environmental exposure scenarios that may accompany the production and popularization of silver nanotechnology consumer products. The current understanding of the toxicity of AgNPs points toward a potential threat via the inhalation exposure route. Nanoparticle size, chemical composition, crystal structure, surface area, and the rate of silver ion release are expected to be important variables in determining toxicity. Possible routes of aerosolization of AgNPs from the production, use, and disposal of existing consumer products are presented. It is estimated that approximately 14% of silver nanotechnology products that have been inventoried could potentially release silver particles into the air during use, whether through spraying, dry powder dispersion, or other methods. In laboratory and industrial settings, six methods of aerosolization have been used to produce airborne AgNPs: spray atomization, liquid-flame spray, thermal evaporation-condensation, chemical vaporization, dry powder dispersion, and manual handling. Fundamental uncertainties remain about the fate of AgNPs in the environment, their short- and long-term health effects, and the specific physical and chemical properties of airborne particles that are responsible for health effects. Thus, to better understand the risks associated with silver nanotechnology, it is vital to understand the conditions under which AgNPs could become airborne.
Airborne nanoscale particles pose a threat to human health because of their abilities to deposit in all regions of the respiratory tract, be taken up by cells, and translocate to sensitive organs via the blood or lymph.1 Particles with at least one dimension smaller than 100 nm are typically described as “ultrafine” when occurring naturally or incidentally (e.g., secondary aerosol that condenses from gases or soot that forms during combustion) and as “nanoparticles” when purposefully engineered. Because of their antibacterial properties, silver nanoparticles (AgNPs) have become one of the most popular types of nanomaterials today. In terms of the number of consumer products and the volume of annual research investment, only carbonaceous nanomaterials exceed silver.2 Like all nanomaterials, AgNPs may present an inhalation toxicity hazard should they become airborne,3,4 a threat that has received inadequate attention and that is the focus of this review.
An important feature of nanoparticles is that, on a mass basis, more atoms are available at the particle’s surface to interact with its surroundings. At this scale, unique physicochemical characteristics appear, and reactivity is largely increased in comparison to the nanoparticles’ bulk counterparts.4–7 With silver, antiseptic efficacy increases as particle size decreases because of the higher surface area per unit volume and subsequently enhanced surface reactivity. 4,8–10 As shown in Figure 1, a 4-nm particle has 50% of its atoms on the surface, whereas a 30-nm particle has only 5% of its atoms on the surface.11,12 This orderof- magnitude difference exemplifies why surface forces are of critical importance in nanoparticles. These novel properties present opportunities for introducing and improving many products.
Bulk silver has historically been used in close contact to humans, in cutlery, jewelry, and currency. Ancient civilizations knew about silver’s antimicrobial potential,13 and colloidal silver has been used for centuries to heal wounds and preserve materials with no obvious toxic effects to humans. Silver compounds were heavily used as antiseptics in World War I, before the development of modern-day antibiotics.14 Soluble silver compounds (e.g., silver salts) have been used for treating mental illness, epilepsy, nicotine addiction, gastroenteritis, and infectious diseases.13,15