Introduction

Nanotechnology is one of the fastest developing fields, and as such, it is becoming more accessible to a broad spectrum of consumers. This is related to the dissemination of everyday products, in the production of which nanostructured materials are used (Chuankrerkkul and Sangsuk 2008). The prefix “nano” comes from Greek and means “dwarf”, which refers to the order of magnitude of matter that has led to the interest in nanotechnology (Kovvuru et al. 2012). Nanotechnology deals with designing, obtaining and using materials whose most important feature is to have at least one dimension not exceed 100 nm (Osuwa and Anusionwu 2011). In cases where the size of the material ranges between values appropriate for individual atoms (10−9 m) and crystals (10−7 m), the properties of such a structure may differ significantly from the typical properties of the corresponding single atoms and crystals (Roduner 2006).

From a commercial point of view, nanomaterials are profitable products. Their value is undoubtedly influenced by the ability to modify their properties. This is accomplished by manipulating the size and shape of the nanoparticles. Designing the sizes of obtained nanomaterials and in desired shapes is possible by using a variety of methods to manufacture them. The method used to obtain nanomaterials is the most important factor affecting the properties of the nanostructures; for example, in order to obtain silver nanoparticles, a reduction reaction is used that may be either chemical, photochemical or electrochemical in nature. The quantitative and qualitative composition of the used substrates is of great importance as well. In addition, the final form of nanomaterial is also affected by pH, temperature and the order of component mixing (Khan et al. 2010; Kader et al. 2014). Nanotechnology creates many new potential solutions in various branches of science and industry. It is currently being used, among other areas, in biomedicine, cosmetology, pharmacy, optoelectronics, animal husbandry, crop production, food processing and plastics (Pulit et al. 2011a, b).

The European Commission to the European Parliament, the Council and the European Economic and Social Committee assess that the annual production of nanomaterials in the world is about 11 million tons. Their value is estimated at € 20 billion. What is more, it is anticipated that production of nanomaterials will increase further by 2015 and the market value of products based on nanomaterials will reach around € 2 trillion (Communication From The Commission To The European Parliament 2012). These projections are based on the development of nanotechnology as a field, as a valuable tool in the design of new materials with distinctive properties.

Metallic nanoparticles belong to a group of materials of very special interest. Nanoparticles of silver, gold, copper and platinum, among others, stand out (Pulit et al. 2011a, b).

Compounds commonly used in processes for obtaining silver nanoparticles are widely described in the literature. The group of most common ones includes: formaldehyde, hydrazine hydrate, sodium borohydride, hydrogen, polyvinylpyrrolidone, sodium dodecylsulphate, chitosan and ethylene glycol. However, preparation of silver nanoparticles using the above-mentioned compounds involves their negative impact on the environment. These substances are toxic, irritating and harmful. Some of them are also carcinogenic and may cause allergic reactions. Their bioaccumulation in the environment can affect especially on aquatic organisms. Moreover, it is necessary to take a specific protection while operating with these substances. Hydrazine hydrate is a second category carcinogen substance, which is highly toxic to aquatic organisms. Formaldehyde also exhibits toxic, irritating and corrosive properties. Sodium borohydride in contact with skin releases flammable gases causes burns and is one of the highly flammable substances. Aniline is also known to be a toxic substance, especially in contact with skin. What is more, sodium dodecylsulphate which is commonly employed as stabilizing agent irritates the skin and pollutes the water with sulphur compounds. Polyvinylpyrrolidone may contain unreacted vinylpyrrolidone monomers which are carcinogenic, and ethylene glycol with the air creates explosive mixtures, acts narcotic, damages the central nervous system and irritates the mucous membranes (Pulit et al. 2012). Nevertheless, the paper concerns the silver nanoparticles themselves whose toxic activity has been confirmed by numerous reports regarding this issue.

Silver nanoparticles

Nanocrystalline silver is one of the most studied nanomaterials. Single silver nanoparticles are characterized by a high fraction of surface atoms. The specific surface area of silver nanoparticles can reach several 100 m2/g. Their specific surface properties enrich silver nanoparticles with unusual physicochemical properties quite different from those typical of solid materials (Lu 2013). Silver nanoparticles are known for their antimicrobial properties and are an effective agent for destroying a wide range of Gram-negative and Gram-positive bacteria. They also act bactericidally against strains resistant to antibiotics (Wright et al. 1999). The group of Gram-negative bacteria against in which the biocidal activity of silver nanoparticles is confirmed includes: Acinetobacter (Niakan et al. 2013a, b), Escherichia (Li et al. 2010), Pseudomonas (Niakan et al. 2013a, b) and Salmonella (Petrus et al. 2011). The efficacy of silver nanoparticles has also been confirmed against Gram-positive bacteria, among which one can distinguish: Bacillus (Shahrokh and Emtiazi 2009), Enterococcus (Lotfi et al. 2011), Listeria (Zarei et al. 2014), Staphylococcus (Ahangaran et al. 2012) and Streptococcus (Cheng et al. 2013). Recent studies have shown that the use of silver nanoparticles in combination with certain antibiotics such as penicillin G, amoxicillin, erythromycin, clindamycin and vancomycin has a synergic effect in the fight against Escherichia coli lub Staphylococcus aureus (Shahverdi et al. 2007). Research has shown that the metal in the form of nanoparticles may also be an effective weapon in the fight against viruses, which is achieved by inhibiting their replication (Wijnhoven et al. 2009). Its activity has been confirmed even against HIV-1 (Elechiguerra et al. 2005) and the influence virus (Mehrbod et al. 2009). The effectiveness of the processes leading to the destruction of the viruses closely depends on the shape and size of the nanoparticles (Elechiguerra et al. 2005). Silver nanoparticles are also not inert against certain fungi. Studies have confirmed that it is an effective and fast-acting agent, destroying such types of fungi as Aspergillus, Candida and Saccharomyces (Wright et al. 1999).

Progress in the development of nanotechnology and the rapid growth of new methods for the preparation of silver nanoparticles has meant that, in the last two decades, interest in introducing silver nanoparticles into the structures of consumer products has increased. The properties of silver nanoparticles have attracted the attention of many industries. This has mainly concerned fields of science and industry in which its biocidal effect is particularly desirable. Silver nanoparticles have been applied in the food, textile, construction, medical and pharmaceutical industries as well as other antiseptic areas of application (Okafor et al. 2013; Abou El-Nour et al. 2010). Silver nanoparticles are also used in the energy industry (Bonsak et al. 2011) and in biomedicine, in which they act as optical receptors in labelling biological materials (McFarland and van Duyne 2003). Figure 1 shows the industries in which silver nanoparticles are most commonly used.

Fig. 1
figure 1

Categories of products containing silver nanoparticles (Fauss 2008)

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Products with silver nanoparticles

The widespread use of silver nanoparticles means that they can easily pass it into the environment, polluting waters, soil and air. After entering the ecosystem, silver nanoparticles could pose a serious threat to living organisms.

Silver nanoparticles in surface waters and groundwater

Water is the fastest medium for spreading contamination over large areas. After the introduction of silver nanoparticles into various reservoirs, there is a risk of their transfer out of vast areas, which poses a threat not only to aquatic organisms but also organisms whose natural habitat is coastal areas far from the source of contamination. In particular, silver nanoparticles, present in various types of products that directly contact wastewater, are believed to penetrate the surface water and groundwater. Some examples of products with silver nanoparticles that can be a direct source of water pollution are described below.

Yan and Cheng described a method for generating silver nanoparticles-coated granules that could act as antibacterial and antifungal agents. In US Patent 6379712 B1, the authors indicate that the product deactivates a wide variety of bacteria, including S. aureus, Chlamydia trachomatis, Providencia stuartii, Vibrio vulnificus, Bacillus subtilis, and Streptococcus paratyphi. The composition of the matrix granules allows their use as a component of various types of hygiene preparations, such as ointments, lotions, or washes for the mouth, eyes or other infected body parts (Yan and Cheng 2002).

Silver nanoparticles are also used in order to reduce the attachment of bacteria to the surfaces of dental implants, preventing the formation of biofilms on the implant surface that may cause infection. The presence of silver nanoparticles in dental materials allows contamination and infection of teeth to be avoided (Sivolella et al. 2012; Allaker 2010).

Cosmetic products are also exposed to bacteria and fungi, in particular during their production or storage. Besides providing hygienic conditions of production, attention has also been paid to the possibility of using various types of biocides to achieve biocidal properties in the finished product treatments. Nanostructured silver has been used for this purpose. Ha and colleagues in their patent application provided a way of using silver nanoparticles as a component of dyes in coloured cosmetic mixtures. The product is used in dyeing cosmetic products, particularly in foundations, lipsticks, eye shadows, eyebrow pencils and nail polishes (Ha et al. 2009).

A soap with silver nanoparticles is also known. A patent on its method of preparation was submitted in 2011. The authors report that the soap is characterized by foaming properties and has soothing, smoothing, moisturizing and strong disinfecting properties. Thanks to the silver nanoparticles, the product also has anti-inflammatory activity and protects the skin against unwanted microbial action. Silver nanoparticles can penetrate deep into the skin and remove any bacterial contamination from it, which inhibits the formation of acne (Yaoguang et al. 2011).

Silver nanoparticles have also found an application in the production of toothpaste or tooth cleaning gel. At concentrations up to 0.04 mg/g, silver is an effective ingredient in preventing the progression of bacteria, causing odours and dental cavities. The authors stipulate that the best biocidal effect is achieved when the average size of the nanoparticles is <15 nm (Holladay 2013).

Another patented method of taking advantage of the valuable properties of silver nanoparticles is impregnating them into the structures of socks, trousers and other fabrics. The presence of silver nanoparticles in such products prevents the penetration of bacteria into hard-to-reach places as well as inhibiting their growth, which is the most common cause of odours and other pathogenic symptoms. The authors of the patent indicate that socks with silver nanoparticles not only have preventive properties but can also have health benefits because they can be used in the treatment of diseases caused, in particular, by S. aureus, E. coli and Candida albicans (Cheng and Xiong 2003).

Silver nanoparticles embedded in textiles leach into wastewater as a result of being laundered. Other products also get into wastewater directly, which creates a risk of escape of silver nanoparticles and their interaction with living organisms. Blaster and colleagues have analysed the interactions of silver nanoparticles released from plastics and textiles with the ecosystem in the Rhine River. As a result of their work, it was established that silver nanoparticles have been absorbed by the mud and sewage, which in the future may have been spread onto agricultural fields, which in turn can lead to bioaccumulation and toxicological threat (Blaster et al. 2008).

Moreover, silver nanoparticles may not only exist in the aqueous environment in the form of metallic particles. One of the main aqueous environment risk issue is the fact that silver nanoparticles may be dissolved to silver ions. It happens under aerobic conditions in low pH value. The following chemical equation may describe the dissolving process while the silver nanoparticles and oxidant are present in acidic environment:

$$ 4 {\text{Ag}}_{{ ( {\text{s)}}}} {\text{ + O}}_{{ 2 ( {\text{aq)}}}} {\text{ + 4H}}_{{ ( {\text{aq)}}}}^{ + } \Leftrightarrow 4 {\text{Ag}}_{{ ( {\text{aq)}}}}^{ + } {\text{ + 2H}}_{ 2} {\text{O}}_{{({\text{l}})}} $$
(1)

The rate of dissolution is affected by particles size—it is much faster, while silver nanoparticles average diameter decreases (Levard et al. 2012). What is more, the biological activity of silver nanoparticles is also dependant on their “age”. It is strongly affected by the number of silver ions which are released (Kittler et al. 2010).

Products which are enriched with silver nanoparticles and during their life cycle have contact with water environment, such as hygiene preparations, dental implants, toothpastes or textiles may be a source of environment pollution with silver nanoparticles.

Silver nanoparticles in soils

Silver nanoparticles may penetrate into the soil due to leaching, mainly from building materials. Leaching is determined by adverse weather conditions and insufficient bonding of nanoparticles with their material. At the same time, it should be noted that nanoparticles need to have a medium level of mobility, since they should be available to microorganisms. The following products can be a source of soil contamination with silver nanoparticles.

In US Patent 0272542 A1, a method of using silver nanoparticles by introducing them into materials that can then be used as roofing, insulation, siding, and upholstery for houses is described. The authors of the patent indicate that the silver nanoparticles present in this type of material inactivates bacteria, fungi or algae, which can cause considerable damage in building materials (Horner et al. 2012).

A paint containing silver nanoparticles is also known. The patent by Kwon et al. describes a method for the preparation of an opaque material which, thanks to the presence of silver nanoparticles, exhibits biocidal properties (Kwon et al. 2006).

Kaegi and colleagues evaluated the intensity of silver nanoparticles release from paint covering the facade of a building specially designed for this purpose. The facade was exposed to natural weather conditions over a period of 1 year. The experiment consisted of measuring the concentration of silver nanoparticles in the rainwater surrounding the building. In the initial phase of the research, strong leaching of silver nanoparticles was noted. At that time, the maximum concentration of silver was 0.145 mg/dm3. It was found that after 12 months, about 30 % of the silver nanoparticles had leaked into the environment. The nanosilver particle sizes were <15 nm (Kaegi et al. 2010).

The incorporation of silver nanoparticles in the structures of new building materials, particularly polyurethane foam, provides many new opportunities for its use in known construction and finishing materials. Polyurethane foam is widely used as an insulating material in windows, doors, etc. The success of this type of application offers a wide range of possibilities for the use of polyurethane foam characterized by its biocidal properties due to the presence of silver nanoparticles. Prashant and Pradeep described a method of enriching polyurethane foam with silver nanoparticles that was achieved by physical adsorption of nanoparticles on the foam’s surface (Prashant and Pradeep 2005).

Nia described an example of applying silver nanoparticles in rural industry, particularly in activities that improve the rate of plant growth. The behaviour of various plants after contact with silver nanoparticles was examined. Citrus, grains, fruits and olive trees were the tested plant species. In the course of the experiment, it was found that spraying the silver nanoparticles dust on the plants caused faster growth and the creation of longer roots. What is more, studies showed that the use of silver nanoparticle inhibited the occurrence of diseases affecting roots (Nia 2009).

Silver nanoparticles contained mainly in building materials, such as paint, siding and roofing may be washed out of them with rainwater and directed into the soil, where they may be accumulated for a long time.

Silver nanoparticles in the air

Silver nanoparticles can be released into the air from different types of devices that are used to clean the air. Air pollution by microorganisms occurs mainly in slaughterhouses and meat processing plants. Material which is processed there is a great medium for the growth of bacteria.

Maintaining antiseptic conditions in industrial facilities that, due to the nature of their production, are particularly vulnerable to microbial contamination is an important issue. An innovative approach is the use of silver nanoparticles to purify the air in meat plants. The air blown into the production halls transfers microorganisms onto meat products that may be a conducive environment for their development. Microorganisms can penetrate meat products during slaughter, storage or processing. Accordingly, the blowing air must be filtered. In order to avoid the risk of microbial growth in air conditioning systems, a method of impregnating air filters with silver nanoparticles has been developed. The results of microbiological tests have confirmed that soaking bag filters with silver nanoparticles allows for almost total elimination of microbial contamination of air (Kowalski et al. 2010).

The current problem also includes optimizing the conditions of rearing and breeding the farm animals. Animal faeces and feed are a source of air pollution, in particular odours, including ammonia and hydrogen sulphide. Gases present in the air impose a high burden on the environment. Studies have been carried out whose aim was to determine the level of ammonia emissions from sheep manure after treatment with a mineral sorbent with silver nanoparticles. The results confirmed reduced gas emission under the conditions of use of the developed product (Schiffman 1998).

Glover and colleagues described the phenomena that silver nanoparticles may transform under different air conditions. They discovered that formation of new silver nanoparticles may occur and is strictly dependent on air humidity. If humidity is greater than 50 %, new silver nanoparticles create in the neighbourhood of existing silver nanoparticles. There are three process stages. In the first step, metallic silver which are located on the outer layer are oxidized and dissolved to silver ions according to the chemical equation no. 1. After that, ions diffuse and are adsorbed on water layer. Finally, they are reduced into metallic silver via chemical or photochemical way which leads to creation of new silver particles. This phenomenon is important when it comes to finding dependences between toxicity and particles sizes. New-formed silver nanoparticles have smaller than before-existing particles, so their toxicity also may be stronger (Glover et al. 2011).

The flow of silver nanoparticles to the air is possible as a result of operation of air conditioners or air filters, which in their structure have silver nanoparticles. Silver nanoparticles present in the air environment may be inhaled by living organism in an uncontrolled manner.

The impact of silver nanoparticles on living organisms

Once silver nanoparticles reach water, there is a high probability of their penetrating the structures of aquatic organisms. This is particularly dangerous, since many living organisms that comprise the first stage of the food chain live in water and silver nanoparticles can thereby easily get into organisms ranking higher in the trophic hierarchy.

Silver nanoparticles are stable after coating them with chemical compounds. But after penetrating silver nanoparticles into the living organism, their aggregation may be also controlled by solution pH value, ionic strength or ionic constituents. That means that the stability and activity of silver nanoparticles is also affected by composition of body fluid (Sharma 2013).

Asghari and colleagues studied the effects of silver nanoparticles on Daphnia. These arthropods belong to the group of organisms most sensitive to pollution of aquatic environments. Due to the fact that Daphnia are the base of the trophic chain in many aquatic ecosystems, any changes in their quantity or quality may affect the populations of other aquatic organisms. The team conducted tests on the toxicity of silver nanoparticles to Daphnia based on a standardized immobilization test (OECD Guidelines for the Testing of Chemicals 2004).

The tested silver was present in an aqueous suspension at a concentration of 200,000 mg/dm3, comprising spherical nanoparticles having an average size of 5–25 nm. A different aqueous suspension with a concentration of silver nanoparticles equal to 4,000 mg/dm3 was also tested. This suspension was characterized by spherical particles with an average size of about 16.5 nm. The researchers also used silver nanoparticles in the form of a powder having an average particle size of about 20 nm. The team transferred the powder into an aqueous suspension at a concentration of 400 mg/dm3. After exposing Daphnia to the above products, discoloration of their bodies was noted, which could have indicated the accumulation of silver under their cuirasses. Furthermore, nanoparticles were found in their gastrointestinal tracts, indicating that they were being consumed. In some cases, the intake was significant enough that it inhibited normal swimming. An irregular path of movement was observed, and in the later stages of the tests, the Daphnia migrated to the bottom of the beaker. This type of disorder was induced by silver nanoparticles added to the slurry at a concentration of 0.002 mg/dm3. The minimum concentrations of the three used silver nanoparticles formulations causing 100 % mortality in the Daphnia after 48 h were 0.006, 0.00325 and 0.275 mg/dm3. The slurry of the silver nanoparticles powder had the highest toxicity. The authors suggested that the response of Daphnia depends on the physicochemical properties of the used nanoparticles (Asghari et al. 2012). It is worth to notice that some of used silver nanoparticles were obtained by adding hydrazine solution to silver nitrate solution which was done in order to reduce silver ions. Therefore, the harmful effect may have been induced by toxic properties of hydrazine since hydrazine hydrate is a known to be carcinogenic substance (second category), which is highly poisonous to aquatic organisms (Hydrazine Hydrate Datasheet 2014).

Bilberg and colleagues studied the effect of silver nanoparticles on the breathing of the Eurasian perch. Fish gills naturally have direct contact with the surrounding water, making them a potential target for silver nanoparticles. The researchers examined the degree of oxygen consumption while perch were exposed to silver nanoparticles. Each exposure lasted for 3 days. Spherical silver nanoparticles with an average size of about 80 nm were applied. The study involved five groups of six fishes exposed to silver nanoparticles at concentrations of 0.063, 0.129 and 0.300 mg/dm3. Perches that were not immersed in water with silver nanoparticles were used as a comparable group of fishes. Oxygen consumption was measured using an automatic respirometer. During exposure, the silver concentration was also measured in the water. Within 20 h of exposure, oxygen consumption decreased by 65, 83 and 67 %, respectively, with the above-mentioned concentrations of silver nanoparticles. The final results revealed that exposure to silver nanoparticles reduced tolerance to hypoxia (oxygen deficiency) (Bilberg et al. 2010). Bilberg and colleagues in their studies used silver nanoparticles powder that was stabilized by polyvinylpyrrolidone coating. Despite the fact that this substance is considered as safe, polyvinylpyrrolidone may contain unreacted mers of vinylpyrrolidone which are carcinogenic (Polyvinylpyrrolidone Datasheet 2014).

Massarsky and colleagues evaluated the effects of silver nanoparticles on embryos of zebrafish, a freshwater fish in the carp family. Aqueous suspensions with concentrations of silver nanoparticles ranging from 0.03 to 1.55 mg/dm3 were used. The average size of the nanoparticles was about 9 nm, but detailed studies revealed the presence of larger aggregates. It was observed that increasing the silver concentration triggered greater toxic activity, which manifested as increased mortality in the embryos. Fifty per cent mortality occurred when silver nanoparticles were used at a concentration of 1.18 mg/dm3. A lower percentage of brood fish was also observed in fish exposed to silver nanoparticles. Moreover, it was also noted that exposure to silver nanoparticles reduced the embryo heart rate. It is particularly worrisome that researchers have noted physical malformations in exposed embryos, particularly at higher concentrations of silver nanoparticles. They concerned mostly distortion notochord, pericardial oedema and degeneration of the body. Despite the fact that the mechanism of the toxic effect of silver nanoparticles on fish is incompletely understood, the team suggested that the direct cause of the anomaly may be oxidative stress induced by silver nanoparticles (Massarsky et al. 2013).

Silver nanoparticles present into the soil can also penetrate the tissues of plants grown in it, which then are fed to humans and animals. Furthermore, silver nanoparticles may also penetrate the tissues of animals such as annelids, which in turn are eaten by other organisms higher in trophic chain. The impact of ingested silver nanoparticles on humans has not been described in the literature; however, there are numerous studies of the effects of silver nanoparticle consumption by animals.

Loghman and colleagues studied the toxicity of silver nanoparticles in broiler chickens. The studies were conducted on 240-day-old males. Every day, for 42 days, silver nanoparticles suspensions at concentrations of 4, 8 and 12 mg/dm3 were added to the feed and drinking water. The silver nanoparticles had an average size of 18 nm. After the experiment, the chickens were slaughtered and their livers were examined. The control sample showed no pathological changes. Histopathological examination of liver samples from chickens fed silver nanoparticles showed several types of changes. At the lowest concentration of silver nanoparticles, it was found to have accumulated in the hepatocytes, which were also swollen and congested. Higher concentrations of silver nanoparticles resulted in further thickening of the central vein and considerable fatty degeneration. At the highest concentration of the silver nanoparticles, tissue fibroplasia and focal necrosis of hepatocytes (pathological apoptosis) were observed (Loghman et al. 2012).

Koohi and colleagues studied the toxicity of silver nanoparticles in rabbits. The first stage of the research concerned assessing the effects of the nanoparticles on the skin; in the second stage, the reactions of other organs, including liver, kidney, heart and brain were studied. In this study, silver nanoparticles having a particle size equal to 10, 20 and 30 nm were used. The silver nanoparticles suspension had a concentration equal to 8,000 mg/dm3, and its working volume was 0.5 cm3/individual. The skin of the rabbits was exposed to the silver suspension for 3 min, 1 and 4 h. The effect of exposure time and the concentration of the applied suspension of silver nanoparticles were studied. The suspension was applied to the shaved skin by adhering a piece of soaked gauze to an area of 6 cm2. In the case of 10- or 20-nm particles applied for 4 h, skin erythema and oedema were noted. After 14 days of exposure, skin hyperkeratosis and the appearance of abnormal epidermal papillae, as well as fibrosis, congestion, redness, swelling and glazing of intracellular collagen in the dermis, were observed. Liver function tests revealed cell death and pathological fatty changes as well as congestion of the central vein. Congestion of the spleen was also diagnosed. The researchers also noted the occurrence of cerebral oedema and changes in the meninges. It was concluded that exposing the rabbits to smaller silver nanoparticles caused more adverse effects than to larger ones (Koohi et al. 2011). It was notified that mono ethylene glycol (100 %) was used as solvent for 20- and 30-nm silver nanoparticles. It is known that ethylene glycol induces narcotic effect, causes damage to the central nervous system and to the spinal cord as well as irritates the mucous membranes (Ethylene Glycol Datasheet 2014). Therefore, irritant action may have occurred due to the properties of this substance as well.

Heydarnejad and colleagues studied the direct impact of silver nanoparticles dressings, reflected in histopathological changes in the livers of laboratory mice. The tested mice were 8 weeks old. The scientists created similar wounds on the animals’ backs that were covered by a dressing impregnated with silver nanoparticles. Each dressing contained 50 μl of a silver nanoparticles suspension at a concentration of 10 mg/dm3, with an average particle size of 30 nm. A gauze with distilled water served as the control sample. The experiment lasted 14 days and histopathological examination of livers was performed at 2, 7 and 14 days after exposure. In the case of the control sample, no pathological changes were observed. After treatment with silver nanoparticles, disturbing symptoms such as the central vein extension, congestion, cell swelling, an increase in inflammatory cells and fatty degeneration were all noted. The intensity of these changes was increased with time (Heydarnejad et al. 2014).

Silver nanoparticles present in the air can easily penetrate into the interior of living organisms via the process of respiration. Scientists have limited knowledge of the effects of inhaled silver nanoparticles in the case of human organisms. However, studies conducted on the effects of inhaled silver nanoparticles have been conducted in animals.

Stebounova and colleagues studied the effects of inhaled silver nanoparticles in mice. Silver nanoparticles were present in the air at a concentration of 0.0033 mg/dm3, with an average particle of about 10 nm. Exposure lasted for 10 days, 4 h per day. Six-week-old male mice were used in the studies. In the exposure chamber, the mice breathed air that contained the sprayed aerosol silver nanoparticles suspension. The control sample consisted of filtered air. After killing the mice, the average amount of silver retained in their lungs was measured. Studies have shown that silver nanoparticles do not dissolve in the inside and extracellular solutions. It was found that on day 1 of the study, the average silver concentration was 3.1 × 10−5 mg/g of lung dry weight, whereas in the control mice, its concentration was at the limit of detection. The researchers observed that exposing mice to the silver nanoparticles-containing aerosol induced pulmonary inflammation (Grassian and Thorne 2011).

Kim and colleagues conducted studies on the genotoxicity of silver nanoparticles in female laboratory rats. In vivo studies were performed in accordance with the OECD standard (OECD Guidelines for the Testing of Chemicals 2008). Over 90 days, for 6 h per day, the rats breathed air with suspended silver nanoparticles having an average size of 18 nm. Silver concentrations ranged from 0.7 × 109 to 2.9 × 109 particles/dm3 of air. It was noted that the silver nanoparticles accumulated mainly in the lungs and liver of the rats, but their genotoxicity was not confirmed (Kim et al. 2011). Sung’s team and colleagues also performed a similar study. For 90 days, rats were exposed to laboratory air with silver nanoparticles having an average size of 18 nm. As before, silver concentrations ranged from 0.7 × 109 to 2.9 × 109 particles/dm3 of air. After completion of the study, a significantly reduced lung volume was found. It was concluded that with increasing concentrations of silver, the incidence of pneumonia also increased. Alveolar inflammation and other changes in the physical appearance of the rats were also noted (Sung et al. 2008).

In the work of Roberts and colleagues, laboratory rats were also the subjects of the research. The aim of the study was to assess threats to the respiratory and circulatory systems that might occur as a result of inhaling silver nanoparticles. Rats were stored in a chamber containing air with silver nanoparticles that were sprayed into it at concentrations ranging from 1 × 10−4 to 1 × 10−3 mg/dm3. The average particle size was 35 nm. It was found that about 0.014 mg silver/individual rat was deposited due to their breathing the aerosol. The possibility of different diseases such as pneumonia, pathological changes in cell morphology, pulmonary alveolar damage, constriction or dilation of blood vessels and changes in heart rate and blood pressure were also checked. It was observed that after only a short time of inhalation, silver nanoparticles did not induce significant changes in the tested parameters; only an increased heart rate was observed. Nevertheless, the accumulation of silver nanoparticles in the bodies of the rats was confirmed. The authors suggest conducting more research over a wider range of silver nanoparticles concentrations, which will undoubtedly provide significant new phenomenon (Roberts et al. 2013). The above data are summarized in Table 1.

Table 1 Summary of the toxic impact of silver nanoparticles on the organisms tested
Full size table

Conclusion

Silver nanoparticles can be seen as a threat or as an ecotoxic product that accumulates in the trophic chain (SCENIHR. The appropriateness of existing methodologies to assess the potential risks associated with engineered and adventitious products of nanotechnologies 2006).

In order to identify emerging threats from silver nanoparticles, one should consider four factors: hazard identification, estimation of the degree of toxicity, assessment of exposure and hazard characteristics (Linkov et al. 2008).

In recent years, the number of available products containing nanoparticles of metallic silver in particular has increased significantly. Humans are exposed to its effects by direct contact with everyday objects. Silver nanoparticles toxicity may be affected by its physicochemical properties such as size, particle shape and the intensity of the particles’ biological reactivity. Silver nanoparticles can penetrate into the body through the skin, respiration and digestive systems. Studies have previously revealed that silver nanoparticles tend to accumulate in various organs, especially the liver, kidneys, lungs and others. The presence of silver nanoparticles in the liver may be particularly dangerous. This organ controls the quality and quantity of substances penetrating the body; however, it is not able to excrete silver nanoparticles, which can lead to its penetrating other organs. The accumulation of silver nanoparticles in the lungs may also have negative effects that can manifest in the future.

Undoubtedly, it is necessary to conduct further research into the toxicity of silver nanoparticles on living organisms. Alarming data about the malicious activity of silver nanoparticles are served up by public media. However, full scientific information is not yet available, as the developed applications are incomplete. The realization of the full research cycle is highly needed. On this basis, it will be possible to determine the actual amounts of silver nanoparticles penetrating into the environment. In the next stage, it is necessary to determine the degree of accumulation of silver in living matter. Knowledge about the effects of silver nanoparticles on living organisms should also be expanded. Implementation of the full cycle of research will provide the basis for further actions aimed primarily at protecting the environment. If the alarming data are confirmed, it will also be necessary to develop a method to restore the environment and protect it against possible threat from nanomaterials accumulating in it in the future.