In summer , Logitech brought an external iPad keyboard powered by light on the market, representing the first major commercial use of dye-sensitized solar cells. In , there were about nanotechnology-based consumer products that are commercially available in over 20 countries .
Micro- and Nanostructure of Titania Coatings Modified with Functional Ceramic Nanoparticles
Sources of nanomaterials can be classified into three main categories based on their origin: i incidental nanomaterials, which are produced incidentally as a byproduct of industrial processes such as nanoparticles produced from vehicle engine exhaust, welding fumes, combustion processes and even some natural process such as forest fires; ii engineered nanomaterials, which have been manufactured by humans to have certain required properties for desired applications and iii naturally produced nanomaterials, which can be found in the bodies of organisms, insects, plants, animals and human bodies.
However, the distinctions between naturally occurring, incidental, and manufactured NPs are often blurred. In some cases, for example, incidental NMs can be considered as a subcategory of natural NMs. Molecules are made up of atoms, which are the basic structural components of all living and nonliving organisms in nature. Atoms and molecules have been naturally manipulated several times to create intricate NPs and NSMs that continually contribute to life on earth.
Incidental and naturally occurring NMs are continuously being formed within and distributed throughout ground and surface water, the oceans, continental soil, and the atmosphere. One of the main differences between incidental and engineered NMs is that the morphology of engineered NMs can usually be better controlled as compared to incidental NMs; additionally, engineered NMs can be purposely designed to exploit novel features that stem from their small size. It is known that metal NPs may be spontaneously generated from synthetic objects, which implies that humans have long been in direct contact with synthetic NMs and that macroscale objects are also a potential source of incidental nanoparticles in the environment.
Photochemical reactions, volcanic eruptions, and forest fires are some of the natural processes that lead to the production of natural NPs as mentioned. In addition, skin and hair shedding of plants and animals, which is frequent in nature, contributes to NP composition in nature.
Dust storms, volcanic eruptions, and forest fires are events of natural origin that are reported to produce high quantities of nanoparticulate matter that significantly affect worldwide air quality. Similarly, transportation, industrial operations, and charcoal burning are some of the human activities that lead to the emergence of synthetic NPs.
Dust storms and cosmic dust: The Eagle Nebula stars are light years away from Earth and are born with a disk-like cloud and the ability to form solar systems accompanied by dust and gas mostly hydrogen . Diamond, of a few nanometers in diameter, has been observed in the Murchison meteorite, which is a perfect example of the nanoparticulate origin in planetary system objects other than stars . Different types of NMs are present throughout the universe which are mixed, sorted and modified into several forms.
Electromagnetic radiation, pressure gradients, dramatic temperature, physical collisions and shock waves help in energizing and forming NPs in space . Dust storms are the main source of NPs in desert and terrestrial regions. Studies supported by satellite images revealed that dust storms in one region can migrate the nano and micro-sized minerals and anthropogenic pollutants to thousands of kilometers away from their origin. The consequence of aerosol particles on the environment and climate was extensively reviewed by Buseck and Posfai.
They mentioned that widespread transport of aerosols across oceans have a major effect on life, including the life forms at the bottom of the food chain . Another study by Al-Dabbous and Prashant Kumar revealed the presence of 5— nm range airborne NPs during summertime and dust events in busy roadsides terrestrial of Kuwait .
Asthma and emphysema are two prominent health problems in humans that are caused by terrestrial airborne dust particles [50,51]. Dust NPs containing metals have the capability of damaging lung tissues by producing reactive oxygen species . A case study shows that the quality of air in Asia and North America is heavily disturbed during every spring season due to dust storms occurring in the Gobi desert [52,53].
More recently, Shi et al. Cosmic dust is a collection of extraterrestrial dust particles that widely exist in space on the nanoscale. The astronauts and aeronautic instruments are severely threatened by cosmic dust . Lunar dust is smaller compared to typical terrestrial dust, with excess sub-micrometer particles. Lunar dust, with a few magnetic NPs, can settle on the space suits of astronauts by electrostatic attraction and damage them [59,60]. They have been known to cause irritation in the lungs and eyes of Apollo astronauts by becoming airborne .
Studies have found that through the intratracheal route, lunar materials lead to pneumoconiosis and fibrosis formation in rats . Dust particles on Mars can damage the solar panels of the exploration robots via accumulation and affects the power source for sensing, communication, and locomotion . Astronauts who are frequently on longer space missions have prolonged exposure to cosmic dust with an increased risk of respiratory disease. Dust particles also cause damage and mechanical failure in spacesuits and airlocks .
Volcanic eruptions: Eruption of volcanoes leads to the propulsion of an enormous amount of aerosols and fine particles into the atmosphere with sizes ranging from micrometers to several nanometers . The released NPs spread throughout the world and settle in the stratosphere and the troposphere, which are the lowest atmospheric layers. However, the effect of NPs will be significant in areas within a certain range 10 km from the volcano.
Rietmeijer and Mackinnon reported that volcanic eruptions in the s resulted in the release of bismuth oxide NPs into the stratosphere and were detected even in . Particulate debris from volcanic eruptions affects human, animal, and plant activities by blocking and scattering the sunlight. The volcanically erupted particles may possess heavy metals that are toxic to humans .
It is caused by the entry of NPs into the body . Forest fires and ocean water evaporation: Lightning and human activity are the main causes of forest and grass fires across the world. Ash and smoke are released by these forest fires and can spread over long distances, affecting the standard of ambient air quality by increasing the number of small particles in the air . It has been shown that that black carbon and soot in large quantities are carried and deposited over the Himalayan glaciers by Asian brown clouds.
Many forest fire cases have been reported to transport micro- and nanosized particles through smoke and ash, and are known to cause respiratory problems in humans and animals . Smoke containing very small particles can worsen pre-existing cardiopulmonary conditions in patients .
Options for Forming of Nanostructured Surface Coatings
Sea salt aerosols are a different type of natural NPs formed due to water evaporation and ejection of wave-produced water droplets from seas and oceans into the atmosphere . Usually, the size of these salt aerosols ranges from nm to few micrometers, and are formed via temperature change and evaporation-mediated natural precipitation.
These small sea salt aerosols act to transfer microorganisms and pollutants that may increase casualties in plants, animals, and humans via adverse health effects. Figure 3: a SEM image of flaming smoke collected during a Madikwe Game Reserve fire in South Africa on August 20, , showing aggregated carbon particles; b TEM image of flaming smoke collected in a Dambo fire in Zambia, on September 5, , showing aggregated carbon particles  , copyright , the American Geophysical Union. Simple combustion during cooking, in vehicles, fuel oil and coal for power generation  , airplane engines, chemical manufacturing, welding, ore refining and smelting are some of the anthropogenic activities that lead to NP formation .
NMs such as carbon NPs  , TiO 2 NPs  and hydroxyapatites  are present in commercial cosmetics, sporting goods, sunscreen and toothpaste. Thus, these synthetic NPs are a new genre of NPs that may induce adverse environmental and human health effects.
Nanoparticles from diesel and engine exhaust: In cosmopolitan cities and town, the main source of atmospheric micro- and nanoparticles is automobile exhaust . Amongst the types of automobile exhaust, diesel engines release 20— nm sized particles whereas gasoline engines release 20—60 nm sized particles [89,90].
It has been found that CNTs and fibers are released as by-products during diesel and gas combustion processes . Thus, pollution from vehicles is a major cause of nanoparticulate contamination in urban atmosphere . The hazardous effect of automobile exhaust depends on the composition of the particulate mixture . Recently, fine particulate matter, especially carbon nanotubes of anthropogenic origin, was found to be present in the broncho-alveolar lavage fluids from asthmatic Parisian children. The results showed that the presence of carbon nanotubes in cells can cause granulomatous reactions, oxidative stress and inflammation, leading to fibroplasia and neoplasia in lungs.
The results also suggested that humans are routinely exposed to carbon nanotubes and showed that the outcome is similar to the vehicle exhaust samples collected in Paris, ambient air samples from the USA, a spider web sample in India and in ice core . Also, benzo[ a ]pyrene, which is a polynuclear aromatic hydrocarbon and a carcinogen, is present in diesel exhaust, which makes it more toxic than gas engine exhaust . Cardiopulmonary mortality [97,98] , childhood cancers due to prenatal and postnatal exposure to exhaust  , myocardial infarction  , and proinflammatory, prothrombotic and hemolytic responses  are some of the health problems that are observed in humans due to high exposure to exhaust in highly populated cities.
Cigarette smoke and building demolition: Cigarette smoking and building demolition are anthropogenic activities that lead to the spread of NPs into the atmosphere. Cigarette smoke has a complex composition of about , chemical compounds in the form of NPs ranging from 10— nm .
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Other than building debris, lead, glass, respirable asbestos fibers and other toxic particles from household materials are released as nanosized particles around the site of building demolition . Cigarette smoke can lead to chronic respiratory illness, cardiovascular disease, pancreatic cancer  , genetic alterations  , middle ear disease and exacerbated asthma .
It is noteworthy that there is a chance to reverse the risks of myocardial infarction associated with inhaled NPs after smoking cessation .
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The hazardous effect of demolition particles and their long-term effects towards humans are still unknown. However, respiratory symptoms such as a cough and bronchial hyperactivity were found among firefighters who participated in the rescue mission during World Trade Center on September 11, . This indicates that extensive studies should be carried out amongst workers of demolition sites to identify the ill-effect of particles that are dissipated.
Nanoparticles in biomedical and healthcare products: NMs are incorporated in cosmetics and sunscreens as antioxidants  and antireflectants . Mostly, NPs used for commercial applications are engineered NPs that are produced using physical  , chemical  and biological methods .
As engineered NPs are attached to a firm surface, the risk of detachment and causing health issues is lessened .
Other than cosmetics, NPs have been extensively used in commercial products ranging from personal care products to paints . Titanium oxide NPs larger than nm are broadly utilized as a white pigment in cosmetic creams and sunscreens . Similarly, Ag NPs have been used in diverse applications including air sanitizer sprays, wet wipes, food storage containers, shampoos, and toothpastes . Several NPs are under research and evaluation of additives in personal care products. In spite of the emerging growth of products with different types of nanomaterials, their hazardous effects on humans are largely unknown.
The extensive studies reported that Ag NPs demonstrated a size, morphology, and dosage-dependent higher cytotoxicity to humans and animals cells than asbestos [91,].