How Nanoparticles Are Generated
What Are Particles and How Are They Produced?
As already mentioned, a few natural sources of particles are present on our planet. Volcanic eruptions are the most dramatic and showy among them. To be sure, unlike what happened a few tens of millions of years ago, not many volcanoes are active nowadays, but when occasionally they belch out, the environmental consequences, though generally short-lived, are often very visible. The year 1816 was known as the 'year without summer' because of the severe climate abnormalities caused by the eruption of the Dutch East Indies Mount Tambora, which reached its climax in April 1815 and continued to emit fumes and particulate matter for about three more years. Its dust occupied a large portion of the atmosphere in the northern hemisphere, thus preventing the sun rays and heat to reach the lower atmosphere and the ground in the usual and, for us, necessary way and causing, among other problems, the scarcity or the utter lack of crops. About two centuries later, in 2010, the Icelandic Eyjafjallajbkull erupted and its dust forced some European airports to close for a few days. About 1500 are the still potentially active volcanoes, more or less one-third of which, mainly located in the so-called Pacific Ring, have erupted in historical time. In any
Advances in Nanopathology: From Vaccines to Food
Antonietta Morena Gatti and Stefano Montanari
Copyright © 2021 Jenny Stanford Publishing Pte. Ltd.
ISBN 978-981-4877-29-9 (Hardcover), 978-1-003-05622-5 (eBook) w w w. j e n ny sta nf o rd. co m case, apart from rather rare exceptions, volcanic dust is not a great danger to health and the environment.
Rock erosion due to ageing and climate (wind, rain, ice, etc.) or, more seldom, landslides are another source of particulate matter. Those particles are generally bigger than volcanoes and, because of that, are not particularly aggressive, since, if inhaled, they are blocked in the upper respiratory tract or, if ingested, they are often too coarse to cross the wall of the digestive system (though those walls are rather permeable) and are eliminated with faeces.
In spite of their size, large enough to be easily visible, sand grains mostly raised by the wind from desert areas can travel over very long distances. Sahara sand, for example, reaches the East Coast of America and can be found in the red rain which occasionally falls in Europe. The Mongolian Gobi Desert's sand is often carried by the wind to Beijing and other Chinese cities.
Forest fires, very rarely natural in their cause and much more often set by arsonists or reckless goers, are a further source of dust. As we are writing these pages (early 2020), the hugest forest fire in the history of man is taking place in southeast Australia, with the particles produced by combustion which have already reached South America, covering no less than 12,000 km, and for a comparatively long time the situation appeared to be out of control. Millions of hectares of land were involved, and both the heat and the disappearance of vegetation prevented the formation of cloud bodies and, therefore, the fall of rain. What is happening is the outbreak of lightning without thunderstorms which triggers new fire outbreaks.
In most circumstances, it is man who is responsible for the formation of particularly polluting dust, and that is a rocketing increase whose entire consequences can only be a matter of speculation, since there is no experience in that matter, except for acute cases. One of them, for example, was the so-called Great Smog which affected London from Friday, 5 December, to Tuesday, 9 December 1952. Particularly cold weather combined with atmospheric high pressure and windless conditions collected airborne pollutants mostly due to the use of coal as a fuel for home heating. That combination of pollutants formed a thick layer of smog over the city, which, according to recent evaluations, caused about 12,000 fatalities.
As happened then in London, most man-produced particles originate from combustion. Temperature is usually high enough to break the molecules of what is being burned, reducing them to smaller molecules or to atoms. If the formation of particles from those fragments of matter is completed immediately in the neighbourhood where the combustion occurs, those particles are called filterable, because, if a filter is placed downstream of the phenomenon, it can ideally capture them. When, instead, the formation occurs at a farther distance from combustion, small molecules and atoms pass freely through the filter and stick together. In that case the particles are condensable. In both eventualities, the elementary composition of the resulting particles is different from that of each of the original substances which have been burned. The atomic elements are obviously the same, but their combination is random, more or less as if we had a mosaic, we disassembled it and then recombined its tiles with eyes closed. It should be noted that the formation of particles also takes place by burning material which is not solid but liquid, such as, for example, automotive or power plant fuel, or gaseous, such as, for example, that generated by the combustion of industrial methane.
The third category of particulate matter created at high temperature is the secondary. Combustion releases different gases, depending on the composition of what is being burned: particularly carbon oxides, sulphur dioxide, nitrogen oxides, ammonia and a quantity of different volatile organic compounds. Once in the atmosphere, they find oxygen, also as its allotropic form ozone, free radicals, water vapour and a mixture of gases varying from place to place, from situation to situation and from time to time, occasionally changing in a matter of hours. In a time which in some instances takes days, catalysed by the sun and, more in particular, by the ultraviolet rays, a condensation reaction takes place among all those gases, forming new particles, precisely the secondary ones. Besides being definitely more numerous than filterable and condensable primary particles, the secondary ones have an important feature: organic pollutants, like, for instance, dioxins and furans, stick to them and are carried by them even to relatively long distances. Due to the time which condensation takes and because of diffusion phenomena, secondary particles can be found pretty far from the actual source of the substances by which they have been generated, and the pollutants adhering to them can have origins which have little or nothing to do with that of the particles themselves.
Though, at least hopefully, not a usual form of dust pollution typical of man, the collapse of the Twin Towers in New York on 11 September 2001, is a particularly important one, immediately followed as it was by a huge cloud of dust, much of which had a particularly fine size because of the very high temperature reached. According to some information sources, apart from the many thousands of tons of concrete, the rest consisted of more than 2500 contaminants: roughly 50% non-fibrous material and construction debris, 40% glass and other fibres, 9.2% cellulose and 0.8% of asbestos, plus lead and mercury, not to mention the variety of metals contained in computers and other electronic systems. That dust lingered for a long time in the atmosphere before, eventually, falling to the ground, probably also far away horn the place where the cloud started. It is reasonable to assume that some of the dust has entered into many air-conditioning pipelines with all its obvious consequences.
A much more usual man-made high-temperature production is that coming from explosions.
Of this, as of 9/11, more will be said later.
Not all man-produced particles come from high-temperature processes. If not equally important in terms of quantity, friction can produce dust as well. Turning works in factories and workshops and the brake pads of cars rubbing against discs, for example, are dust producers.
The particles we deal with and which are discussed in this book are solid and inorganic. Their size varies from a few tens of nanometres up to a few tens of microns. By official definition, nanoparticles (or ultrafine particles) have a size ranging from 1 to 100 nm. Again officially, fine particles are sized between 100 nm and 2.5 pm, while coarse particles cover a range between 2.5 and 10 pm. In our research field, that classification has no great meaning. So, in general, we call nanoparticles those whose size is smaller than 1 pm and microparticles the ones larger than that size. It is just a matter of personal habit and of convenience, which, of course, is of no consequence, though deemed important by those who approach science in a bureaucratic way.
If particles vary greatly in size, their shape is at least as variable. A sphere or a spheroid is often the result of high-temperature processes, particularly when metals like, for example, iron are involved. In a way, those shapes remind one of soap bubbles: a formation which is hollow inside and is surrounded by an outer, thin, solid crust That crust, the outer surface of the sphere, is generally composed of juxtaposed crystals, which make the structure very fragile, and because of that, it crumbles easily, thus producing 'shards’ whose size is obviously much smaller than the original particle and whose shape looks geometrically irregular.
If temperature is not high enough to produce spheres, the shapes of the particles can’t be guessed.
As already briefly mentioned, size is roughly inversely proportional to the temperature of formation: the higher the temperature, the smaller the particle.
It must be kept in mind that, because it is extremely important and will be useful later, for their evaluation as environmental pollutants, particles are considered as if they were spheres and, therefore, have a radius. It is also to be recalled that the volume of a sphere is proportional to the third power of the radius (the volume of a sphere is calculated multiplying 4/3 by jt by the radius cubed). Therefore, if a sphere has a radius 10 times greater than that of another sphere, its volume is 103, that is, 1000, times greater.
When the origin of a particle happens to be a friction, that particle has often, though not in all cases, sharp edges.
All these notions, along with the chemical composition of the particulate matter, are essential when one has to look for the origin of a particular pollution.
Very often, the inorganic particles produced by pollution are not easily degradable either in the environment and the biological tissues. So, they resist virtually unchanged for very long periods of time, in many circumstances far exceeding any human lifespan. This fact entails important problems, for example, their accumulation. Even if one tries to reduce pollution, in our case the introduction of particles, in the environment or in the body, their numbers and their density are inevitably destined to increase. Corrosion phenomena converting metals to a more stable form (e.g. oxide, hydroxide, sulphide) can transform particles into something else, occasionally no less toxic to the organism than the original.
The laboratory generation of nanoparticles, the so-called engineered nanoparticles (to be distinguished from incidental nanoparticles), is completely different In that case, the particles are produced on purpose for industrial use and, therefore, have the desired shape and composition. Thus, when in a polluted sample all particles equal in shape and composition are found, one is almost certainly faced with engineered material. This concept is particularly important when, later in this book, we will talk about the poll utants of drugs, especially vaccines. In those cases, as far as we are concerned, we never found particles which could make one think of a purpose-made material, and everything we found appeared to be incidental pollution from inaccurate production methods and controls.