Take a deep breath. You just inhaled about 1 million particles. Figure 1 shows various types of airborne particles, also known as aerosols, under a microscope.
Most of the particles in the atmosphere are smaller than 100 nanometers, or about 1000 times smaller than the width of a human hair. Scientists call such particles, “nanoparticles.” Airborne nanoparticles are not new. Ever since there have been fires and windblown dust, very small particles have been suspended in the atmosphere. So what is the difference between “engineered” nanoparticles and “incidental” nanoparticles? And if there is a difference, does it matter?
Have you ever tried to make breadcrumbs from a slice of bread? Maybe you toasted the bread or let it get stale and then tried to smash it with a rolling pin or some other tool? Or maybe you were making graham cracker crumbs for a cheesecake? These are intentionally “engineered” crumbs. You also know that if you eat a piece of toast or a graham cracker, you inevitably leave behind some crumbs. These are “incidental” crumbs. Likewise, engineered nanoparticles are intentionally manufactured in order to serve as an ingredient in some product, whereas incidental nanoparticles are unintentionally produced as a byproduct of some other activity. For example, driving a gasoline-powered car produces incidental nanoparticles from combustion of the fuel.
Presently, many engineered nanoparticles are made of certain types of metals and other materials that are not found as commonly in incidental nanoparticles. Engineered and incidental nanoparticles may also differ in shape. Titanium dioxide is a naturally occurring mineral that is processed to make titanium dioxide nanoparticles for use in sunscreen, paint, and other products. However, titanium dioxide is not abundant in the atmosphere. On the other hand, soot nanoparticles, which consist mainly of carbon, are commonly found in the atmosphere as incidental nanoparticles. Carbon appears in engineered nanomaterials such as fullerenes, carbon nanotubes, and graphene, shown in Figure 2, in which it has a more regular shape and structure than in soot.
Many studies have shown that inhalation of incidental nanoparticles is associated with premature mortality, heart disease, lung cancer, asthma, and other health effects, but scientists still do not know exactly what it is about the particles is harmful. If it is merely the presence of some small, foreign objects in the lungs, then the difference between engineered nanoparticles and incidental ones may not matter, but if the chemical composition or shape matters, then the difference could be important. In everyday life, we don’t need to worry about breathing nanoparticles in the air if it is relatively clean, as in most areas in the US, but we should be careful about being too close to exhaust or exercising vigorously on a polluted day.