One meter consists of 1000 millimeters (mm). One millimeter equals 1000 micrometers (µm), and one micrometer equals 1000 nm. Thus, 1 nm = 1×10-9 m.

How do nanomaterials compare in size to other objects regarded as “small”? Dust mites have a diameter of approximately 200 µm. Human hairs have a diameter of 60-120 µm. Thus, both are 1000 or more times larger than nanoscale. The smallest known bacterial species, such as the genusMycoplasma, have a diameter of approximately 300 nm (.3 µm), which is still greater than nanoscale. Some smaller viruses (e.g. Parvoviruses, diameter ~25 nm) exist at nanoscale, but most viruses are larger. Typical double-stranded DNA has a diameter of ~2.5 nm.

Nanomaterials are among the smallest materials that can exist, because the smallest unit of elemental matter that retains the properties of the element (the atom) is not much smaller than nanoscale. Due to the uncertain position of the electron cloud around the central nucleus, scientists can only estimate the diameter of atoms. However, most estimates of atomic diameter range from .05 to .25 nm (0.5 to 2.5 Angstroms).

Nanomaterials divide roughly into two main categories: ambient (or “natural”) nanoparticles, and engineered/manufactured nanomaterials. The rest of this Chapter will use the term “nanoparticles” when referring specifically to nanoscale natural (non-engineered) substances. The term “nanomaterials” will be used as a blanket term for all nanoscale substances. In most scientific uses, the terms are interchangeable.

Ambient nanoparticles are also known as “ultrafine” particles in standard industrial hygiene terminology. Sources include diesel engine exhaust, welding fumes, and other combustion processes. Most grinding and crushing processes are incapable of producing nanoparticles, unless fine bead mills are used. Ultrafine/nanoparticles have a larger surface area per unit volume than an equal volume of same composition larger particles. This can lead to different physical, chemical, and biological response properties.

Other natural nanoparticles include smaller viruses and rickettsia, and intracellular proteins, nucleic acids, and organelles.

Figure 18.1
Left: Diesel engines produce particulate soot, which includes ultrafine amorphous carbon (inset).
Right: Welding produces metal fumes, many of which are ultrafine/nanoscale (inset).

Engineered or manufactured nanomaterials are deliberately created and used for a structural/functional purpose. Engineered nanomaterials can include both homogeneous materials and heterogeneous structures with specific applications in computing, medicine, and other disciplines. The next section will examine several examples of engineered nanomaterials.