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154 CHAPTER 8 1950 Guide of organic substances such as tobacco, wood, coal, oil, tar and other carbonaceous materials. The term smoke is commonly applied to the mixture of solid, liquid and gaseous products of combustion, although the technical literature prefers to distin guish between such components as soot or carbon particles, fly-asn, cinders, tarry matter, unburned gases, and gaseous combustion products. The finest particulate constituents are much less than 1 micron in size, often in the range of 0.1 to 0.3 mi cron. Mists and Fogs are liquid particulate air contaminants. Misti are very small airborne droplets of materials that are ordinarily liquid at normal temperatures and pressures. They may be formed by atomizing, spraying, splashing, mixing, violent chemical reaction, electrolytic evolution of gas from a liquid, or escape of a dissolved gas upon release of pressure. Very small droplets expelled or atomized into the air by sneezing constitute mists containing microorgan isms that become air contaminants. Fogs are limited by some classifications to airborne droplets formed by condensa tion from the vapor state. This arbitrary distinction between mist and fog is of minor importance, as both terms are used to indicate the particulate state of airborne liquids (occasionally termed aerosols). Fog nozzles are so named because of their ability to produce extra fine droplets as compared to the mist from ordinary spray devices. The highly volatile nature of some liquids quickly reduces their airborne droplets from the mist to the fog range, and eventually to the vapor phase until the air becomes saturated with that liquid. Many droplets in fogs or clouds are micro scopic and submicroscopic in size, and may be conceived as the transition state between the larger mists and the vapors. Vapors and Gases are non-particulate air contaminants. Vapors are the gaseous phase of substances that are either liquid or solid in their commonly known state, examples being gasoline, kerosene, benzene, carbon tetra chloride, mercury, iodine, camphor. Vapors may be changed to the solid or liquid form by increasing the pressure, decreasing the temperature or applying both proc esses simultaneously. They are removed from the air by condensation with les3 difficulty than are the gases. Gases are normally formless fluids which tend to occupy a space or enclosure com pletely and uniformly at ordinary temperatures and pressures. The following sub stances qualify as gases: oxygen, nitrogen, carbon dioxide, carbon monoxide, hydro gen, ammonia, sulfur dioxide. Gases, likewise, may be solidified or liquefied by the proper control of temperature and pressure. The preceding classification is not suitable for the airborne living or ganisms, which range in size from the submicroscopic viruses to the largest pollen grains, not considering the smallest insect life. Bacteria range from about 0.2 to 5 microns in size, fungus spores from 1 to 10 microns, and pollen from 5 to 150 microns. SIZES OF AIRBORNE PARTICLES Fig. 1 is a graphic tabulation of the properties of airborne solids and liquids arranged according to size on the micron scale. There are 25,400 microns in 1 inch. Particles larger than 10 microns are unlikely to remain suspended in air currents of moderate strength, but settle out by gravity at speeds dependent upon the shape, size and specific gravity of the particle, wind velocity, orientation of the collecting surface, and topography. These larger particles are of major interest to the engineer in the solution of nui sance problems, but it is usually the smaller particles, or those below 10 microns, that remain in the air long enough to be of hygienic as well as economic significance. Industrial dust particles are predominantly of the order of 1 micron in size. Tremendous numbers are also present in the submicroscopic range below 0.5 micron, but those below 0.1 micron are not believed at Air Contaminants dresent to be of practical importance, possibly due to their exceedingly small mass in comparison with the balance of airborne matter. In fact, articles this small may become the permanent atmospheric impurities that have little, if any, opportunity of settling because of the continual motion imparted to them by air currents and the molecular activity of gases (Brownian Movement). .Fio. 1 Sizes and Characteristics op Airborne Particulate Matter The survey1 of atmospheric pollution in 14 American cities conducted from 1931 to 1933 indicated the average size of outdoor dust particles to be 0.5 micron, as collected by the Owens jet dust counter and measured under the microscope. Inability of the light field microscope to reveal