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152 CHAPTER 8 1958 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 (listinguish between such components as soot or carbon particles, fly-ash, cinders, tarry matter, unburned gases, and gaseous combustion products. The finest particulate *1 constituents are much less than 1 micron in size, often in the range of 0.1 to 0.3 mi- Mists and Fogs are liquid particulate air contaminants. Mists 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 micro- \ organisms 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 , W air becomes saturated with that liquid. Many droplets in fogs or clouds are micro- -j scopic-and submicroscopic-in size,..and_may be .conceived .as tho 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 less', 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 substances 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 fronriv. about 0.2 to 5 microns in size, fungus spores from 1 to 10 microns, and pol-|-5 len 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 in4:; 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 aa'jr economic significance. 44 Industrial dust particles are predominantly of the order of 1 micron injj size. Tremendous numbers are also present in the submicroscopic range- below 0.5 micron, but those below 0.1 micron are not believed at present Air Contaminants 153 to be of practical importance, possibly due to their exceedingly small mass in comparison with the balance of airborne matter. In fact, particles 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). LAWS OE SETTLING IN RELATION TO PARTICLE SIZE (LtWtS Or DEMARCATION APPRO*.) "particles" FALL WITH INCREASING VELOCITY Vc.-v/-23ds' 3Kst c-Vtlocity cm/sc Velocity ft/Wn d"Diom. of parC = Z4.9VdS7 tick in cm. STOKES LAW D-Diam. of par ticle in Microns r Rodim of partide incm. c g-981 chl/mc.1 acceleration S7 *i Density of particle FOR AIR XT 70* F. VDtni^i of Air c -300,4603^1' C -00392 3,0* (Very Small relative to s,) 7-Viscosity of dr in poises CUNNINGHAM'S Oir at 70* F. FACTOR A* 10"* cm. C-c`(|,k) (Mean free cVcOrSTOKlSUM K .8T0.es path of gas molecules ) PARTICLES HOVE LIKE GAS MOLECULES A* Distance of motion m taw t R-Ga constant 0.516 xlOT a=-\/5J A V h 3JT7r T * Absolute Temperature NNumber of Oos foolccuks in one fnol&06*l0P p Compiled by W. G. Frank and Copyrighted, used by permission. IG- 1. Sizes and Characteristics of Airborne Particulate Matter froXmllei93iVfni\ 059 t?ajspheric PolIution in 14 American cities conducted to be 0.5 miern m,,lcate^,the, average size of outdoor dust particles under thp 5 ^ collected by the Owens jet dust counter and measured icroscope. Inability of the light field microscope to reveal