Aerosols dispersion, airflows

Air contaminants in solid or liquid state (aerosols), e.g., wood dust, welding smoke, or oil mist, are all in principle directly visible. The dispersion of those contaminants and the airflow patterns around the source may therefore be studied without any special tools. It is, however, not always possible to see the contaminant if, for example, the concentration in the air is low, the size of the particles is small, or the lighting is poor. The fact that the contaminant can t be seen may stem from the acceptable low level of the concentration but that can of course not be used to conclude that the control is acceptable. That conclusion depends not only on the contaminant s toxicological qualities but on how visible it is iit air. The ability to see the particles directly is also, as said above, a function of their size. Small particles, able to be transported deep into the thinner airways of the lungs, are many times also difficult to see directly. 

At Biosafety/Laboratory Containment Levels 3 and 4, the laboratory is maintained at a negative air pressure in relation to the external atmosphere. This ensures a continuous airflow into the laboratory. Exhaust air is not recirculated to any other area in the building but is discharged to the outside through a HEPA filter (or equivalent) and dispersed away from occupied areas and air intakes. Any equipment that may produce aerosols is contained in devices that exhaust air through HEPA filters. At Biosafety/Laboratory Containment Level 4 the differential pressure/directional air flow is monitored and alarmed to warn of any malfunction of the system. 

The dominant transport mechanism for both aerosol and gaseous agents in the atmosphere is advection associated with the bulk motion of the atmosphere. Since airflows in the planetary boundary layer exhibit signihcant turbulence under most conditions (though turbulence may be suppressed under conditions of temperature inversion), this will cause aerosol releases to disperse into a plume or puff that expands... 

Neglecting the gravitational force and particle inertia leads to the zero-order approximation that V u that is, the aerosol follows the streamlines of the airflow. This approximation is often sufficient for most atmospheric applications, such as turbulent dispersion. However, it is often necessary to quantify the deviation of the aerosol trajectories from the fluid streamlines (Figure 8.13). 

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