Direct Observation of Aerosols Enhanced by Special Light

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. 1 hat con­clusion depends not only on the contaminant’s toxicological qualities but on how visible it is in 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.

The lighting conditions are in many ways crucial to see an aerosol. A general increase in the intensity of light will to a certain degree increase the visibility of the panicles. More important, however, is the quality of the light and the geometrical relations of the light source, the contaminated air volume, and the observer.

An ordinary photography spotlight, desk lamp, or electric torch will im­prove the visibility of the aerosol a lot if used in the best way. Figure 12.1 il­lustrates this with a smoke source with only general lighting (a), with the use of a spotlight close to the observer (b), and with the spotlight arranged oppo­site with a screen placed to avoid dazzling the observer u ). The effect of the extra light is obvious, especially when the geometry is optimal.

If a spotlight with a parabolic reflector is used, resulting in mainly par­allel beams of light, the effect will be further improved. The effect is the same as when the sun (parallel light beams) is shining through the window, making all airborne particles visible. This effect is called the Tyndall effect and is utilized in special dust or Tyndall lamps. ’ The dust lamp is based on the effect: that wThen parallel beams of light are directed to a cloud of fine particles, the most intensively scattered light is in the forward direction. The angle of this scattered light is small, and it is therefore of special im­portance that the anglelamp-dust cloud-observer be close to 180°. A devi —

Direct Observation of Aerosols Enhanced by Special Light

Arion between 5° and 15° is optimum. This implies that some kind of shield is necessary between the lamp and the observer.

This kind of spotlight enhances the visibility markedly and is very useful for the visualization of fine particles emitted from a relatively small source. Figure 12.2 illustrates the effect of a dust lamp used to visualize wood dust emission and exposure when sanding wood.

A drawback with the dust lamp compared to the photography spotlight is the limited volume of interest that will be illuminated. The cross-section of the light beam can never be wider than the diameter ol the reflector in the lamp. A large reflector may certainly be used but will often be impractical for the pur­pose. With a lamp of the type in Fig. 12.2, it will therefore not normally be possible to sec the emission of the contaminant at the same time as the expo­sure is visualized. This drawback, however, is many times compensated bv the better capacity to visualize low concentrations of aerosols.

Typical examples where the use of extra light in general and the dust lamp in particular is very useful include checking that enclosures and local exhausts are effectively capturing emitted particles from, e. g., crushing, grinding, or sanding operations; checking for leakage from partly or totally enclosed

Direct Observation of Aerosols Enhanced by Special Light

Processes; or studying to what extent emitted particles reach the breathing zone. Another important use is to improve the visibility of smoke emitted to visualize airflow, etc. (see Section 12.2.2). The principles for the use of light for that purpose are the same as described above.