Air contaminant measurement

Another way of looking at this is to talk about equivalent aerodynamic diameter (e. a.d.) . What we mean is that if a parhcle has an e.a.d. of 7 micrometres it falls at the same speed as a 7 micrometre sphere with a density of 1 g/ml. This approach means that we get away from size and shape and look at what the parhcle does that is, how easily does it sediment or fall out. Experiments have shown that in dust which reaches the alveoli, virtually no parhcle has an e.a.d. of more than 7 nhcrometres - 50% of particles with e.a.d. of 5 micrometres get that far, and nearly 100% of parhcles with e.a.d. of 0.1 micrometer or less do so. 

If we are interested in what reaches the lower lung then sampling devices should collect dust on the same basis. Sampling devices have been designed which do this, such as the horizontal elutriator as the reference device (heavy and bulky), and the mini-cyclone (lightweight and portable). 

You may have read that it is asbestos fibres of up to 3 micrometer real diameter which pose a threat to the lower lung. This is so because an asbestos fibre (a fibre is a particle at least three times as long as wide) of 3 micrometer diameter falls out at a speed the same as a spherical particle of ordinary dust with an equivalent aerodynamic (not real) diameter of 7 micrometers (it is not a true sphere but is called spherical because it is very roughly the same diameter whichever way you measure it). 

Some workplace dusts are neither fibrous nor spherical , but platy that is, roughly speaking, a thin disc. Talc is an example. 

We call all dust which can enter the nose irtspirable that is, dust with an e.a.d. from just above zero to approx. 30-50 micrometres e.ad. Generally speaking, the health effects of spherical particles of concern on the Itmg depend on the mass or weight of dust taken in, and this depends on the concentration in the air and the time over which it is breathed. 

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