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crushing, grinding, milling, drilling, demolition, shovelling, conveying, screening,
bagging, and sweeping. Dust particles are usually in the size range from about 1 to
100 μm in diameter, and they settle slowly under the influence of gravity."

However, in referring to the particle size of airborne dust, the term "particle diameter"
alone is an over simplification, since the geometric size of a particle does not fully explain how it
behaves in its airborne state. Therefore, the most appropriate measure of particle size, for most
occupational hygiene situations, is particle aerodynamic diameter, defined as "the diameter of a
hypothetical sphere of density 1 g/cm 3 having the same terminal settling velocity in calm air as the
particle in question, regardless of its geometric size, shape, and true density."
The aerodynamic diameter expressed in this way is appropriate because it relates closely to
the ability of the particle to penetrate and deposit at different sites of the respiratory tract, as well
as to particle transport in aerosol sampling and filtration devices. There are other definitions of
particle size, relating, for example, to the behaviour of particles as they move by diffusion or under
the influence of electrical forces. But these are generally of secondary importance as far as
airborne dust in the workplace is concerned. In aerosol science, it is generally accepted that
particles with aerodynamic diameter >50 μ m do not usually remain airborne very long: they have
a terminal velocity >7 cm/sec. However, depending on the conditions, particles even >100 μ m may
become (but hardly remain) airborne. Furthermore, dust particles are frequently found with
dimensions considerably <1 μ m and, for these, settling due to gravity is negligible for all practical
purposes. The terminal velocity of a 1- μ m particle is about 0.03 mm/sec, so movement with the air
is more important than sedimentation through it. Therefore, summarizing in the present context, it
is considered that dusts are solid particles, ranging in size from below 1 μm up to at least 100 μm,
which may be or become airborne, depending on their origin, physical characteristics and ambient
conditions.

Examples of the types of dust found in the work environment include:
 mineral dusts, such as those containing free crystalline silica (e.g., as
quartz), coal and cement dusts;

 metallic dusts, such as lead, cadmium, nickel, and beryllium dusts;
 other chemical dusts, e.g., many bulk chemicals and pesticides:
 organic and vegetable dusts, such as flour, wood, cotton and tea dusts,
pollens;
 biohazards, such as viable particles, moulds and spores.

Dusts are generated not only by work processes, but may also occur naturally,
e.g., pollens, volcanic ashes, and sandstorms.
The most dangerous for humans are particles 3-10 μm in size that get into
lungs through breathing and stay there; accumulating, they cause diseases. Particles
larger than 10 μm stay in the nose and the throat and those smaller than 3 μm are
exhaled.
Fibrous dusts, such as asbestos and other such materials, have been shown to present
special health problems primarily related to the shape of the particles. In relation to health,
particles with diameter < 3 μm, length > 5 μm, and aspect ratio (length to width) greater than or
equal to 3 to 1, are classified as "fibres" (WHO, 1997). Examples of fibres include asbestos
(comprising two groups of minerals: the serpentines, e.g., chrysotile, and the amphiboles, e.g.,
crocidolite - "blue asbestos"). Other examples include synthetic fibrous materials such as rockwool
(or stonewool) and glass wool, as well as ceramic, aramid, nylon, and carbon and silicon carbide
fibres.

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