Dust at the Workplace – When size does matter!

It is important for everyone in the Health and Safety Environment to understand the nature of dust and when and where it may be a threat to safety. An effective dust suppression system is critical in a dusty work environment.

Dusts are solid particles ranging in size from below 1 µm up to around 100 µm, which may be or become airborne, depending on their origin, physical characteristics and ambient conditions.

[There are also other aerosols (such as fumes and mists), with very fine particles resulting from chemical reactions in the air, or with air pollution outside the workplace. However, in many cases similar principles of control apply to these as to dusts.]

Examples of hazardous dusts in the workplace include:

Mineral dusts from the extraction and processing of minerals (these often contain silica, which is particularly dangerous);

• Metallic dusts, such as lead and cadmium and their compounds.
• Other chemical dusts, such as bulk chemicals and pesticides.
• Vegetable dusts, such as wood, flour, cotton and tea, and pollens.
• Moulds and spores.
• Asbestos is a mineral fibre, which is particularly dangerous, and is found, for example, in maintenance and demolition of buildings where it had been used as insulation material.

Size fractions

In occupational hygiene, particle size is usually described in terms of the aerodynamic diameter, which is a measure of the particle’s aerodynamic properties. Whether or not an airborne particle is inhaled depends on its aerodynamic diameter, the velocity of the surrounding air, and the persons’ breathing rate.

How particles then proceed through the respiratory tract to the different regions of the lungs, and where they are likely to deposit, depend on the particle aerodynamic diameter, the airway dimensions and the breathing pattern.

If a particle is soluble, it may dissolve wherever it deposits, and its components may then reach the blood stream and other organs and cause disease. This is the case, for example, of certain systemic poisons such as lead. There are particles which do not dissolve but cause local reactions leading to disease; in this instance, the site of deposition makes a difference.

When a relatively large particle (say 30 µm) is inhaled, it is usually deposited in the nose or upper airways. Finer particles may reach the gas-exchange region in the depths of the lungs, where removal mechanisms are less efficient. Certain substances, if deposited in this region, can cause serious disease, for example, free crystalline silica dust can cause silicosis.

The smaller the aerodynamic diameter, the greater the probability that a particle will penetrate deep into the respiratory tract. Particles with an aerodynamic diameter > 10 µm are very unlikely to reach the gas-exchange region of the lung, but below that size, the proportion reaching the gas exchange region increases down to about 2µm .

The depth of penetration of a fibre into the lung depends mainly on its diameter, not its length. As a consequence, fibres as long as 100 µm, have been found in the pulmonary spaces of the respiratory system.

Whenever exposure to airborne dust needs to be quantitatively evaluated, instruments must be used which select the right size range for the hazard concerned.

There are conventions for the size ranges of particles to be measured; it is usual to collect either the inhalable fraction, i.e. everything that is likely to be inhaled, or the respirable fraction, i.e. the particles likely to reach the gas-exchange region of the lung. For example, if silica is present, it is necessary to measure the respirable fraction of the airborne dust.

From the above it is clear that the size of particles does matter – and also “cements” the need for dust suppression at the workplace!

Note: The official symbol for the micron or micrometer is μm, sometimes simplified as um. A micron is defined as one-millionth of a meter, a little more than one twenty-five thousandth of an inch.