All Dust? An overview on the classification, effects and containment of small particles

Properties of dust

Emissions present in solid form are known as dust. This term covers all fine, solid particles that occur in different sizes in gases and especially in air. The particles themselves can be of the most various origin. Their suspension duration – depending on size – can also vary.

Not all dusts are alike!

Depending on how and where dust arises, certain properties result from it with corresponding effects to humans and the environment. If these properties are harmful, solutions must be found to eliminate the dust.
When dusts are processed - or in general when they occur - it must be noted that, depending on composition and occurrence, a hazard may arise for employees. In terms of occupational safety, limit values are defined as a function of particle size and particle shape. Compliance with these limits is checked by measuring the concentration in the air.

Differentiation according to particle size and shape

• Inhalable dust (E-dust)

  This dust can be breathed in through mouth and nose. Particles with a size of up to 100µm are retained by nose hairs or the mucous membranes.
  Example: Pollen or cement dust

• Alveolar dust (A-dust)

  Alveolar dust is fine dust that can penetrate as far as the pulmonary alveoli (aveolar), as the particles are only 5 µm (max.) in size.
  Example: Welding fumes

• Ultra-fine dust (U-dust)

  U-dust stands for "ultra-fine dust". These are nanoparticles with a size <100 nm.
  Example: Products of combustion processes, in lacquers and coatings

• Fibers/fiber dusts

  Fibers are elongated particles of inorganic or organic materials. Here, all fibers >5 µm in length and with diameters <3 µm and such that exceed the length-to-diameter ratio of 3:1 pose a danger, as they can penetrate into deeper airways.
  Example: Asbestos, glass wool, cotton dust

Differentiation according to physical properties

If dusts are not only generated in production but are used to manufacture a product, the following characteristics must be taken into account:

• Sticky dusts
  With sticky dusts, the particles adhere to each other and to surfaces. This can lead to bridge formation in the production plant equipment.
  Example: Coating of tablets

• Abrasive dusts
  Due to their material strength and surface quality, the abrasive dust particles can cause damage to the production plant equipment over time. For this reason, the flow velocities and wall thicknesses must be taken into account when dimensioning the pipelines.

• Hygroscopic dusts
  These particles draw moisture from out of the air and then adhere together or liquefy. They are then no longer dry and pourable, which can affect further production steps.

• Hydrophobic dusts
  Hydrophobic dusts do not bond with the moisture in the air.
  
  
• Agglomerating dusts
  By spray-wetting fine particles, adherence leads to the formation of small grains, which improve the transport properties and/or the processing of the materials. Due to the agglomeration, the grains are better pourable than fine particles, enabling easier transport. When dimensioning the piping and production equipment, the flow velocities must be taken into account as plug formation can occur in the production plant equipment.
  
  
• Pourable dusts
  The finer the particles are, the less pourable the dust is, allowing the material to be whirled up more easily. The pourability must be observed when dimensioning the production plant equipment.
  
  
• Bridge-forming dusts
  Bridge formation must be avoided in production plant equipment, as this can lead to malfunctions in the process. When designing production plant equipment, bridge- forming dusts must be considered and the design must be adapted accordingly.

The effects of dust

The effects of dust depend on various factors. Dust itself, for example, can pose a problem simply because of the amount of dust present. The human body, especially the respiratory tract, has various protective systems to counteract the dust as long as the amount is within limits. However, the effect also depends on the aforementioned properties - for example how far particles can make their way into the organism.

To determine the hazard potential, not only the size of the particles but also their composition is important. Depending on the material contents, even larger particles that only reach the upper respiratory tract can pose a hazard. For this reason, the toxicological properties must be taken into account:

• Sensitizing dusts
  Such dusts can trigger allergies and thus represent a burden for humans.
  Example: Organic dusts from plants and animals
  
  
• Fibrogenic dusts
  Fibrogenic dusts can lead to scarring of the lung tissue and have lasting effects on lung function.
  Example: Quartz dust
  
  
• Caustic dusts
  The particles of caustic dusts can destroy human tissue by forming acids or bases.
  Example: Bird droppings
  
  
• Toxic dusts
  Inhaling toxic dusts can damage internal organs, such as the lungs.
  Example: Toxic dusts of lead, cadmium, ...
  
  
• Carcinogenic dusts
  These dusts can cause cancer.
  Example: Oak dust, asbestos
  
  
• Radioactive dusts
  Radioactive dusts can attack the genetic material, cause tumors, settle in mucous membranes or bronchial tubes.
  Example: Tritium

Containment of dust

To reduce the spreading of dust as well as hazards caused through dust, the STOP principle, which we have already reflected upon in our blog article on employee protection, is employed:

• Substitution of hazardous substances - Checking if there are substances that are less toxicologic
• Technical measures - Upgrading systems with appropriate safety solutions
• Organizational measures - Planning to ensure that employees are not exposed to dangerous substances for too long
• Personal protective equipment - Using these if all other measures are not applicable

The first step is to try to prevent the occurrence of dust. This can be done by selecting appropriate material properties - as an example, less dust is produced when using agglomerating substances instead of fine substances.

If dust nonetheless occurs, the hazard can be reduced by using fewer toxic substances.

Technical measures include solutions, which absorb occurring dust. This involves encapsulation of dust-generating process steps and using water-spray systems or fans.

An efficient method in protecting humans and the environment is dust extraction: Here, the dust-laden air is directly drawn off and cleaned in dedusting systems using filter elements adapted for the application. Extracted dust remains in the filter system and can be disposed of accordingly.

Depending on particle size and properties, there are different possibilities for selection, as shown exemplary in the following overview. When planning a filter system, it is therefore important to communicate the dust properties to the filter-system manufacturer for proper filter selection and dimensioning.