Inhalable, Respirable and Visible Dusts: Know The Difference

Visible Dusts – as the name implies – are what you can visibly see. The particle sizes covered in this range will vary according to a few factors such as the colour of the mineral or metal and lighting but generally the visible component of visible dust clouds will be of a larger particle size and even potentially not inhalable.  Where there is visible dust however, there is likely to be inhalable and respirable dust. 

Inhalable Dust is a further classification – It is the component that will enter the Nose & Mouth During Normal Breathing. 

Respirable Dust is a further classification of inhalable dust and consists of the fraction between “Zero” and 18 µm. This fraction is the most harmful as they are dusts that are able to reach lower bronchioles & alveolar regions of the lung. Due to their small size gravity doesn’t work very well on them and so they resist dropping out of the air we inhale and so are carried further into the body – probably permanently. It is important to note (as shown below) the smaller the particle size the more respirable it is. 

 

Why is the Above Relevant? For a water droplet to adhere to a dust particle (and therefore increase the particle’s mass which will let gravity drop them out of the air) a water droplet needs to be the same size or smaller than the dust particle.

So if we refer to the graph below and use a 10 μm water droplet size as an example: A 10 μm water droplet would be able to capture dust particles 10 μm and bigger. But looking at how much more inhalable a dust particle is as the particle size decreases that may not even be a 40% reduction in the respirable dust hazard! Remember the smaller the dust particle the further it will penetrate into the lungs. So the hazard could now disproportionately increase as the user unwittingly believes the air is cleaner and so reduces other dust mitigation practices.

So in short – if you’re going to try and reduce the dust hazards in your process with water do it properly and use the best-performing technology available to you where you know you’re getting the smallest droplet size (average and distribution). 

 

A Graph of the Differences Between Fogging and Misting with Respect to Dust Particle Size Capture

Please note the “Averages” shown above are for illustration purposes. Some misting systems may have substantially larger water droplet size averages. Some misting systems may generate a small % of their droplets at close to the upper droplet sizes found in a fog – which may sound impressive – however this % should be proven as it is a very important factor. 

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