What is dust?
You probably see it each and every day. As you pass by the window, when a stream of sunlight shines in, tiny, previously invisible particles appear suspended in the air. Or, looking into the bag of your vacuum cleaner, where great swathes of grey cotton candy lie swirled up immobilized. Or even, when staring out on a dry day into the distance, the wind kicks up and spreads a fine haze of miniscule particles across your field of vision. We have a noun common to all of these phenomena: dust. It is ubiquitous in our daily lives. What it actually is, however, is far from a well-known fact.
How can dust be defined?
The following is an exposition of two of the ways in which we can understand dust: firstly, a quantitative understanding of dust provided by an exposition of the various scientifically grounded categories within which dust pollution levels are recorded, as set and popularized by the Environmental Protection Agency (EPA) of the USA; and secondly, a qualitative understanding of dust through an enumeration of four of its most distinctive qualities.
As the old adage goes, the key to winning any battle is to know thy enemy. In the battle against dust pollution, each and every way of knowing just what dust is is important.
We begin with the quantitative understanding of dust. The proper technological term for dust is particulate matter (PM). The name makes sense: dust really just is tiny particles of matter. But a new technological name is only the first step on the road to knowing just what dust is. Or rather, knowing what particulate matter is.
Source: 2001 EPA (p. 2.-11), taken from: Source: Adapted from Wilson and Suh (1997); CD, p. 2-35.
Types of dust are generally categorized in terms of their size in micrometers, or μm (1m x 10-6). They start from the very, very small, and continue to the incredibly small. Before the 1980s, the categorizing of dust particle size wasn’t very precise. Total Suspended Particulate (TSP) was used to stand for all the dust particles in the air, for all the particulate matter (PM).
TSP was a catch-all term for each and every suspended dust particle present in the air, and was unintentionally based on the inlet-size of filters used to measure air quality – in practice this could range from 0-100 μm in size. Today it is used as an umbrella term encapsulating all other groupings.
What is PM10?
In 1987, however, the Environmental Protection Agency (EPA) of the United States set about defining a more precise categorizing schema for dust, based upon the potential health issues their size could allow them to cause. They first defined PM10: those “inhalable coarse particles” of a size of 10 μm and below. This category can be composed of pollen, dead skin cells, and is visible to the human eye only in very dense concentrations. The particles diffract the light, creating the hazy effect one might see hanging over a city. This category is now known as identifying “coarse” particles, in opposition to the introduction in 1997 of an even more precise category: PM2.5.
What is PM2.5?
PM2.5, measuring at 2.5 μm and even less, has been defined as being comprised of fine particles. These particles are small enough to be respired, and thus pose the greatest health risk of all the different dust categories. While high concentrations are likely to coincide with the visual impairment caused by high concentrations of PM10, PM2.5 particles are in fact invisible to the human eye. This makes PM2.5 the most dangerous of dust categories: it can be all around you, in every breath you take, without you even noticing. Relying on our senses isn’t enough – systematic, scientifically informed measures need to be taken.
What is PMUltrafine?
A further, even smaller category has been added in recent years. This is ultrafine particulate matter. Advancements are now being made into the study of these particles, their behaviour and their health effects, with the results still forthcoming. They are even smaller than PM2.5, but have a much shorter lifetime, as they generally either settle quickly or agglomerate themselves into bigger particles. It is expected that in the coming years studies into their airborne behaviour will again alter the regulatory standards enforced by environmental agencies around the globe.
“What is particulate matter?”: a quantitative answer.
These four categories taken together comprise a general understanding of dust in terms of its scientific categorization. The development of a precise and human-oriented categorization system has been an important step towards understanding the health threats posed by dust, and the following necessity of putting anti-dust measures in place. It thereby represents a further step forwards in the battle to reduce our levels of dust pollution below harmful levels.
This is not the only way to understand what dust is, however. A useful way of understanding anything is by a listing of its qualities, its particular characteristics which give it its uniqueness. This listing as a whole gives a qualitative understanding. Let’s undertake one for dust, focusing on the particular peculiar qualities which make it such a danger.
Dust is made up of solid matter. This contrasts it with the air, made up of gaseous elements, in which it floats. Solids are rigid and hard, and don’t spread out across a container like gases or liquids do. This is why they pose such a problem when they enter our respiratory system: they can build up to form physical blockages.
A dust particle is small. Generally we start to count something as a dust particle, or PM, coming down from an aerodynamic diameter of less than around 400μm, although particles above 100μm are far less likely to remain airborne for extended periods of time. From then on the size plummets down to the unimaginably small, with the smallest category measuring particles of a width of less than 0.1μm.
A dust particle is also lightweight. It is so lightweight that only a relatively small force is needed to send it up skyward. If the wind kicks up, or a truck passes over a dusty road, the particulate matter will be shot up into the air. Because it’s so light and small, giving it a low density, it can stay suspended in the air for hours, days and even months, depending on its size.
4. Heterogeneous Composition
A bunch of particulate matter is not just made up of one material. Dust is made up of lots of things. The generality of the term “particulate matter” is here useful: a dust cloud could have grains of sand from the beach, particles of quartz from a construction site, as well as flakes of human skin and animal fur all floating together suspended in the air. To the eye they are indistinguishable, if at all perceptible. To the body, however, the different chemical compositions can result in differing effects.
“What is dust?”: a qualitative answer.
These four qualities of dust don’t exhaust dust’s peculiarities. But they are sufficient, when taken together, to lend dust its menace. This was the aim of our qualitative understanding: to know, in common speech, just what properties of dust are undesirable, and why. For now we can say that, though the qualities enumerated taken one by one are harmless, and would be no cause for concern, or even interest if alone, as a quartet they form a deadly tune.
This is due to the following causal chain:
As the dust is so lightweight, it can become easily airborne. As it’s so small, we usually can’t see it in the air. This means we can’t simply use our sensory input to avoid it. Its solidity means that inhaling it poses threats to our respiratory system; it cannot be easily dissolved or filtered out by our defences, especially if inhaled in large quantities (See here for a description of our respiratory system’s defences). And its heterogeneity means that we can’t just develop a blanket defence against a certain type of particle, or rest easy if we find out that a certain type of material poses little to no threat to human health.
Adding these qualities up makes dust a troublesome foe: evasive and unpredictable, invisible yet potentially deadly, with huge diversity in type.
“What is dust?”: an overarching conclusion.
Our gained understanding need not be cause for despair, however. Our quantitative understanding of dust allows us to talk of it clearly, to demarcate that which we try to to control, and that which can cause us no harm. Possessing the right vocabulary helps our testing and detection methods become more precise, and helps us hone in on the really important data.
The qualitative understanding, on the other hand, can inform our development and application of dust controlling measures. Controlling one of the four qualities mentioned so that their realisation is prevented will result in a negation of the dust’s threat.
We’ve taken both forms of understanding here at LAMI®, and instilled them into our approach and technology. If you want to find out which quality of dust we target, or the means by which we target both PM10 and PM2.5, please feel free to contact us for more information. Alternatively, if you just want to know more about dust, check out some of our other articles.
EPA, 2019. Policy Assessment for the Review of the National Ambient Air Quality Standards for Particulate Matter. [Draft] Available from:
EPA, 2001. Policy Assessment for the Review of the National Ambient Air Quality Standards for Particulate Matter. [Draft] Available from:
PublicLab, (n.y.). Introduction to Particulate Matter. Available from:
Encyclopedia.com, 2020. TOTAL SUSPENDED PARTICLES (TSP). Available from: