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The Respiratory System’s Defences

The average person who is moderately active during the daytime breathes about 20,000 litres (more than 5,000 gallons) of air every 24 hours…

Humans breathe in air to acquire oxygen. Of course, the air is not just made up of oxygen. A typical piece of Earth air is composed of 78% nitrogen, 21% oxygen, and 1% of other elements, such as argon, helium and hydrogen. This is just accounting for the gases naturally found in it, however. It doesn’t take into account any foreign contaminants that may be floating in the air; it doesn’t take into account the presence of dust, or particulate matter (PM). A hearty breath of clear, crisp air might contain large concentrations of PM, alongside other harmful agents such as bacteria, fungi or viruses (See here how these threats often come in tandem).

We are not defenceless against this threat, however. Even though our senses aren’t aware of the danger, our bodies have evolved so as to ward off this unobservable danger. Let’s take a look at each stage of our respiratory system and their respective defences against dust.

Mouth and Nose

woman breathing using her defences against dust
The mouth and nose are the first points of entry for any foreign airborne matter.

First, we have the elements we are most familiar with: the mouth and nose. These are the orifices through which the oxygen we need to survive is inhaled; they are also the points of entry for toxic airborne material, such as fine dust. Their first line of defence is simple: they can be closed off so that no toxic materials can enter, by shutting the mouth or pinching one’s nostrils. This is of course only a temporary defence measure, as most humans can only hold their breath up to 30 seconds at a time.

Their secondary measure is the hairs found in the nasal passages. These act as physical entrapping agents. If any solid material is inhaled and is large enough to collide with these agents, they stick. The irritation one feels just before one is about to sneeze is often caused by the trapping of some solid material by the hairs in the nasal passage. Their trapping causes the irritation, the irritation causes the sneeze, and the rapid exhalation of air through the air passages blows the offending solid material outwards. This is a very effective line of defence, on the condition that the offending material is large enough for it to reliably come into contact with the hairs.

Larger coarse dust particles, those too big to fall into the PM10 category, are likely to be trapped here. (See here for a guide to the different dust particle categories). If the particle is small enough however, the probability of collision will be so reduced that they could start to reliably infiltrate deeper into our bodies. Here, however, they meet yet more lines of defence.

Upper Airways

woman coughing body defence dust inhalation

The next stage of the respiratory system is the trachea or windpipe, which leads down from the mouth and nose cavities towards the lungs, where it is attached to two bronchial tubes. These are the tubes which lead to each individual lung. Lining the inside of these tubes are minute little hairs, called cilia. They constantly transport mucus upwards, beating more than 1000 times a minute.

Mucus is one of the main defensive agents the respiratory system possesses. It is a sticky substance one coughs up when under the weather, and what fills up your nose, otherwise known as snot. Mucus’s function is vital, if its appearance unsightly. It acts as a kind of glue, a trapping agent, which, upon contact with any solid material in the airways, adheres it to itself. Foreign contaminants thus stuck are then gradually sent back up to where they can either be swallowed or coughed out, negating any threat.


PM, a mixture of particles and droplets in the air, can raise the virus attachment to respiratory epithelial cells and deposit deep in the lung due to the small size and larger surface-to-volume ratio.

Su et al, (2019) The short-term effects of air pollutants on influenza-like illness in Jinan, China, p. 6.

But, as with the nasal hairs, minute particles can avoid the mucus in the windpipe. These particles are most likely to be no wider than 2.5μm, falling into the PM2.5 category. They are those particles which are most likely to continue on into the lungs.

The lungs are those organs responsible for the intake of oxygen. They are filled with over 400,000 alveoli, which act as dense little air sacs for the oxygen to latch onto. At this point of the respiratory system there can be no mucus-based defence, as it would inhibit the alveoli’s ability to capture the oxygen. Instead, if dust particles or other foreign substances penetrate this deep into the lung, the immune system takes over.

Alveolar macrophages are produced in and inhabit the lungs. They are a type of white blood cell, which, when presented with a foreign contaminant in the lungs, attach themselves to them and eventually ingest them, preventing them from either multiplying, if the contaminant is living, or from forming a physical blockage that could inhibit the proper functioning of the respiratory system. While very effective, the phagocytic system can be overwhelmed simply through a too large a quantity of contaminants to ingest. When this happens our respiratory system’s defences can be ultimately overwhelmed, leading to respiratory diseases.

black haired man wears anti dust mask as defence
Living in an area with high particulate matter pollution rates will weaken your body’s defences. Wearing a face mask is a temporary counter-measure to add to your body’s natural defences.


This has just been a cursory representation of the defences our respiratory systems possess. And while they may seem comprehensive, and are indeed extremely effective normally, a consistent exposure to dust pollution will lead to inevitable damage of the respiratory system. Too much dust and the defences become overwhelmed, with respiratory conditions and diseases following as the dust lodges itself within our bodies.


Su et al., (2019). The short-term effects of air pollutants on influenza-like illness in Jinan, China. BMC Public Health [online]. 19 (1319), 1-12. [Viewed 18.05.2020]. Available online: