To live, we need the air we breathe, the food we eat and the water we drink.
But of the three - air, food and water - air is the one we need on a continual basis.
That is, we can go hours between drinking and/or eating but we can't survive for more than a few minutes without air.
On average, an adult human breathes fourteen times per minute or 840 times per hour or 20,160 times per day. It totals to well over seven million breaths in a single year and about 600 million breaths in our lifetimes.
The amazing thing is we do this with little or no conscious thought.
Indeed, we take breathing pretty much for granted despite its critical function in keeping our bodies functioning.
With each breath amounting to around a litre of air, so we process over seven million litres of air through our lungs each and every year.
It is perhaps not surprising we take air quality very seriously.
I mean, even if something is present at part per million levels - that is, one part in a million - over the course of the year, breathing it adds up. After all, even at the level of one part per million, it means we are breathing in about seven litres of the substance over the course of a single year.
Most of the substances in the air are not harmful.
Indeed oxygen, at 21 per cent of the atmosphere, is essential for life.
Nitrogen at 78 per cent and argon at one per cent make up most of the rest of the atmosphere and both are chemically inert within the human body. But there are many substances in our air which are harmful and they surround us on a daily basis.
For many of these harmful compounds, a single dose of seven litres would be quite overwhelming, and could result in serious injury or death. For example, breathing in seven litres of carbon monoxide at one time is deadly.
It is only because they are present in low concentrations they are relatively safe.
Furthermore, most of these substances do not accumulate in the body. This is particularly critical.
Our bodies are assaulted daily be a wide variety of compounds that could potentially do us harm. They arrive in our food, our drinks, and, yes, in the air we breathe.
But our bodies have natural defences to deal with the vast majority of these compounds - provided they are present only at low levels.
Our defence mechanisms can handle day-to-day doses. It is only when they get overwhelmed that we run the risk of injury.
However, there are some airborne pollutants that our bodies have difficulty dealing with.
One of the major concerns is "particulate matter" - the PM 10s and PM 2.5s - as they can have a profound impact on our health.
These are fine dust particles and soot resulting from such things as incomplete combustion - such as diesel exhaust - and road dust.
The term PM 10 applies to particles that are 10 microns in size or smaller, while PM 2.5 represents particles that are only 2.5 microns in size or even smaller.
Because of their small size, the lungs have trouble expelling these particles, so that they can accumulate and even penetrate the cells lining the lungs or enter the circulatory system. This can lead to a wide variety of medical conditions ranging from asthma to heart failure.
For example, the results from a 2016 study indicate that for every five micrograms per cubic metre increase in the concentration of PM 2.5, there is a corresponding increase of 20 per cent in coronary artery calcium deposits, leading to a higher risk of coronary disease.
Five micrograms per cubic metre of air is 0.005 parts per million of PM 2.5 or five parts in a billion parts of air. It is not a very large amount.
Another study found a 22 per cent increase in the risk of hypertension in people for the same increase in particulate matter for populations living in heavily-polluted areas.
Put in more pragmatic terms, the extraordinarily high levels of particulate matter found on bad days in areas in China such as Beijing lead to the estimate that outdoor air pollution contributes to 1.2 million premature deaths each year in China alone.
Particulate matter found in heavy smog is the fourth leading risk factor for deaths after dietary risks, high blood pressure and smoking.
I should point out particulate matter are not specific chemical compounds, nor is their effect a consequence of their chemical components. Particulate matter act as physical irritants damaging the lungs and causing health issues regardless of what they are made of. This is one of the reasons breathing in smoke of any sort is so bad for us.
The presence of particulate matter can seriously erode the quality of the air we breathe, one breath at a time.
