Indoors, there is an old trade-off for the heating and cooling of a building: energy expended on temperature control versus input of good fresh air from outside.

Nowadays, energy efficiency is winning the debate, but is it at the expense of optimal air quality for breathing?

Air, whether from indoors or outdoors, contains mould spores and other allergens, but these are generally present at natural or normal levels that are generally not harmful to people. Water intrusion changes all this, allowing levels of mould spores to easily become elevated if not caught soon enough.

Bringing some outdoor air to the indoors is a practice recommended by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), perhaps on the assumption that indoor air may accumulate dust and other particulate matter, and may even accumulate chemical molecules if these are being released inside the building consistently or episodically.

If there is no other reason to ventilate buildings, we need oxygen to breathe, and accordingly airtightness as a goal of the building industry is questionable as a one-size-fits-all approach, even when it saves on energy bills. Similarly, carbon dioxide buildup is also of mild concern when buildings are airtight. These ideal gases equilibrate between spaces more readily than larger particles do, but even they can’t move through solid materials by magic.

Besides fundamental gases, what else does a body of air contain anyway? In the mid 1970s, experts from the World Health Organization and the American Chemical Society met at the International Conference on Environmental Sensing and Assessment. Smog was the concern at the time, Los Angeles smog in particular. They thought, “if only we could see what else, besides gases, is in air at the microscopic level, then we could set about controlling its composition.” The goal was to fix the smog, as it were, across the country. Sampling machines and modern electronic instrumentation had drastically improved in the 70s and were, by then, up to the task.

The air outdoors can be modified by government legislation incumbent on industry. On the other hand, indoor air is modified by a building owner, architect or engineer, and is often not subject to as much regulation. It has been argued that the inside air comes essentially from outside, but there is also an argument that the indoor air is a special flow of its own, and that what is creeping under the door is only gases exchanging, not so much particles.

Legislation is currently in place to keep outdoor air reasonably clean, thanks to that meeting in the 1970s. One reason for now taking a close look at indoor air today is that there are also unwanted things present in indoor air: irritants, allergens, and things that make us feel unwell.

Chemical compounds, for instance, are present in indoor air as well as outdoors. It’s just a question of amount, and guidelines do exist on what are normal or tolerable amounts. At high levels, chemical constituents such as formaldehyde are problematic. Chemicals may come from the off-gassing of flooring materials, paints and furniture coverings, and even plastic building materials. So rather than all chemical and biological constituents of air coming from the outside, some can actually be getting generated indoors. In fact, even moulds can produce chemical toxins, while the spores themselves are larger. Mycotoxins, as the toxins of mould are called, aggregate in particles smaller than a spore, and so are more readily breathed in.

The allergens in air, such as mould spores, which are biologically derived, attract less attention than chemicals do and are the subject of even fewer building guidelines than chemicals are, lost in the murky quagmires of microbiological history. Perhaps it’s simply because lab microscope magnification for mould tends to stop at 40 or 60 times magnification. As such, there has been – and still is – a sharp division in our minds between what’s visible, real life, readily confirmable under a low power microscope, versus what’s regarded as chemistry; the inference that atoms, molecules and compounds exist. Toxins, for instance.

In reality, particle sizes in air are a spectrum of sizes, ranging from atoms all the way up to spores and pollen grains. There are no gaps in the spectrum; there are particles at every size, we have simply lacked the instruments to see all the small ones effectively. Since we couldn’t see them directly, we need to use some tool. Handheld meters are now available that detect the presence of very minuscule particles.

With particles, it’s a numbers game rather than presence versus absence. The size spectrum tells us that there are some of every size present, and so the focus instead falls on exactly how many of each there are at each size. Many committees have weighed in and will continue to weigh in, and this collaboration is exactly what is needed. Consensus in the quantitative sciences is so apt, it could even be called elegant.

The dust that plays within shafts of light has inspired and educated many a backyard innovator. Visible floating dust still has plenty to show us. The building engineer’s tradeoff of energy versus ventilation can raise the challenge of air filtration, a luxury enjoyed mainly in business premises. The task: to actually get the dust particles out, and/or prevent them coming in.

If filtration were to be the solution, then the vast majority of the community are in trouble, as they may never be in a position to afford air filtration, and they need energy efficiency even more than the top end of town does. The great leveler is that energy efficiency trends gradually become common across society. Some trends may even begin with domestic architects – not just commercial ones – and so trends and alternate solution methods may travel either way.

Those who have looked at dust and wondered what’s in it include those who have looked toward the horizon and couldn’t see it for the smog in the way. The USEPA and the LA smog researchers are the people who we can thank for establishing just what is in air, other than pure gases.

What they found is that there are basically two particle size categories in air, and these two categories have a most amazing inbuilt distinction. Particles are either “fine” (meaning small), being 0.1 to 2.0 micrometres, or they’re “coarse” (meaning large), being 2.0 to 20 micrometres and above, and rarely the twain shall meet. The LA smog researchers gave us a picture, in the form of a graph, of whats found in air. It showed two peaks of particle sizes in the distribution. There are a lot more of the fine particles than there are of the coarse particles. The tiniest particles can grow in size by fusing together. Being liquid droplets, they tend to attract each other.

The reason particles from one size category can only rarely fuse with particles from the opposite size category is because fine particles are wet and the coarse particles are dry. Physical forces keep the fine wet particles aloft, while the coarse dry particles settle out more quickly. If some fine wet particles do join with a coarse dry particle, the coarse particle just gets even heavier and settles even more quickly.

Delving deeper, many of the dry particles are what’s called hydrophobic, meaning their characteristic is to repel water. Mould spores are in this coarse hydrophobic or dry fraction. Due to environmental selection, the mould spores actually need to be in the dry fraction. If they got bound up with every raindrop or with every iota of humidity, they would settle out very quickly due to sheer weight, and would not have a chance to spread.

More specifically, moulds lived on the forest floor before indoors became a thing. To spread, they relied on raindrops hitting them. If instead of becoming part of the rain drop, they could instead use the raindrop as a propellant, like a rocket ship or booster, then they could travel far, and so that is where environmental selection has pushed them.

The coatings on mould spores are like the coatings on the legs of water skaters. When a rain drop hits a patch of mould, the rain drop stays put. Meanwhile the energy that the drop had as it fell miles from the sky, all gets directed into the motion of the mould spores outward and upward, as well as some remainder of the energy dissipating as heat. That series of events may propel the mould a few metres up, and that’s enough to get them into the airstream, where they can be carried away by the wind.

The optimal situation from a mould’s point of view is to spread very far and start growing somewhere else where more resources may be found in the form of nutrients. Clearly there is no rain factor indoors like this, but like rain, human movement tends to create gusts that spread spores and keep them continually aloft. It’s thanks to the long history of mould spore design that the spores have this amazing ability to stay aloft.

Chemicals, by contrast, are not alive and are not subject to environmental selection on their behavior; it’s not survival of the fittest. They happen to be carried in the fine particle size category that happens to stay aloft for long periods, and this is simply on account of their tiny size. So these two kinds of particle then – chemicals and mould spores – each bear the characteristic of being airborne. It’s precisely because particles can hang about for long periods that they can build up in indoor air.

We find chemicals generally in the fine faction, which is generally wet, but there can also sometimes be large wet droplets in the coarse fraction – ocean spray, and 100 per cent humidity during rain being the main examples. These large liquid droplets settle out within hours to days, leaving only the dry particles remaining in the coarse fraction: insect parts, pollens, mould spores, minerals, carbon and the like.

Where does this leave us with building ventilation? We can’t all go and live back on the forest floor where ventilation is perfect. Perhaps a compromise of using excessive ventilation indoors may work for some people, but in general we’re all going to gravitate toward common solutions – middle ground and greater control.

Engineers get this instinctively, and they know what to do about it. They produce solutions. That’s where we get the ASHRAE standard 62 from, and its why we have it. The engineers are telling us the practical to-dos, and the why. If only we’d listen to those guidelines.

Oddly, throughout the rise of civilization, residents and builders have focused mainly on the costs involved in building, oblivious of the danger of spores, dangers that were arguably well understood by ventilation engineers. Meanwhile, many property managers have been unsure of how to eradicate those spores already growing indoors. Not being able to see what they were breathing in, residents in damp buildings have had essentially no idea that mould was potentially making them sick.

Builders and architects too, couldn’t always imagine what happens when they leave the scene. Water moves in on its own, without anyone causing it to. Is it time to act in prevention, renovation and remediation? Instrument advances have made us aware of what the two classes of particles found in air are composed of. Mould spores and other irritants are standing up to be counted. To document or not to document? Are we sure we want to see what’s there?