Face masks, face screens, and breathing

There’s no more controversial issue in the prevention of Covid-19 than face masks. Unfortunately, as is so often the case now, a great deal of nonsense and non-science has been spread, and has been passed around as dogma. This article looks at one often-neglected area: face masks, screens, and the mechanics of breathing. Can a face mask cause difficulty breathing? Do masks result in the build-up of carbon dioxide? And why isn’t there good research into face masks?

Dead space

The respiratory system has to bring in fresh air, with around 20% oxygen and no carbon dioxide, into the small alveoli deep in the lungs, and remove that air when the local blood supply has extracted oxygen from it, and put out carbon dioxide for removal. That’s what breathing does: every few seconds, your chest expels much of that ‘old’ air from the alveoli, then brings in fresh air to replace it.

The alveoli are deep in the lung tissues. The way in and out for air is at the mouth and nose, with one tree of airways to move air both in and out. This means that not all of the ‘old’ air is expelled from the lungs; some is left in the airways, to mix with incoming fresh air in the next breath. The volume in the airways which doesn’t perform gas exchange with the blood, but which still contains air, is the dead space (those with prior knowledge will be aware of different types of dead space, and how to measure them).

We function well with natural dead space, typically around 150 mL. Increase the dead space significantly and it has effects on the efficiency of breathing. If you were to breathe normally (at rest) with the added dead space of a wide bore plastic or cardboard tube of around 1-2 feet (30-60 cm) length, you’d see the effects. At the start, you’d be breathing at rest, slowly and relatively infrequently. Over time your rate and depth of breathing will steadily increase because the tube increases your respiratory dead space. The air inside that tube, and in your airways, becomes richer in carbon dioxide and lower in oxygen. The body’s control systems respond to that by increasing the frequency and depth of breathing.

The conclusion is that we mustn’t do anything to increase dead space significantly, or it will cause problems with breathing.


From the alveoli up to the nose and mouth, the airways get steadily larger until they reach free air. This minimises the resistance to moving air through those tubes. If you suffer from asthma, you’ll have episodes when smaller airways narrow and make it hard for you to breathe. During an asthma attack, it becomes a real effort to move the air that you need, because of this increased respiratory resistance.

The same happens when the resistance is applied to the inlet and outlet by breathing through a dense filter, such as that in an N95 mask. If you were to hold two layers of that material against your mouth, you’d notice how much harder it is to breathe through those filters. Doing this for hours at rest is hard enough, but when you have to work physically hard, increased respiratory resistance becomes tiring. The muscles which perform breathing become fatigued, and you feel shattered.

The conclusion is that we mustn’t do anything to increase respiratory resistance significantly, or breathing will fatigue.

What face masks do

Face masks, as opposed to full-face respirators and face screens, are designed to fit closely over the nose and mouth, to provide a physical barrier between your respiratory system and the air around you. Because they fit closely, they don’t increase dead space significantly. Clinical or fabric face masks are usually quite thin and aren’t fine filters. Because they aren’t fitted to seal against the face, air also moves in and out around their edges. Although they can increase resistance, for someone with a healthy respiratory system that shouldn’t prove fatiguing.

Those face masks should, if kept clean and dry, act as effective barriers to the passage of droplets containing virus outward from the wearer to anyone close by, and inward from someone close to the wearer to the wearer themselves. Their respiratory cost for doing so is very small, insignificant for most healthy people.

The snag with those clinical masks is that they aren’t effective filters for smaller particles, particularly those in aerosols, in either direction. To protect the wearer and those close to them from exposure to aerosols, you need a much finer filter which is sealed against the face, like N95 masks or, even better, full-face respirators.

Both N95 masks and full-face respirators achieve better protection at cost to your breathing. They both increase dead space within the mask and increase resistance because you can’t breathe except through their filter system. As a result, performing physical work while wearing them is more fatiguing, as you’re likely to be breathing more (to compensate for the increased dead space), and because it takes more effort to move the air through their filters.

Full-face respirators must be carefully designed to ensure that added dead space is kept to a minimum. Putting the inlet and outlet high up by the eyes, for example, increases dead space much more than putting them close to the mouth and nose.

One simple indication that a sealed mask or full-face respirator has maintained an effective seal is that, following wear for any length of time, that seal leaves marks around the face – as you may have seen on ICU staff when they’ve just completed a shift.

The very best protection from droplets, aerosols and even airborne chemicals (and radioactive material) is provided by a powered external filter and sealed respirator system, sometimes known as a ‘whistling handbag’, worn by fast jet pilots in the military. An equivalent system used in heavily contaminated areas is an airflow respirator or suit.

These have large filters which can remove aerosols effectively, fans to draw the air through those filters and blow the air inside the respirator. By blowing clean air in they have no added dead space or resistance. As the air inside the respirator is at a slightly higher pressure than ambient, they only leak clean air out, and don’t let contaminated air in. They’re also incredibly expensive and require a lot of routine maintenance – but that’s the price of performance.

What face screens do

Face screens appear far more basic, but in many respects can perform as well as more basic face masks. They place a completely impermeable but transparent barrier between the wearer’s airways and eyes and anyone who gets too close. They prevent passage of droplets as well as or better than basic face masks (they’re completely impermeable, and when wetted the droplets either dry in place or run down). They provide essentially no barrier to aerosols, though.

Well-designed face screens are wide open at the sides, by the ears, and below the jawline. They may impose a small increase in effective dead space, which shouldn’t affect the wearer, but don’t increase respiratory resistance at all. They also don’t interact with eyewear, and provide droplet protection to the eyes, which ordinary face masks can’t.

The problem with cloth face masks

It’s incredibly frustrating that the type of face mask most commonly worn by the public has been the subject of such little research. There are good and careful studies of the potential benefits of clinical masks, including ‘regular’ and N95 types, but next to nothing on cloth masks.

One of the biggest problems here is that there is no standard for cloth face masks. Look around at those worn by others and you’ll see some made carefully from what appears to be close-woven fabric, fitted well to the face. Others are no more than an old garment or scarf wrapped roughly around the right area. Clinical masks are tested and manufactured to comply with one of a range of standards which assess their ability to protect without adverse effects on the wearer. They are also single-use, whilst cloth masks are normally washed and worn again many times, during which their fabrics will almost invariably deteriorate in performance.

I have seen cloth face masks which appear to be superior in design and performance to approved clinical masks, and all too many which are tokens rather than protection. Because most recommendations, rules and laws specify the wear of a ‘face covering’, people also use the wrong types of mask, such as those designed to protect the wearer from dust when working with wood, etc.

There is no evidence that all of those face masks will provide better protection than not wearing a mask at all. Sadly, some can’t and won’t. Most masks intended to protect the wearer from external dust have an exhalation valve which concentrates outlet air, including any aerosol, and isn’t even designed to block the emission of larger droplets. A loose woollen scarf may not trap many droplets containing virus. Requiring people just to wear ‘face coverings’ helps no one, least of all the wearer. As sadly too many care sector workers have discovered, there are many occasions when a properly worn, standard-compliant clinical mask hasn’t protected them, and too many tragic deaths as a result.

But until people understand face masks, screens, breathing and protection this will just remain a mess of disinformation and misunderstanding. It’s also an invaluable surrogate for politicians: maintaining isolation and distancing has high economic costs; shifting responsibility onto people to choose and use protective masks instead costs the state almost nothing, and puts the blame onto the wearer if they still get or spread Covid-19. The moral is surely that, outside of care settings, you should maintain full distancing and hygiene regardless, and that a mask can then only augment your protection. The moment that you rely on your mask as sole protection you have lost control of the risk.