How improving indoor air quality is one of the best ways to limit virus transmission

Good indoor air quality – and by that I mean proper indoor ventilation that exhausts stale air from a building and replaces it with fresh air from outside – is one of the best ways to prevent the transmission of influenza viruses such as SARS and Corona, according to medical studies, because it dilutes the amount of airborne virus particles in the air.

I start by underlining that this article has a number of medical journals as its basis, and I caveat right away that I’m not a medical professional myself.

Good indoor ventilation and its link to preventing virus transmission

Research following the last worldwide influenza epidemic (SARS) demonstrated that good indoor ventilation is better at preventing you catching a virus than wearing face masks (which are single use and therefore environmentally unsustainable), because fresh air dilutes the virus in indoor environments.

The research also showed that maintaining high levels of indoor ventilation is even better at preventing virus transmission than washing your hands more than ten times a day (although it’s still good to do this, of course).

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How to protect yourself against airborne viral infections like Corona

A British Medical Journal study from the SARS virus in 2003 measured the effectiveness of hand washing, mask wearing and N95 masks.

Hand washing ten times a day had an Odds Ratio (OR) of 0.45, meaning that (in simplistic terms), whatever the transmission risk, it approximately halved it.

Face masks were better at preventing transmission, at 0.32 OR, and N95 masks, which nurses and doctors use, had an OR of 0.09, so a one in ten transmission risk.

The same data is shown below from a research document called “Common Cold“.

Another medical report (Room ventilation and the risk of airborne infection transmission in 3 health care settings within a large teaching hospital – behind a medical paywall) stated that if you spent 15 minutes in a room with a patient who had influenza, you would have a 30 per cent of catching the virus if the room had a typical ventilation rate (o.3 air changes per hour, like a well-ventilated British home).

This aligns with on-the-ground data, which showed that nurses wearing face masks treating SARS patients in Canada back in 2003 had an infection rate of 8 in 32.

If you wear an N95 mask, this risk of transmission goes down to 10 per cent, as above.

But crucially, when doctors increased the ventilation of the room significantly (4 air changes per hour), they lowered the infection risk to 3.6 per cent – better than wearing a mask or washing hands, by far.

When they increased the ventilation to 10 air changes per hour, the infection risk dropped to almost zero.

Increase the ventilation in a room and the transmission risk drops

The conclusion from the study was that the faster you exchange indoor air with outdoor air in a room, the lower the infection risk becomes. Basically, you dilute the virus in the air and that makes it more difficult to infect you.

Or put another way, if you spent all of your time outdoors, an influenza sufferer would have to literally sneeze into your face for you to catch it. The airborne virus is too diluted otherwise. As such, you don’t need to wear a mask if you’re walking around outside, as the risk is almost zero anyway. Plus, there’s now a global shortage of face masks, which means hospitals (which actually need them to prevent transmitting diseases and bacteria to sick patients) can’t get any.

The real problem with any virus is that in our daily winter lives we pack ourselves into tubes, buses, busy offices, coffee shops and pubs, with the windows firmly shut against the cold outdoor air.

That’s why social distancing is working.

Viruses spread indoors in poorly ventilated rooms

Viruses spread more easily in winter because we spend 90 per cent or more of our lives indoors when it’s very cold. We worsen the transmission risk by sitting in poorly ventilated rooms, with the windows and trickle vents firmly closed against the cold. A virus flourishes in warm, humid air, and we get sick.

The medical community has known for thousands of years that one of the best environments to improve the health of a sick patient is one with lots of space, clean, fresh air and sunlight, such was the design philosophy behind the ancient and modern sanitariums.

The air change rate of a room or building is directly tied to the transmission rate of airborne respiratory diseases.

Better ventilation lowers the risk of viruses spreading. This isn’t fear-mongering, it’s the basics of healthy buildings.

How can you promote good indoor air quality to prevent Corona transmission risk?

The easiest way to increase ventilation would be to open all of the windows, but this isn’t comfortable for most – particularly in winter. However, putting on an extra jumper and opening the window a little is the best way to get the air changes needed to dilute the virus.

Staying off buses and tubes on the daily commute is another way to reduce risk – perhaps by working from home, cycling or walking where possible. I’ve written before about how our offices and homes are being built and maintained in such a way as to make us ill.

It’s often been said that the facilities manager of your office building has more of an influence on your health than your doctor or GP – that’s more true than ever today.

Designing a home or office with health in mind

The best way to design a building with good indoor air quality and health in mind would be to consider mechanical ventilation with heat recovery (MVHR), as this will increase indoor ventilation rates without compromising warmth, and the windows can still be kept shut whilst maintaining good base ventilation.

MVHR systems are also better at controlling the humidity of the building, to keep it within the 40 – 60 per cent Relative Humidity range that suppresses viruses, allergies and respiratory illnesses.


A Passivhaus ventilation system is designed to an air change rate of 0.5 when it’s in boost mode, but many ventilation systems have enough capacity to double this to 1 air change per hour without too much trouble.

Buildings with good quality ventilation systems would help significantly in the fight against viruses, creating a warm but freshly ventilated indoor environment for occupants of homes, offices, schools and hospitals everywhere – and that can only be a good thing.



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