Tips for preventing COVID-19 indoors

April 2020

In this post we provide some advice on how to prevent and reduce the spread of COVID-19 indoors. Recommendations to be applied not only during confinement at home, but especially in a few weeks' time when, if all goes well, we return to our usual workspace.

The first thought: the size of this virus (like many others) is between 0.07 and 0.16 µm, about 70 to 80 times smaller than an F8 or F9 ventilation filter. However, the coronavirus generally travels in so-called bioaerosols, small suspended particles of microbial, plant or animal origin that can spread through the air or remain deposited on a surface.

How to prevent COVID-19 indoors
COVID-19 propagation pathways. Source: REHVA

What precautions can we take?

Avoiding contagion by direct contact: We have heard this many times. Avoid direct contact with people who are not part of our confinement. Try not to touch surfaces that may have been exposed to COVID-19 and wash your hands frequently, always avoiding touching your eyes or mouth.

Avoiding airborne transmission: This is more complicated. Whether by talking, coughing, sneezing or even breathing, water droplets are generated which, if infected, can contain the virus. Larger droplets (>10 µm) do not reach further than 1-2 metres, so a safe distance is sufficient .

The problem is that droplets below 5 µm are lighter and remain suspended in the air, moving with the vortices and currents that form inside the space. Although there is no scientific evidence on the matter, logic tells us that this can also be a method of transmission and measures should be taken. Which ones? Good ventilation (a high priority these days) and a mask. There is much controversy about the mask, but the comparison of the evolution of the number of people affected between the Czech Republic and Portugal (with the same population and date of the first death from COVID-19) seems to endorse the positive effect of this preventive method (Portugal currently doubles the number of people affected in the Czech Republic).

Do temperature and humidity affect the spread of the virus?

According to the REHVA briefing note, COVID-19 is highly resistant to a wide range of temperatures and humidity, which makes it more dangerous than other viruses. It can withstand 14 days at 4°C (so watch out for refrigerated ones), one day at 37°C and 30 minutes at 56°C.

In our comfort range of 20-25°C and 40-60% relative humidity the virus is very stable and only becomes weak at humidities above 80%. Studies have shown that low relative humidity (below 40%) favours the spread of the virus through the air and reduces the resistance of our immune system, so maintaining a well-humidified environment will always reduce the consequences of possible infection.

What other sources of contagion can we avoid?

- Mechanical ventilation systems:
As mentioned above, filters cannot retain the virus (given their size), but they can reduce the number of virus-laden droplets in suspension. Generally the source of emission is going to be inside the building, so there are a number of issues to consider:

  • Avoid the operation of heat recovery units. Their operation usually helps to improve energy efficiency by pre-heating or pre-cooling the air entering the building, thanks to the thermal level of the outgoing air. Rotary recovery systems are obviously a source of contagion, contaminating the incoming air with the outgoing air. While this does not affect plate heat recovery units, it is not difficult for a micropore to affect the incoming air. For this reason, as long as there is a risk of propagation , it is advisable to bypass the recuperator, if possible, given that temperatures in April, May and June are not extreme.
  • Be careful with filter changes (especially the extraction filter). If COVID-19 has been emitted inside the building, it is certain that a bioburden has been retained in the exhaust filter, which must be removed with the utmost caution and ensuring safe conditions for both personnel and the environment where it is located.

Cross ventilation should be encouraged to ensure that the entire indoor air volume is completely swept. For more information you can consult previous monitoring experiences in our Case Studies section.

- Air conditioning systems:
All air conditioning systems with recirculating air can accumulate bioburden in the exchange coils. They should be avoided as far as possible and appropriate treatment should be considered to remove this load. It is highly recommended to keep these systems above 60°C for at least one hour and to clean with disinfectants that disinfect in a suitable way.


Filters (even HEPA type) will not be able to eliminate the virus but, as mentioned above, they can reduce the virus load in the environment by retaining particles.

There are indications, although not scientifically proven, that UV treatment systems eliminate or at least reduce the activity of the virus.

- Toilets:

It is very important to keep drain traps primed and to keep the toilet seat closed before flushing. Otherwise, small droplets in suspension are generated from the faecal water and can spread the virus. Of course, wash your hands and ensure good ventilation.

Since there is currently no vaccine or foolproof remedy to prevent this disease, the best way to prevent the coronavirus is not to expose oneself to it. Staying indoors is the best way to avoid the spread of the disease but, as we have seen, there are also risks of infection indoors. It is therefore advisable to take these precautionary measures into account, both in our homes and in our workplaces.

Share it on

Linkedin Twitter Facebook