Covid19 – Finding a good air quality solution

March 22, 2021 – 7 Min

The WHO and governments worldwide have recommended good indoor ventilation ever since they recognised that Covid19 was predominantly transmitted via airborne particulates and that the transmission risk from fomites (surfaces) was low.

This article looks at the principal factors that need to be considered when assessing what ventilation or air purification is required in order to develop an effective Covid19 airborne transmission risk mitigation strategy. It concludes that the following key issues need to be considered:

  • The occupancy of an enclosed space.
  • The size and environmental conditions of the room.
  • The types of breathing and other behavioural factors of the room’s occupants.
  • The Healthcare sector has much higher air purification requirements, leading to different measurements. 


A previous article discussed the different methods of Covid19 transmission, referencing key scientific research that demonstrated the main routes of transmission were from directly inhaling both large droplets and smaller airborne aerosols. It concluded with strategies to limit transmission, including wearing masks and maintaining safe distances, as well as ventilating rooms with fresh outside air.  And, when those solutions were not possible, to purify the air by mechanical means with high quality filters, all of which will eliminate the build-up of droplets and aerosols within the room.

There are many risk factors to consider when trying to mitigate the risks of Covid19 airborne transmission:

  • Enclosure: higher transmission risk when indoors, increasing with poor ventilation.
  • Crowding: more people within a confined space means greater probability of a virus host being present. More people results in more aerosol generation, an important factor in small poorly ventilated spaces.
  • Environmental Conditions:  cool, dry, dark conditions increase virus survivability rates.
  • Symptoms: it is harder to detect asymptomatic infectious people – they may not have a cough or fever.
  • Proximity: infection risk increases the shorter the distance between people or when individuals are face to face in the absence of basic PPE (masks).
  • Duration: infection risk increases the longer someone is in close proximity to an infected person.
  • Activity: speaking, loud speaking, singing, aerobic activity all increase individuals’ breathing rates and thereby heighten risk of aerosolized viral emission.


The recently updated UK HSE website reflects new guidance to combat the risks of Covid19 aerosol transmission, recommending to assess ventilation and improve fresh air ventilation where it is sub-standard, and if that is not possible, to implement mechanical air purification units (but only those using HEPA and UVC technologies). This is very similar to the recommendations made by the SAGE Committee in their November 2020 report. 

Key Ventilation Issues. As shown by the risk factors above, the key issues to consider in terms of ventilation are:

  • The number of people in an indoor space. 
  • The air flow from existing ventilation systems.
  • The size and environmental conditions of the indoor space. 

The other risk factors (Symptoms, Proximity, Duration and Activity) are all behavioural issues addressed by conduct policies and practices (PPE deployment, masks, social distancing in spaces etc).

UK Governmental guidance (i.) stipulates that good ventilation is achieved by providing 10 litres of fresh air per second per person (l/s/p) in a room, which is equivalent to 36 cubic meters per person per hour (ii.)

In view of the Covid19 pandemic, ASHRAE, the American Society of Heating, Refrigerating and Airconditioning Engineers, states that facilities of all types should follow the minimum outdoor airflow rate of (iii.):

  • 3.8 l/s/p for most retail buildings
  • 10 l/s/p for gyms, health clubs, aerobics rooms & weight rooms
  • 3.8–5 l/s/p for educational buildings 

Room Size as a Determinant of Infection Probability. Figure 1 below shows the aerosol concentration in two very different sized rooms with different occupancies levels and varying degrees of ventilation: 1, 5 and 10 l/s/p respectively). As can be seen, when there is a good amount of air ventilation (10 l/s/p), the result is (iv.):

  • a significant reduction in particle concentration in the smaller Office.
  • no discernible difference to already low concentration in the larger Sports Hall. 

This makes intuitive sense given that there is only 15m3 of space per person in the small 20 person Office, versus the much greater ~55m3 of air per person in the larger 160 person Sports Hall. Thus, the first factor to assess is the number of people occupying a room, and whether the 10l/s/p air ventilation is being achieved or not. 


Various studies show that the number of aerosols emitted changes with different human behaviour and activity (v. & vi.). Figures 2 and 3 show how breathing, singing, talking and coughing activities lead to changing amounts of aerosolized particle emissions.  What stands out from the data below is, firstly, the wide range of results from each activity and secondly but no less importantly, the exponential increase of some activities compared to others (note the log scales). 

But the type of emission (breathing, singing etc) is only one behavioural factor to consider – the other is its duration. Whilst coughing once causes a large exhalation of aerosols, exercise can be ongoing for a long period of time, so the total amount of aerosols emitted from exercising will far exceed a few coughs. 

Exercise causes both deep and rapid inhalation. A research study has indicated that deep exhalation could yield a 4x to 6x increase in aerosolized particle emission compared to normal breathing while rapid inhalation a further 2x to 3x increase in emission, yielding a maximum 18x increase (vii.). Therefore, concerns about high aerosol particle concentrations within indoor sports centres, fitness centres and gyms are justified.

As mentioned previously, ASHRAE recommend 10 l/s/p for indoor sports facilities, 2.6x the ventilation recommended for retail spaces, which is similar to Dutch state guidelines, which recommends 11.1 l/s/p, 3.2x that recommended in other buildings (viii.).

In conclusion, human behaviour is a key factor to consider within a confined space, bearing in mind that in some countries there is a requirement that ventilation within indoor sports facilities should be a 2.5x – 3.2x multiple of places where normal breathing might occur. 


When the WHO finally recognised airborne transmission as the greatest risk of spreading Covid19 at the last quarter of 2020, they recommended that indoor ventilation should achieve at least 6 “Air Changes per Hour” (ix.). Requiring a certain level of Air Changes per Hour (ACH) is an alternative way to recommend an adequate amount of ventilation. 

The calculation of a room’s ACH involves three factors:

  • the amount of air that is introduced into or purified in a room (in m3) 
  • the time of the ventilation purification (60 minutes)
  • the overall volume of the room (in m3)

In the example above, purifying 300m3 of air for one hour in a 100m3 room would result in delivering 3 ACH for that room. 

An ACH standard might seem a vague recommendation as it does not consider the room’s occupancy level or the breathing behaviour of the people in the room. However, if a very large amount of ventilation/purification is required in small rooms, independent of the occupancy, then ACH is an easier standard to set. Indeed, the term ACH is used in the Healthcare sector where high ACH rates are recommended to prevent infection to immunocompromised individuals recovering from illnesses, operations or those with chronic illnesses (e.g. pulmonary diseases). ACH guidance is also used where Aerosol Generating Procedures (AGPS) are performed, e.g. by Dentists, whereby large amounts of aerosols are released into the air, increasing the risk of infection.

Public Health England guidance states that ‘two air changes are pragmatic’ for hospital wards, but it does not state whether there is evidence this is sufficient to reduce risk. Intensive Care Units might have up to 12 ACH, similar to that of Dentists, whilst a hospital operating theatre will have 25ACH (x.). To put this extremely high amount of ventilation into perspective, 25 ACH in a 270m3 operating theatre with 10 people present would achieve the equivalent of 375 litres of air per second per person. 

Given the need to implement extreme high levels of ventilation and air purification, it is understandable why ACH standards are used to determine ventilation and purification rates in the health sector.


Rensair determines the best air purification solutions for their clients who have poorly ventilated facilities by taking into account their particular premises, occupancy rates, room measurements and use case. Each and every solution: 

  • is uniquely designed to meet every client’s air purification requirement 
  • is developed using Rensair’s extensive knowledge of the latest government regulations and scientific research
  • uses the experience gained from working with both the Healthcare sector and a wide range of global businesses
  • uses extremely high-quality HEPA and UVC technology in a patented hospital-grade portable air purification unit whose independent laboratory test results demonstrate their efficacy in cleaning the air independent of where placed in a room. 
  • contact Rensair should you wish to find out more or determine a solution for your business, or
  • book a Consultation with one of Rensair’s Experts where we can discuss your air purification requirements at a time that suits you – this can be done either via a video or phone call. 
  1. CIBSE Guide B Heating, Ventilating, Air Conditioning and Refrigeration (2016)
  2. 10 litres per second = 36 cubic metres per hour [(10 * 60 * 60)/1000]
  3. ASHRAE: ventilation for acceptable indoor air quality ANSI/ASHRAE Standard 62 (2019)
  4. Prof C. Noakes, Hazards Forum Seminar (17/03/2021)
  5. Comparing the Respirable Aerosol Concentrations – Gregson et al.
  6. The effect of respiratory activity on total aerosol emissions – Wilson & al.
  7. The mechanism of breath aerosol formation – G.R. Johnson and L. Morawska
  8. Ventilation and air cleaning to limit aerosol particle concentrations in a gym during the COVID-19 pandemic. – Blocken et al.
  9. WHO’s Science in 5 on COVID-19 – Ventilation – 30 October 2020
  10. Personal protective equipment during the COVID – Cook

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