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New World Health Organisation information emphasises effective ventilation in buildingsJuly 2020

The World Health Organisation has recently published information about the airborne transmission of COVID-19. This new information further emphasises the importance of having good, effective ventilation in a building.

Ensuring correct air change rates in an occupied space has always been an important design consideration and perhaps perceived as only being critical in healthcare settings.  Given our ‘new normal’, the emphasis on correct air change rates across all occupied settings is becoming crucial.

The air change rate (ACR) in a room is a function of the ventilation supply or extract flowrate (in l/s or m3/h) and a function of the volume of the room (m3).  The resultant air change rate, normally expressed as air changes per hour (ACH) (h-1), gives us an indication of how many times per hour the ventilation system provides the room with new fresh air.  

Ventilation is paramount to dilute the concentration of airborne contaminants and it is important to understand how contaminants are diluted, in order to establish procedures, i.e. time that has to pass between patients entering a room in a hospital.

By means of visualising the air change rate in a room, if we have an empty glass simulating our room, which is sitting under a tap acting as our ventilation system and the tap is turned on the equivalent air change rate is a count of how many times the glass can be filled with water  in a certain period of time.  If some or all of the water misses the glass and goes directly down the drain, the air change rate is effectively reduced. As in the case of a ventilation system, consideration would also need to be given as to what the consequences may be if the tap water were contaminated.

If the source of contaminants is still in the room, the ventilation system will eventually provide a steady state concentration of contaminants, which is diluted if compared to the source. If there are (airborne) contaminants in the room, but the source has left (e.g. the patient has left the surgery) the ventilation system will purge the contaminants with every air change.

The decay of the concentration can be expressed by the equation:

C = Cmax . (e -t/т)

C = [Cmax, 0] as t = [0, ∞)


  • t  is the time from the start of the rise or decay (s)
  • τ is known as the “half-life” or air change (s) 
  • C is the concentration at time t (ppm)
  • Cmax is the maximum concentration during rise or decay (ppm)

τ is calculated by:

т = V/Q


  • V is the volume of the room (m3)
  • Q is the ventilation rate (m3/s)

The initial concentration before the decay, is the maximum concentration Cmax.   When t=τ (half-life or 1 air change), then the concentration in the room has reduced to 0.37Cmax. and when t=3∙τ, then the concentration C= 0.05.Cmax. This means that after 3 half-lives, the concentration in the room has been reduced by 95%. Therefore in practice, 3 τ is considered to be the time to achieve steady state conditions, for a rise, or the time to clear the room, for a decay.

We do have to take into account the fact that the air supplied must be fresh, i.e. not recirculated and that the ventilation must be effective;  fresh air must not be short circuited into the extract and it must reach everywhere in the room, to avoid areas of high contaminant concentration (stagnation).

The mechanical air change rate provided by the ventilation system is not often the same as the effective air change rate.  The concept of a “mixing factor, k” can be used to describe the quality or effectiveness of the mixing process in a space.   Mathematically, Q'=k∙Q where k = 1 for ideal mixing and k = 0.5 for poor mixing.  The effect on decay levels is shown in the following equation:


    τ’ is the room’s “half-life” (s)

    V is the room’s volume (m3)

    Q is the mechanical, theoretical ventilation rate (m3/s)

    Q’ is the effective ventilation rate (m3/s)

Substituting the k value into the decay equation, the following expression is obtained:

C = Cmax . (e -t.k/т)

For example, for a k factor of 0.6,  when t=τ(half-life), then the concentration in the room has reduced to 0.55 Cmax, instead of the original 0.37Cmax and when t=3∙τ, then the concentration is still 0.17.Cmax, compared to 0.05 for k=1.

This means that it will take longer to purge the contaminant from the room i.e. If the ACR of an isolation room is 10 ACH, for a k value of 0.6, the effective ventilation rate would be 6 ACH.

BSRIA has studied several hospital isolation rooms, with dedicated ventilation. The k factor in those rooms, which were considered well-mixed, was k= 0.8, calculated using gas tracer methodologies.

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