Ventilation and aerosol transmission

Last updated: 15 Jan 2021

Why is ventilation important in reducing the risk of COVID transmission?

Coughing, sneezing, talking and breathing produce particles of saliva of various sizes. If a person has COVID-19, these particles will carry the virus.

The larger particles, which tend to be known as droplets, will fall to the ground or onto surfaces – the larger they are the more quickly they will fall. Physical distancing and cleaning surfaces will mitigate the risk of transmission via these larger particles.

Smaller particles, known as aerosols, are of concern because they can stay buoyant and are dispersed by the air, travelling further than larger particles. Transmission by aerosol can happen at distances beyond 2m in the same enclosed space, especially if the ventilation is poor and duration of exposure is sufficient.

A well-ventilated space reduces the concentration of viruses in the air and hence the probability of infection. Ventilation should therefore be integral to the COVID-19 risk mitigation strategy for multi-occupant workplaces.

How do you judge the risk?

The risk of aerosol transmission depends on the interaction of multiple factors, including how much of the virus someone with COVID-19 gives off into the environment, the rate of ventilation in a room, the duration of exposure and the number of occupants.

Providing the ventilation rate remains the same, increasing the number of people or the time spent in the environment increases the probability of airborne transmission.

When is the risk of aerosol transmission highest?

The risk is highest when people share poorly ventilated spaces. Aerosol particles can build up in these areas. These should be prioritised for increasing the ventilation rate as much as possible. There are some simple indicators of poorly ventilated areas:

  • Areas where there is no mechanical or natural ventilation, such as windows or vents that it is possible to open
  • Areas that feel stuffy, have stale air or smell badly (bear in mind though that recirculation units, including air conditioning, can mask poor ventilation by making an area feel more comfortable)
  • Areas with high levels of carbon dioxide, as indicated by CO2 Because ventilation dilutes CO2 exhaled by a room’s occupants, a high CO2 concentration indicates poor ventilation. According to CIBSE, indoor air at 600 to 800 parts per million (ppm) C02 indicates a relatively well-ventilated room. CO2 concentrations greater than 1,500 ppm are indicative of a very poorly ventilated space. However, in low occupancy or large volume spaces, a low level of CO2 cannot necessarily be used as an indicator that ventilation is sufficient to mitigate transmission risks.

I’m a rep – what should I do?

Ventilation is complex, and employers’ risk assessments are likely to draw on professional expertise. Ventilation systems and functionality can vary widely, and will often incorporate systems to heat and cool the building.

Employers should take note of CIBSE guidance

Assessing ventilation in many environments requires engineering expertise, and mitigation measures should be specific to the environment, taking into account the nature of the building and users, the ventilation type, length of exposure and activity.

While health and safety reps are unlikely to have experience of building ventilation, they do have experience of risk assessment. Focus engagement and challenge on risk assessment to avoid getting drawn into a technical field.

There are a number of straightforward things you can ask your employer to demonstrate, or to consider when making enquiries.

  1. What is the ventilation rate in the room? Current CIBSE and REHVA ventilation standards are that, in a room, there should be two air changes per hour or 10 litres per second per person (l/s pp). Those spaces which are occupied by more than one person and do not meet this standard should be have their ventilation upgraded where possible, or the use of the space restricted to a lower occupancy to reduce overall risk of transmission.
  2. In what areas is ventilation lowest? Ventilation at very low levels is inadequate and linked to super-spreader events. In offices and other workplaces, these levels of ventilation may be found in smaller rooms, or stairwells and enclosed corridors. Where there is low ventilation, mandatory facemask/covering is an additional control, as they will reduce the amount of aerosol and larger droplets produced when breathing and talking. Prospect recommends that reps seek to negotiate agreements with employers that staff should wear type IIR surgical facemasks when they are in communal areas, such as corridors, and when they are speaking to others. Consideration should be given to limiting access to rooms and areas where there is no direct supply of outside air, especially if they are likely to be occupied for considerable lengths of time.
  3. What is the occupancy of the area? Occupancy rates are key; air ventilation levels improve by reducing the occupancy levels. Halving the number of people in a room or area will double the per-person rate of ventilation.

Are there any other things to bear in mind?

  • Mechanical ventilation systems should be operated at the maximum design flow rate, even if a space has a lower occupancy than the maximum permitted. As set out in CIBSE guidance, it is recommended that ventilation systems are set to run on full fresh air, as far as possible.
  • Air naturally ventilated rooms (i.e. rooms ventilated passively with the flow of outdoor through openings such as louvers, doors and windows) as frequently as possible.
  • If the area is cold, employers should relax dress codes so people can wear extra layers and warmer clothing.
  • Extend the operating times of ventilation systems to before and after people use work areas
  • Recirculation of warm air is not recommended. Where centralised ventilation systems circulate air to different rooms, it is recommended that recirculation is turned off and a fresh air supply I used instead.
  • Fans should not be used in poorly ventilated spaces