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Clearing the air during the COVID crisis

As we brace ourselves for the COVID crisis becoming a long haul, the COVID-specific HSE guidance on air conditioning and ventilation offers only an overview, with the emphasis on increasing the supply of fresh air as a favoured response to the threat.

Few buildings can rely solely on opening doors and windows, and a warm British summer is short anyway. So, do you know how effective your mechanical ventilation systems are at changing old, potentially-contaminated air for new? Outside of critical facilities like healthcare or laboratories and production facilities and workshops, people tend to focus on the cleanliness rather than the actual efficiency of their ventilation systems.

If they are working properly, then the positive news is that compliance with existing guidance and standards should have your COVID-response covered. So, expenditure now on COVID prevention will, in fact, be a long-term investment in your compliance.

Now, while the building remains wholly or partially unoccupied, is the ideal opportunity to review the way your systems are set up and working, carry out essential inspection and cleaning works and ensure your compliance records are up to date. Ventilation specialists may also have more capacity to help now, after the initial focus on critical and healthcare premises.

If you do already have a compliant inspection and cleaning regime in place, it is likely that you are considering bringing the next scheduled clean forward. However, do take advice as changes in the way you use the systems may be more relevant for you in the short term.


HSE advises that the risk of air conditioning spreading coronavirus in the workplace is extremely low as long as there is an adequate supply of fresh air and ventilation. You can, it says, continue using most types of air conditioning system as normal. But, if you use a centralised ventilation system that removes and circulates air to different rooms it is recommended that you turn off recirculation and use a fresh air supply.

Ensure that you are operating HVAC plant to obtain maximum air change rates and that the system is balanced to achieve uniform air distribution and reduce dead spots.

Soiled ducting and air grilles in ventilation systems can infect the air circulating within the occupied area through direct contamination and/or reducing air circulation, possibly below critical air change rates. Visual inspection and professional cleaning will address these threats.

To give you an idea of why air change matters, look at the sort of numbers that high-risk healthcare premises work on. As an example for dental facilities, the fallow time following generation of aerosols during dental procedures to allow particles to settle, before work can even start on cleaning and decontamination, is one hour for a room with windows and two hours for one without. However, if 10 room air changes an hour take place, this can be reduced to 30 minutes (Reference US Centre for Disease Control, Atlanta, US).


We get asked about fan coil units a lot because they basically recirculate the same air and may send it to other spaces. This shouldn’t be an issue for systems serving individual rooms, assuming the units are cleaned in the normal, compliant way.

Their filters may harbour particles containing microorganisms. So, switching them on and off represents an increased danger because these organisms can multiply while the unit is off and then be emitted in relatively high concentrations when switched back on. So, either run them continuously (selecting lower speeds when the building is vacant) or, if there is no immediate need for heating and cooling, consider turning them right off and relying on natural air circulation.

If you are not sure that your system set-up is working properly, perhaps because it is older or you do not have audit records to refer to, now is a good time to check. Do get in touch. We can advise and help.

Posted: 16 July 2020

If you cannot have fresh air, use mechanical ventilation for clean air
Water and air hygiene  Services in southern and midlands England and Wales


Airmec Essential Services
Linden House
Almondsbury Business Centre
Bradley Stoke
BS32 4QH

Tel: 01454 275050
Company Registration No: 04711853


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