Last reviewed 3 August 2020

Airborne transmission of the Covid-19 virus is revealed to be more of a threat than initially thought, highlighting the need for effective ventilation and air conditioning systems in our workplaces. Vicky Powell examines the latest specialist advice on how to mitigate the threat of the airborne coronavirus.

The science of airborne viral particles

In July 2020, the World Health Organization (WHO) updated its advice on controlling the spread of Covid-19 by announcing that there had been some outbreaks in indoor crowded spaces, such as restaurants, fitness classes and even a choir practice in Washington in the USA.

Analysis of these outbreaks had suggested an important role being played by airborne viral particles — or aerosol transmission, where very small droplets called aerosols are generated — and that this method of transmission should be considered as a health risk along with transmission of the virus by means of larger droplets.

In its statement, the WHO said, “In these events, short-range aerosol transmission, particularly in specific indoor locations such as crowded and inadequately ventilated spaces over a prolonged period of time with infected persons cannot be ruled out.”

Clearly, understanding exactly how Covid-19 is being spread is critical if we are to prevent infection and break the chains of transmission.

According to the WHO, the current evidence suggests that transmission of Covid-19 is mainly through contact with infected people via saliva and respiratory secretions or through respiratory droplets, which are expelled when an infected person coughs, sneezes, talks or sings.

Respiratory droplets from infected individuals can also land on objects, creating “fomites” (contaminated surfaces) and it’s likely that the virus can also be transmitted by people touching these surfaces and then touching their eyes, nose or mouth before cleaning their hands.

However, in the face of the latest evidence, the WHO has noted that airborne transmission of the virus cannot be ruled out, and it is this possibility, of aerosol transmission combined with droplet transmission, which is currently under the spotlight.

Following an open letter from international scientists about the potential risks posed by the airborne Covid-19 virus, the WHO has indicated that it plans to review its advice on the subject. The UN agency has also acknowledged that urgent high-quality research is needed to better understand this transmission route.

In the interim, however, it falls on employers to take the practical steps necessary to protect their staff and the public from these invisible threats, particularly as more confined indoor spaces such as restaurants and gyms begin to reopen.

UK air quality expertise

Fortunately, the UK’s indoor air quality (IAQ) expert community has for some time been ahead of the game in terms of the airborne risks posed by the coronavirus.

The Building Engineering Services Association (BESA) recently said the WHO’s updated stance supports the position taken by many IAQ experts, that facilities managers should be revisiting their ventilation strategies to minimise possible transmission risks as more buildings reopen following the lockdown period.

Nathan Wood, chair of BESA’s Health & Wellbeing in Buildings group, said, “We have been arguing since the early days of the pandemic that ventilation can play a crucial role in reducing the threat of virus transmission.

“We should also consider the wider deployment of air purification technologies, which are now being used more regularly in healthcare settings.”

Explaining the significance of the recent WHO statement, a source at BESA noted that the WHO’s original position was that Covid-19 did not remain in the air but was transmitted via droplets emitted when people cough or sneeze falling onto surfaces. This is why handwashing was seen as such a key prevention measure (and, to be clear, remains as essential as ever).

However, the new evidence suggests the virus can also remain suspended in the air for some hours. Essentially, the larger droplets will fall by gravity, which influences the 2m social distancing measures to reduce spread. However, fine aerosols can remain airborne for several hours and this could require additional tactics for reducing transmission rates.

Nathan Wood said, “Not only do we have this significant admission from the WHO, but many internal spaces have been changed to allow for social distancing. Putting up partitions, for example, will change the pattern of air distribution.”

Clearly, it is not only restaurants and gyms which are relevant here but also premises such as offices and schools.

In fact, schools have become a key area of concern, according to BESA. A survey of 20 UK classrooms carried out by National Air Quality Testing Services (NAQTS) revealed very low air change rates in many that could increase the risk of virus transmission.

Douglas Booker, Chief Executive of the NAQTS, said, “Our study showed that some classrooms had air change rates below 0.5 per hour and that even small increases in flow rates could significantly reduce the risk of infection. Raising airflows from zero to 100 cubic metres per hour cuts the risk by up to 60%. However, there are gradually diminishing rates of return from increasing it beyond that level.”

During a recent webinar hosted by BESA, Douglas Booker also emphasised it was important to focus on what was “reasonably achievable”, particularly in settings such as education where budgets are constrained.

The good news is that low cost adjustments to ventilation are a good investment according to the UK’s air quality experts.

Mr Booker said, “This crisis is an opportunity to improve the IAQ performance of buildings for the long term, but we need to focus on affordability. There is a lot of money going into shiny new academies, but that risks increasing inequality even further by leaving older school buildings behind.”

He also cautioned against the use of “increased amounts of harsh chemicals” to clean indoor spaces as these could create “another type of indoor pollution”.

He said, “We must not over-react to this crisis by over-sanitising our indoor environments. Simply ramping up cleaning procedures could create unintended consequences that do not address the full range of infection risks. Don’t throw the baby out with the bathwater.”

Expert advice on ventilation

The Chartered Institution of Building Services Engineers (CIBSE) has recently published a suite of free specialist guidance to help employers and their building managers as the UK moves out of lockdown. The guides are continually being updated with the latest changes and evidence.

However, perhaps the most key piece of advice for managers and employers is to understand your workplace’s ventilation or air conditioning system, consulting the manufacturer’s guidance as needed.

It may also be necessary to ask advice from your heating, ventilation and air conditioning (HVAC) engineer or other indoor air quality advisor.

The general CIBSE guide entitled Safely Re-occupying Buildings advises employers to ensure high levels of ventilation in all occupied areas of the building.

It says, “To minimise the risk of airborne transmission, it is important to maintain higher ventilation rates and to consider increasing ventilation rates in toilets and circulation spaces such as stairwells.”

However, the other useful publication entitled Covid-19 Ventilation Guidance gives far more detailed guidance for building managers and operators to minimise the risks of airborne transmission of Covid-19.

The overarching advice is to increase the air supply and ventilation, supplying as much outside air to dilute and remove the virus as far is reasonably possible.

Here the guidance emphasises the importance of understanding your particular type of air conditioning or ventilation system. For example, some air conditioners take in air from outdoors and expel it again, while others, called split units, recirculate the same air.

Therefore, a split air system in an office may be good at warming or cooling the indoor workspace but does not provide any outside air into the room. Without a dedicated source of outside, these systems could be responsible for recirculating and spreading airborne viral particles into the sphere of socially distanced office workers, for example.

Consequently, it may be best to turn the system off or use it only with open windows, even if this makes the room hotter or cooler than is ideal and is more expensive. (The argument is that the benefit to staff and public health currently outweighs the reduction in energy efficiency of an open window.) Where a window can’t be opened, experts advise it may be better to turn the split unit off altogether. Fresh air is key for a healthier workspace.

Advice from CIBSE states, “It is recommended that any ventilation or air conditioning system that normally runs with a recirculation mode should now be set up to run on full outside air where this is possible.”

Advice from the regulator

Following the WHO change in stance, the Health and Safety Executive (HSE) has recently updated its guidance on air conditioning and ventilation at work.

The regulator has updated its original statement, which stated that the risk of transmission of the coronavirus through ceiling and desk fans and air conditioning was “extremely low”.

The guidance now states that the risk of spreading Covid-19 in the workplace through air conditioning, ceiling and desk fans is “extremely low” as long as there is an adequate supply of fresh air and ventilation. In line with the advice from other health experts, the importance of a fresh air supply is emphasised for the workplace.

Asked for comment on the issue for this feature, an HSE spokesperson said, “Employers are required to ensure an adequate supply of fresh air through natural or mechanical ventilation, and this has not changed. Improving ventilation may help reduce the risk of transmission of coronavirus in poorly ventilated areas.”

The following key areas of focus from the HSE may be useful.

  • In the case of ventilation, all workplaces need an adequate supply of fresh air, which can be achieved through natural ventilation (through doors, windows, vents) or by controlled means such as by a powered fan.

  • In some cases, general ventilation can be improved by opening doors, etc, but HSE is not proposing to issue additional guidance on the subject. Those in control of premises retain a legal duty (see the guidance on the Workplace (Health, Safety and Welfare) Regulations 1992 — regulation 6 in particular) to ensure effective ventilation.

  • For fully mechanical centralised systems, which both deliver air to and extract air from multiple rooms, it is best practice to avoid recirculation of air where possible. All centralised mechanical ventilation systems should have the facility to turn off recirculation and use only a fresh air supply.

  • For mechanical systems in individual rooms (or a portable unit), where recirculation modes enable higher rates of supply of fresh air to be provided to a space, then these devices should be allowed to operate.

  • Most air conditioning systems do not need adjustment; however, where systems serve multiple buildings, or you are unsure, advice should be sought from your HVAC engineers or advisors.

The HSE guidance is based on the latest information and will likely be updated as and when new evidence about the virus becomes available.

Healthier indoor air at work

Poorly ventilated workspaces that lead to poor indoor air quality are not healthy for anyone and, in time of the coronavirus, they could be life threatening.

Therefore, the growing awareness of the risks posed by poor indoor air quality and airborne pathogens such as the coronavirus inside buildings should, in the long run, be welcomed.

As Douglas Booker, Chief Executive of the NAQTS, recently said during a BESA webinar, “If a pandemic that forces people to spend almost all of their time indoors does not change attitudes to indoor air quality, then nothing will.”