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Exposing the silent killer; poor Indoor Air Quality.

As employees return to their offices, companies may unwittingly be increasing the risk of COVID-19 transmission by failing to make operational changes to improve indoor air quality.

Better ventilation systems and improved air quality will play an important role in preventing airborne infection and ensuring a safe and healthy working environment. Companies looking to reoccupy their buildings will need to review their ventilation systems to ascertain whether they need to be serviced or adjusted. Occupants need as much fresh air as possible, but it is paramount that it is not circulated from one occupied space to another, as this may increase the risk of transmission. This will become vital as we approach the colder winter months, which may potentially coincide with a second peak of infection.

Francesca Brady, Head of Environmental Research at AirRated, says, ‘While uncertainties remain regarding the relative contributions of the different routes of transmission, current evidence is sufficiently strong to warrant engineering controls targeting airborne transmission as part of an overall strategy to limit the risk of infection indoors. Appropriate building controls include effective ventilation, enhanced by particle filtration, avoiding the recirculation of air and avoiding overcrowding. The implementation of these strategies throughout buildings, in conjunction with other methods of mitigation, including social distancing and hand hygiene, will be an important measure to reduce the likelihood of transmission, thereby protecting occupants.’

‘Evidence is sufficiently strong to warrant engineering controls targeting airborne transmission as part of an overall strategy to limit the risk of infection indoors’ Francesca Brady, Head of Environmental Research at AirRated

The Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA), has released a guidance document calling for engineers to stop recirculating air in buildings within areas experiencing an outbreak of the virus.

As well as increasing ventilation and improving filter efficiency, HVAC systems need to be adjusted to ensure that temperature and humidity remain at optimal levels.

The Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA), has released a guidance document calling for engineers to stop recirculating air in buildings within areas experiencing an outbreak of the virus. SARS-CoV-2, the virus that causes COVID-19, is transferred through droplets of various sizes, which are expelled by coughing, sneezing, and perhaps even talking. Larger droplets tend to land on surfaces and do not usually travel very far, but smaller droplets, which usually occur in lower humidity, can remain in the air for longer periods of time and travel further distances, presenting a significant risk. Turning off systems that use local recirculation can therefore avoid resuspension of virus particles within your space, reducing the risk of transmission. As many of these systems are essential for temperature control, it is important to consult an HVAC engineer to see if it is practical to implement this strategy. 

Adam Taylor, IAQ and HVAC Technical Advisor, explains: ‘To make current buildings safer, where possible, diluting airborne contaminants by increasing ventilation rates should be the first step to reduce the risk of airborne transmission. Supplying as much outside air as is reasonably possible during mid-season should not incur a significant energy penalty, in peak summer and winter however, existing heating and cooling systems may not have the capacity. Where possible, opening windows is an effective way of boosting air exchange rates, although this is not possible in many modern spaces and the quality of air that will be introduced via the open windows and the draughts created need to be considered.’

Building owners are encouraged to improve the efficiency of the filters serving any critical re-circulating HVAC systems, to ensure that they can effectively remove the fine particles produced by respiratory aerosols. A key consideration for filtration systems is the size of particulate matter (PM), the microscopic aerosol particles made up of solids and liquids, that needs to be eliminated. 

‘Whilst these filters are better at removing particles with a diameter between 0.3 and 1 micrometres, they also require greater air pressures to drive air through the filter. It is therefore essential to ensure that the capacity of the HVAC system is sufficient to accommodate the better filters without adversely affecting the way in which the system functions.’ Adam Taylor, IAQ and HVAC Technical Advisor

The current minimum requirements for most commercial office buildings are simply not high enough to filter out the PM droplets containing the SARS-CoV-2 virus.

Companies that fail to make changes to improve indoor air quality within their offices not only risk contributing to a range of underlying health conditions, but may even be facilitating the spread of the virus amongst their workforce.

Mechanical and electric HVAC systems use filters that receive a ISO16890 classification based on several factors, including the size and amount of PM that is able to pass through them. In the US, MERV ratings are more common, with 16 representing the best, filtering out more than 75% of the smaller PM which range from 0.3 to 1 micrometres in size. Standard commercial office buildings are required to use filters with a MERV 6 rating, in accordance with ASHRAE’s standards, but at this level, smaller droplets under 3 micrometres can still pass through. Consequently, the current minimum requirements for most commercial office buildings are simply not high enough to filter out the PM droplets containing the SARS-CoV-2 virus. To begin filtering out such droplets, a MERV 10 rating or higher is required, although ASHRAE recommends a MERV rating of 13 or higher to be truly efficient at capturing airborne viruses.  

Taylor explains, ‘Whilst these filters are better at removing particles with a diameter between 0.3 and 1 micrometres, they also require greater air pressures to drive air through the filter. It is therefore essential to ensure that the capacity of the HVAC system is sufficient to accommodate the better filters without adversely affecting the way in which the system functions. Upgrading filters on outdoor air systems may have little practical effect on reducing transmission, although this study suggests that the virus can be present on air pollution particles. The use of additional air filters like HEPA (High Efficiency Particulate Air) units, Bi-Polar Ionisation or UV-C (ultraviolet) lights can also help to reduce the amount of infectious aerosols in the air, but these should be viewed as supplemental to increasing fresh air.

As well as increasing ventilation and improving filter efficiency, HVAC systems need to be adjusted to ensure that temperature and humidity remain at optimal levels. Humidity has a notable influence on the survival rate and transmission of biological pollutants, including pathogens. Like COVID-19, most infectious diseases are spread from human to human by droplet nuclei which are expelled into the air by coughing, sneezing and talking. A decrease in indoor relative humidity of about 20% makes it easier for airborne viral particles to travel. A study has shown that at a relative humidity above 80% or lower than 20%, most coronaviruses are still active after two days at a constant temperature of 20°C. However, at a constant temperature and relative humidity of 50%, less than 1% of viruses survive beyond 2 days. 

Brady explains, ‘Not only can humidity influence the survival of COVID-19, but research has also demonstrated that it can impact our body’s defence mechanisms against such diseases. Outside the cells lining our body’s airways, there are hair-like organelles known as cilia, which have decreased functionality in dry conditions, diminishing their ability to remove viral particles. Nasal mucus, another natural defence mechanism, is also compromised in low humidity, with its viscidity reducing by 50% when relative humidity drops from 100% to 60%. Low humidity levels can also lead to respiratory stress, skin and eye irritation and a range of symptoms that are associated with Sick Building Syndrome. Health issues such as these can put us in a more vulnerable position to fight off viruses such as COVID-19.’


Air quality in buildings has long been a cause for concern, but the COVID-19 pandemic has undoubtedly accentuated the need for healthier indoor air. Companies that fail to make changes to improve indoor air quality within their offices not only risk contributing to a range of underlying health conditions, but may even be facilitating the spread of the virus amongst their workforce.