The battle with Covid-19 has dramatically increased our motivation to manage the indoor environment.

Words Michelle Ludwig

A recipe for

“Once we have an effective vaccine for Covid-19, will we forget about the importance of indoor air quality (IAQ) in buildings for protecting us from infectious respiratory viruses?” asks Dr Stephanie Taylor, a practising physician who obtained a Masters in Architecture and now employs both disciplines as an infection control specialist. While protocols of social distancing and mask-wearing will likely diminish over time, other illnesses are clearly related to the indoor environment. “The understanding that indoor air should be managed to reduce diseases and actually improve our health is an attitude I hope is here to stay,” she adds.


Heating, ventilation and air conditioning (HVAC) systems play a key role in managing indoor air and therefore significantly contribute to healthiness – or not – of indoor conditioned spaces. “HVAC systems can help contain the spread of a contaminant but if not carefully designed, and carefully commissioned and maintained, they can worsen the scenario,” cautions Andre Harms, founder of Ecolution Consulting and senior mechanical engineer. As an example, he references a study analysing a super-spreader event from a restaurant in China, where a single infected restaurant patron further contaminated many others, exacerbated by the air conditioning system. When the contamination impacts were tracked and modelled, it indicated that the aircon system played an integral role in increasing the spread since it was a system that recirculated the air but supplied little to no fresh air or filtration. (please see below)

Simulated dispersion of fine droplets exhaled from index Patient A1 (magenta-blue), which are initially confined within the cloud envelope due to the zoned air-conditioning arrangement. The fine droplets eventually disperse into the other zones via air exchange and are eventually removed via the restroom exhaust fan. Other infected patients are shown in red and other non-infected in gold color. Only a single human body is used to represent all patrons.

HVAC systems can help contain the spread of a contaminant but if not carefully designed, and carefully commissioned and maintained, they can worsen the scenario.

Evolving HVAC

Controlling contaminants and improving air quality in general works best as a multi-layered approach. Mechanical and technological approaches involve applying layers of interventions which include dilution, filtration, disinfection, humidification, supplementation, and proper maintenance. Used in conjunction, these strategies create a recipe for success and healthy indoor air quality.


Fresh air quantities combined with filtration can reduce airborne exposures. Air stagnation may concentrate airborne viruses or dust, so it is critical to keep indoor air as refreshed as possible. Research has shown that increased ventilation in a building can reduce the chance of influenza – a study published in 2019 found that ensuring even minimum levels of outdoor air ventilation reduced influenza transmission as much as having 50-60% of the people in a building vaccinated.

In addition to avoiding things that make us sick, health-related impacts of providing fresh air have also been shown to include significant improvements in cognitive function such as information usage, strategy, and safety decisions. This is why most green building certification schemes, such as Green Star, promote and recognise increased ventilation rates.
Richard Humphrey, the senior mechanical engineer at HFW Consulting, however, asks: “Why should increased fresh air be reserved just for green buildings?” When applying many building standards, keep in mind that they are minimums. Rather consult guidelines such as ASHRAE Indoor Air Quality Guide: Best Practice for Design, Construction and Commissioning, suggests Humphrey. He points out that the South African National Building Regulations standard level for ventilation is outdated, and calculated on densities that are not as typical today and much lower than many best practice codes.


Providing higher rates of fresh air dilutes the concentration of any pollutants or airborne contaminants, thereby reducing exposure. Beyond just a safety intervention, provision of more fresh clean air also improves many human body functions.

Installing a higher quality air filtration system has rapidly increasing benefits to society-at-large, which culminates in quantifiable financial impacts.
Top Left: Respiratory aerosol properties, Top Right: Respiratory aerosol dynamics, Bottom: Financial implications of modifications to building filtration system.


More fresh air is a start, but the next level is to ensure that air is clean. The quality of the air we breathe determines the health of the lungs as well as other organs. Dust, pollen, tobacco, and mould in polluted air, have been linked to increases in death rates. Ventilation systems typically have some sort of filtration for particulate matter (PM), however filters come in varying degrees of filtration and typically rated with a Minimum Efficiency Reporting Value (MERV). MERV ratings are on a scale of one to 20 depending on the minimum size of the particles a filter can be expected to capture. Low-efficiency filters have been typically used to keep lint and dust from clogging the heating and cooling coils of an HVAC system. Medium- and high-efficiency filters are required to address smaller particles such as pollen and bacteria.

Studies of improved filtration levels have demonstrated net benefits – all the financial benefits to society – benefits to the healthcare system, benefits to employers and individuals – across cities with widely varying outdoor air quality, Harms explains. “Installing a higher quality air filtration system has rapidly increasing benefits to society-at-large,” which culminates in quantifiable financial impacts. The study further summarises: The monetised health benefits of filter installations outweigh the operation costs by up to a factor of 10 – the net benefits of improved filtration were greatest for the highest efficiency filters.


The average office is not designed to a very high IAQ compared to other typologies such as a hospital. And the industry is seeing trends towards less expensive, thus simpler, HVAC systems such as split cassette systems, explains Edward Hector, founder and CEO of HVAC systems services company SFI Group. Average filters target particulate matter sized at 10 microns but this is not sufficient for microscopic contaminants such as pathogens.

To target smaller particles, Hector recommends also employing UV-C lights that emit part of the UV range of light useful for germicidal irradiation. “The UV-C light inactivates viral, bacterial, and fungal material, disrupting the functioning and ability to replicate.” HVAC systems can be designed with UV-C lights within that disinfect the air as it passes through, as well as assist with keeping the cooling coils clean from biofilm buildup. These can be retrofitted to existing systems in many cases, Hector points out, but in all cases needs to be carefully designed and calibrated.


Damp and humid air leading to condensation and mould is well understood, however, studies show to air that is too dry is also detrimental to health. In repeated studies of hospital-acquired infection (HAI) rates, many factors of transmission were investigated and relative humidity (RH) was found to be the most influential factor. There is also evidence that humidity can play a role in the survival of viruses such as Covid-19. According to Taylor: “If the relative humidity is below 20%, the average person becomes clinically dehydrated and the human body suffers from its systems being compromised. Bad pathogens love it.” She highlights indoor air hydration – a mid-range humidity of 40-60% RH – is a powerful medicine that is essential for buildings that protect our health.


Supplementing existing HVAC systems with interventions and various technologies can also be effective, especially where retrofitting is limited. For example, when looking at the typical aerosol spread inside a conditioned office environment, using local air purifiers can control the spread and reduce concentrations by more than 50%. So, should there be an infected individual, this would greatly reduce the risk to others.
Viral load is a very important factor in whether people get infected. Harms explains: “Just because one is in a space that might have some infected aerosol does not necessarily mean one will automatically get infected. It depends upon the intensity or the concentration, and duration, therefore these types of mitigation measures become important really quickly [to reduce occupants’ exposure].”


Keeping systems running optimally is key to healthy indoor air quality. After efforts of designing, procuring, and calibrating an effective HVAC system, it can all be undermined if it is not looked after and operated appropriately. “HVAC design can only contribute to wellness in buildings if correctly implemented and operated as ensured by robust commissioning processes,” Harms points out. After initial commissioning, HVAC systems need to have ongoing periodic maintenance to check on things such as the cleanliness of the coils and ducts, the filters changed out per manufacturer recommendations, and monitoring of effective performance.

After Covid-19

Before airborne pathogens entered our headlines, and attention spans, indoor air quality have been considered of paramount importance for health. A silver lining of the Covid-19 experience may be the additional focus directed towards creating and staying healthy in indoor spaces, not just for green buildings but for all. Mechanical ventilation systems can be a detriment if implemented and operated poorly. Alternatively, with consideration of employing multiple strategies, good design and operation, HVAC systems can be highly useful in keeping us healthy, safe and productive for the long term.

1 Yuguo Li, et al. Evidence for probable aerosol transmission of SARS-CoV-2 in a poorly-ventilated restaurant, medRxiv 2020.04.16.20067728; doi:
2 Smieszek T, Lazzari G, Salathé M. Assessing the Dynamics and Control of Droplet- and Aerosol-Transmitted Influenza Using an Indoor Positioning System. Scientific Reports. 2019/02/18 2019;9(1):2185. doi:10.1038/s41598- 019-38825-y
3 “Associations of Cognitive Function Scores with Carbon Dioxide, Ventilation, and Volatile Organic Compound Exposures in Office Workers: A Controlled Exposure Study of Green and Conventional Office Environments,” Joseph G. Allen, Piers MacNaughton, Usha Satish, Suresh Santanam, Jose Vallarino, John D. Spengler, Environmental Health Perspectives, October 26, 2015, doi: 10.1289/ehp.1510037
4 Montgomery, James & Reynolds, Conor & Rogak, Steven & Green, Sheldon. (2015). Financial implications of modifications to building filtration systems. Building and Environment. 85. 10.1016/j.buildenv.2014.11.005.
5 Kim SW, Ramakrishnan MA, Raynor PC, Goyal SM. Effects of humidity and other factors on the generation and sampling of a coronavirus aerosol. Aerobiologia. 2007/12/01 2007;23(4):239-248. doi:10.1007/s10453-007- 9068-9
6 2020. ASHRAE presentation: Bemis P. Applied Math Modelling