Designing HVAC to resist harmful microorganisms

[edit] Introduction

The presence of airborne particles (such as dust), harmful pathogenic microorganisms (such as bacteria) and viruses (such as COVID-19 and other forms of influenza) is inevitable in occupied closed spaces/buildings. Heating, ventilation and air conditioning (HVAC) plays a vital role in making those buildings habitable. Other than providing comfort, it can also impact the wellbeing of occupants by maintaining a healthy indoor environment.

One of the benefits of a well-designed HVAC system is control over harmful pathogenic microorganisms. There are many methods building owners can put in place, including:

• Relative humidity control.
• Increased ventilation.
• High efficiency filters.
• UV lights in ducts.

It is possible to use these methods alone or in combination depending upon project requirements.

[edit] Relative humidity control

Relative humidity is the percentage of water in the air relative to the maximum amount the air can hold. But how does humidity control reduce the spread of viral infections?

The viruses responsible for the most common cold, influenza and COVID-19 spread more easily when the air is dry as viruses and airborne particles stay longer in the air when there is low humidity. For example, people in the Middle East spend most of their time indoors in closed spaces with the air conditioning working, which means viruses spread more easily.

A U.S. Centers for Disease Control and Prevention study states that “At humidity levels of 23%, 70% to 77% of the flu virus particles were still able to cause an infection an hour after the coughing simulation. But when humidity levels were raised to 43%, just 14% of the virus particles had the ability to infect. Most of the flu particles became inactive 15 minutes after they were released into the humid air. The virus just 'falls apart' at high humidity levels.”

Although raising the humidity of an entire building is a challenge, it could be beneficial in hospitals, clinics and the places where there is a high risk of flu transmission. However, the humidity levels should not be higher than 30% to 40%, because then moulds etc. may start to grow.

[edit] Increased ventilation

Ventilation is the process by which stale air is removed and clean air is provided to a space. This may be accomplished by either natural or mechanical means.

According to a statistics published by the Franunhofer Institute for Building Physics, people in industrialised countries spend 90% of their time indoors. As most air conditioning in those countries is of a recirculated type (with 20% fresh air addition only) to conserve energy, this results in an increased chance of airborne particles staying longer in the space.

Ventilation can reduce the concentration of airborne pathogens through removing or diluting airborne droplet nuclei. A higher ventilation rate can provide a higher dilution capability and may help to reduce the risk of airborne infections.

This can be addressed by introducing additional ventilation (~40% instead of 20%) for high-risk areas. This is best done at the design stage, as once the building is constructed, there is not much that can be done without breaking concrete and drilling holes in the walls and slab for duct installation.

[edit] High efficiency (HEPA) filters

HEPA filters are particulate absorbing filters. To qualify as a HEPA filter, the device must remove 99.95% (EU standard) and 99.97% (ASME, US DOE) of 0.3 micrometers sized debris from the air that passes through it. This value is used for filter classification, because it is close to the most penetrating particle size (MPPS) of approximately 0.2-0.3 μm (NASA). Particles that are larger or smaller than the MPPS are filtered more efficiently than the rated efficiency, although in some circumstances particles below the MPPS may act as nucleation sites and form particles close to the MPPS.

Since COVID-19 is 50 to 200 nanometre in size, it is smaller than the MPPS of HEPA filters, and thus the filter will be able to remove it with at least the rated efficiency. Contrary to common misconception, there is no need to produce special filters for blocking smaller viruses.

Note: HEPA filters are not designed to kill viruses and bacteria, which can stay in the filter as long as they survive, which can be up to nine days for COVID-19 in ideal conditions. However, COVID-19 survives best on hard surfaces and tends to die significantly faster on soft, porous materials such as filter media.

[edit] UV lights in AC ducts

Ultraviolet (UV) radiation is a type of energy produced by the sun and some artificial sources, such as black lights and UV lasers.

Introducing UV lamps in the return AC duct and near to the filter can have a significant effect on the indoor air quality, since it may help to kill harmful viruses and deter the growth of moulds inside ducts. Since lamps prevent the build-up of particles both in the system and in the ducts, they may also improve airflow and contribute to energy savings.

[edit] Conclusion

A new air detox mode can be introduced in the HVAC system which has implemented the above-mentioned considerations. The air detox mode could have the following sequence:

1. High humidity for 20 minutes (to kill the bacterial and viruses in the space).
2. Increased ventilation (to avoid mould making and regulate the previously increased humidity).
3. Intake fresh air (which is then recirculated and passed through specially design HEPA filters).
4. UV lights installed inside the air ducts, preferably close to air filters, to kill any residual bacteria and viruses.

This article was written by Muhammad is an MEP consultant, LEED Green Associate. He has experience in the design and development of commercial and residential buildings. He currently lives in Dubai and he loves to write about construction in general and MEP in particular.

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