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Why does humidity matter?

Simply increasing ventilation rates to reduce indoor transmission of airborne viruses may not deliver the right result.

The Covid-19 crisis has thrust the work of building services engineers into the public eye and placed the effectiveness of ventilation systems, in particular, under close scrutiny. However, while the general public may now have a better idea of why controlled, mechanical ventilation is important to maintain healthy indoor conditions, the focus on increasing air change rates has rather over-simplified the message.

Faced with rising rates of indoor transmission, the government and most industry bodies issued advice focused primarily on increasing ventilation rates to achieve minimum air changes of eight litres per second per person. Keeping CO2 concentrations below 800 parts per million was also recommended as an indicator of ventilation effectiveness.

However, the importance of controlling relative humidity (RH) was somewhat overlooked despite it having been recognised as a key factor in achieving healthy indoor environments for many years. Fortunately, several leading healthcare experts have now started speaking up about this oversight.

Dr Stephanie Taylor from Harvard Medical School believes control of RH is key to tackling Covid-19 and in preparing buildings for future health challenges. She told last year’s BESA National Conference that managing the indoor environment was “the best medicine for treatment and prevention” and building professionals were, therefore, the “physicians of the future”.

The ASHRAE Distinguished Lecturer said numerous studies had identified an RH ‘sweet spot’ between 40% and 60% adding that air which was too dry would allow viruses to thrive and be more active.


She added that RH should not be sacrificed to reduce energy use. “We need to get that balance right.”

The body of scientists advising the UK government during the pandemic has also made the link between RH and the welfare of building occupants. The Scientific Advisory Group for Emergencies (SAGE) said mitigating the risk of viral transmission through the air is not simply a case of increasing ventilation rates.

Cath Noakes, Professor of Environmental Engineering for Buildings at the University of Leeds, one of two engineer members of SAGE, confirmed that studies had shown the risk was higher indoors when ventilation provided less than 3 l/s per person and that household transmission was a particular concern. However, she also said the virus thrived in “cool, dry and dark conditions”.

However, current building standards still focus on a simple linear relationship between the outdoor temperature and indoor comfort temperature and there is no mandatory legal requirement to control RH. One positive outcome from the current crisis would be a long overdue change in this area because recommending high ventilation rates without counterbalancing that with RH control can lead to very dry indoor environments, which could have the perverse effect of creating conditions that actually help viruses thrive.

In the cold, dry winters of northern Europe, indoor RH-levels can drop to 5-15% for extended periods, which is great for viruses, but not so good for people. If the humidity is too low, small particles are also more likely to be inhaled deep into lungs and infectious particles survive longer. If the RH remains below 30%, the skin becomes dry leading to itching, cracking and chapping.

On the other hand if the humidity is too high, it encourages mould growth and bacteria that can cause respiratory problems and/or allergic reactions, including increasing the risk of asthma attacks. Very moist air will make people feel chilled in cold weather and hot and sticky in summer.

Humidity also affects the performance of buildings, the durability of building envelope materials and the longevity of equipment. Basically, we need to control humidity so levels are not too low, and not too high.

The moisture holding capacity of air increases dramatically with increases in temperature so, counter-intuitive though it might sound, it is often more economic to raise the air temperature because the increased moisture transport capacity will outweigh the increased energy consumption.

We should also start to consider humidity as an asset and recover it in the same way we do thermal energy to help maintain healthy conditions without paying an energy penalty.

Achieving the right RH for each space will depend on finding a balance that brings together all aspects of HVAC systems – including passive and active ventilation – to deliver healthy air change rates alongside the right temperature balance. It is a complex area, but today’s system designers have all the tools at their disposal – they just have to make the right calculations.

For an in-depth analysis of the humidity issues related to buildings and ways to deliver ideal RH download our humidity white paper at here.