indoor humidity,

Dave Marshall-George of Condair looks at how businesses can adhere to guidance on indoor humidity levels post-lockdown

As part of their Covid-19 “return to work” guidance, many leading authorities in the air conditioning and building services sector have advised employers to maintain their workplace humidity at between 40-60%RH. Studies have shown that this level of humidity significantly improves our respiratory immune system’s defences, as well as minimising the time viruses remain airborne.

These authorities include the UK’s Chartered Institute for Building Services Engineers (CIBSE), as well as leading global building services consultancies, such as Cundall and Buro Happold. Internationally, organisations such as the Australian Institute of Refrigeration, Air Conditioning & Heating (AIRAH) and the Indian Society for Heating, Refrigerating & Air Conditioning Engineers (ISHRAE), are all advocating a minimum lower limit on indoor humidity of 40%RH to combat Covid-19, minimise viral infection and improve employee health.

The good news is that, as Europe progresses through summer, our indoor humidity levels will naturally be in this healthy range of 40-60%RH. Indoor humidity is largely dependent on the outdoor humidity and whether any indoor heating is drying the air. Typically, indoor humidity levels will only drop below 40%RH in the colder winter months, when the outdoor temperature is less than around 10°c and the indoor air heated to above 21°c.

So for any businesses returning to work over the summer months, their initial indoor humidity will be one that is positive for health. One less thing to worry about. However, come October, the indoor humidity across Europe will be falling into the dry danger zone of less than 40%RH. As we spend 90% of our time indoors, a low indoor humidity during the winter months is thought to be a significant causal factor in the seasonality of respiratory illnesses, such as influenza.

Maintain indoor air quality

In order to maintain indoor air quality, in line with the recommended level of 40-60%RH, any heated building must employ humidifiers during the winter. Depending on the complexity, it can take around two months to properly design, install and commission a building’s humidification system. So for businesses to be ready for the 2020 dry winter season, they must act now to ensure they have the humidification systems in place to continue to provide optimal conditions for employee health.

indoor humidity,If a building has a central air treatment system and air ducts to ventilate its rooms then a humidifier can be fitted into the central air handling unit. Air handling units will typically be designed to either have a humidifier incorporated from the start or be able to accommodate a retrofitted system.

For buildings without central air ducts and air handling units, humidifiers can be located directly in rooms. In-room humidifiers can be wall- or ceiling-mounted or even portable units, which can be removed during the warmer summer months.

Commercial humidifiers deliver moisture to an atmosphere by either boiling water to create steam, creating a fine mist that rapidly evaporates, or by directly evaporating moisture from a wetted surface. There are many types of commercial humidifiers that employ one or more of these three mechanisms, and which can be used for both in-duct and in-room humidification.

A humidifier’s output is controlled by a humidistat, in much the same way as a heating system is controlled by a thermostat. A sensor is located either in the air duct or the room, which monitors the humidity level and controls the output of the humidifier accordingly. This ensures that a building’s indoor humidity is maintained at a set level, without over or under humidification.

Humidifier strategy

Operators of large offices or buildings will typically employ a building services consultancy to give advice on what humidifier strategy is right for their building and its business needs. These consultancies will liaise with humidifier manufacturers and suppliers to select and size an appropriate system and installation strategy. Once approved, building services contractors can be employed to fit the humidification system, with either the humidifier supplier or the contractor undertaking the commissioning.

For smaller buildings, with less complex needs, business owners can liaise either directly with the contractor or the humidifier supplier to design and install the system.

Once installed, the humidifier is then operated and maintained by the on-site facilities management team. Like any building services system, all humidifiers will need occasional maintenance, and this can sometimes be outsourced to specialist companies, depending on the complexity.

Any building, no matter how big or small, can be humidified to an optimum level throughout the year. Building humidification is commonplace in the UK but has remained the poorer, less well known, cousin of heating and cooling. Building occupants will immediately recognise if their environment is too hot or too cold, but as humans have no sensory perception of humidity, this aspect of our indoor air quality is frequently not so closely monitored or managed.

From our experience of supplying humidifiers across the UK, I’d estimate that less than 5% of the UK’s built environment is humidified during the winter months. This is a shame, as the wealth of science showing that maintaining humidity at 40-60%RH is an effective infection control mechanism has been available for many years and continues to grow. What is also very disappointing is that humidification for health is used frequently in high-end offices, where there is great focus on minimising staff absenteeism, but isn’t employed by our health service to protect patients and our frontline care workers from respiratory viral infection.

Given the world’s attention on ensuring our working environments are as safe as possible in a post lockdown, Covid-19 world, and the swell in authoritative voices advocating the benefits of 40-60%RH, humidification is set to play its part in maintaining optimum indoor air quality and improving the nation’s health.


Dave Marshall-Georgeindoor humidity,

UK sales manager


+44 (0)1903 850200

Twitter: @JSHumidifiers

LinkedIn: Condair UK


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