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The three keywords for ventilation systems

Health, efficiency and comfort.

The three keywords for ventilation systems.

Today, people spend up to 90% of their time indoors. Our offices and production facilities must meet the requirements necessary to create safe, healthy and productive environments.

The goals of health, energy efficiency and comfort combine to form a new model for the control and management of building ventilation systems.

The supply of fresh air in indoor spaces plays a crucial role in people's health. Not only the right temperature and cleanliness are essential, but also the humidity of the air we breathe.

Moisture and the spread of disease

Most modern hospitals, unlike offices, have implemented very strict cleaning procedures for surfaces and equipment. Nevertheless, air analysis shows that even this is not enough to change the number of bacteria in circulation, which is concluded from the fact that the number of related infections continues to rise. Thus, a large amount of microbes is continuously emitted by people just by talking, breathing or coughing. These microbes are contained in thousands of tiny water droplets floating in the air. Some reports estimate that between 10 and 33% of all pathogens that cause infections are transmitted through the air.

When these droplets enter a room with a relative humidity below 40%, they quickly lose up to 90% of their volume. They quickly shrink to a diameter of 5 µm and manage to hover for a long time, traveling considerable distance in the process. Naturally, this increases the chances of them reaching a new host, rehydrating and infecting that person. However, if the room has a relative humidity between 40-60%, the droplets maintain approximately the same size (≈ 100 µm) and tend to settle much faster, approximately within 1-2 m of the source, where they can be removed much more effectively by traditional cleaning methods.

Indeed, several studies indicate how relative humidity is the most important factor in environmental control in airborne transmission of bacteria and viruses.

In a first study conducted over one year in an American hospital, all environmental parameters in ten rooms were monitored, as well as the medical condition of the patients staying in these rooms. Relative humidity was the variable most closely associated with the number of infections contracted by the patients.

Another study was conducted with dolls, with people with influenza coughing and breathing two meters away from a doll. Aerosol samples were collected near the mouth of the doll at various times and of these, infectivity was assessed, yielding the following results.

The infectivity dropped from 80 to 20% once the relative humidity was between 40 and 70%! This is because the virus can no longer remain in the air for long periods of time in this relative humidity range and thus the life span of these much airborne bacteria and viruses is shortened.

What also plays a role in the transmission of viruses and bacteria is how the human body reacts to low relative humidity.

Influence of relative humidity on our natural defenses

Airborne infections are acquired by breathing in viruses and bacteria, which enter the airways and deposit on the walls or in the alveoli of the lungs, causing pneumonia or blood infections.

Our body's last defense against this aggression consists of the walls of our airways: from the nose to the esophagus and bronchi, a thin layer of mucus is secreted that moistens and warms the incoming air and traps the bacteria and viruses. This layer of mucus is continuously pushed up by the rhythmic movement of epithelial cells with cilia on the walls of the airways. The mucus then reaches the oral cavity and the bacteria are swallowed and rendered harmless by stomach acid. In this way, they become part of the microbiome of the intestines without causing harm.

This process called mucociliary clearance is essential to our health. Under normal conditions, it blocks most of the pathogens we breathe in, but its action is greatly affected by relative humidity. If relative humidity falls below 40%, the mucus layer dries out and the cilia slow down until they stop, allowing viruses and bacteria to enter and infect cells.

Sick building syndrome

Sick Building Syndrome (SBS) affects those in a building who experience health complaints and long-term discomfort that cannot be linked to specific causes or diseases. Often the causes of this condition can be linked to defects or misuse of the air handling system, lack of adequate fresh air, volatile organic compounds (VOC), mold and other materials and substances that release pollutants.

Testing has shown that SBS symptoms are also caused or exacerbated by low relative humidity. These symptoms include deterioration of tear fluid, dryness of the eyes, irritation of the nose and throat, asthma, dry skin, headaches, fatigue and irritability. Low humidity conditions can cause a 3-7% reduction in the performance of individuals performing office tasks such as reading, document editing and arithmetic. When factors such as high temperature and air pollution are added, these symptoms worsen.The optimal relative humidity was calculated at 40% to 60%, which coincides with the level that must be obtained to reduce the risk of infections (40% < rv < 60%) and overlaps with the levels needed to reduce the risk of electrostatic discharge (rv > 35%) and spread of mold (rv < 80%).

If we consider all the effects that relative humidity has, on the human body and keeping buildings livable, we can say that the desired range for this purpose and for the benefit of health and comfort in general, is between 40 and 60%.

Given the desired conditions, an obvious question arises. Are these conditions being met, and are there factors that could change these conditions, making constant climate control necessary?

Florence Nightingale (1820 – 1910)

Ventilation and hygiene are the best ways to prevent infections.

She already knew it but in addition to sufficient fresh air and hygiene, we now know the influence of relative humidity on the longevity and spread of viruses and bacteria and our natural immune system. So we can answer the question with a resounding YES. Unfortunately, the change in the law on January 1, 2007 meant that the law no longer sets requirements for indoor climate comfort. The law limits itself to the requirement that the temperature must not be harmful to health. Whereas initially the energy consumption of a building was considered more important than the well-being or health of the building's occupants, now, as a result of the pandemic, it is hoped that health will take center stage, but with the most energy-efficient solution possible. Incidentally, every employer, including the government, is obliged to provide a healthy and safe working environment for its staff.  But a healthy working environment is determined not only by temperature but also by sufficient fresh air and proper relative humidity. For all these conditions STULZ can provide a solution. Air handling units from STULZ for ventilation, heat pumps from Mitsubishi Heavy Industries for cooling and heating and humidifiers from CAREL for relative humidity. We also offer 24/7 service for all our products so you are never in a cold or dry environment.

Bert Leffers

Humidification specialist at STULZ GROUP B.V. Author of several white papers and articles on adiabatic cooling, humidification and humidity.