Most people spend approximately 90% of their time indoors, and breathe 12,000 liters of air per day. It is therefore important to understand the powerful influence that indoor air quality has on our wellbeing.

The "sweet spot" for indoor air humidity is 40 to 60%, as this dramatically reduces the transmission risk of diseases, and enables the body to better repair and protect itself.

At air humidity below 40%, the water droplets with viruses become smaller and can travel through large open office spaces and survive for hours. Furthermore, many bacteria and viruses become more virulent when exposed to dry air. If the indoor relative humidity is above 60%, mold and mildew begin to form and these can pose health problems.

CO₂ concentration of more than 1000 ppm (parts per million) decreases the brain's ability to concentrate; starting at 2000 ppm and higher, it can lead to fatigue or even headaches. CO₂ level in indoor air is an excellent indicator of space usage and occupancy and therefore an indirect way of potential bio-contamination, such as COVID-19 viruses. If the CO₂ value is high due to increased occupancy and limited air exchange, there is a high potential risk from infectious aerosols.

As air circulation does not filter the air nor supply fresh air into the room, it is not a good solution either. On the contrary, strong unfiltered air movements further spread particles and pathogens within the space. A ventilation system with central air conditioning is needed to provide fresh, filtered, and conditioned air to the zones.

Air handling units usually supply air to several zones in a building. It is important that each room receives the exact amount of fresh air it needs. If the number of people in a room increases, e.g., in a larger meeting room, you would expect the air supply to increase accordingly. Similarly, the polluted air must also be removed from the room. To ensure this, zones and rooms must be supplied individually with variable air volume (VAV) based on air quality measurements.

The polluted and contaminated air must be removed from the room. It is important that each room receives the exact amount of fresh air it needs to prevent an increase in the concentration of infectious aerosols. To ensure this, zones and rooms must be supplied individually with variable air volume (VAV).

To prevent contaminants from entering indoor spaces through supply air ducts, suitable filters must be integrated into the air handling unit and must be replaced when the contamination of the filter increases. Pressure sensors and dynamic airflow measurements can be used to measure the degree of usage or contamination of the filters.

Central air handling units (AHUs) and air distribution systems can centrally control the quality of the supplied air at any time. Temperature, humidity, and air quality sensors permanently measure the conditions of the supplied air and deviations are instantly detected and corrected.

Indoor air quality can be measured either in the room or in the extract (return) air duct. Measuring the supply air doesn't reflect the indoor air quality situation in the room, but the fresh and conditioned air supplied by the air handling unit.

If the indoor air quality is measured in the room, attention must be paid to the mounting location to ensure proper and correct measurement. The installation should be at a height of around 1.5m (5 feet) and at least 50cm (20 inches) from the adjacent wall. It should not be exposed to direct sunlight nor mounted on external walls. Furthermore, avoid installations above radiators, directly adjacent to doors, in recesses (e.g. shelves) and behind curtains.

Alternatively, the measurement can take place in the duct which extracts the used air from the room. While the correct placement of a room sensor requires more attention, the advantage over a duct sensor is that, thanks to its exposure to the occupants, it allows direct, visual, thermal and indoor air quality feedback by means of a display or a traffic light indication.

In systems with variable air volumes for demand-controlled ventilation, the differential pressure fluctuates according to the airflow. In this situation, a pressure switch is not suitable for monitoring the filter status; a differential air pressure sensor is the solution. Air pressure switches are a cost-efficient and reliable way to monitor filters in systems with constant air volume flow.