In buildings, especially those intended for commercial use, to achieve the recommended indoor air quality levels, the inside air needs to be diluted with air-conditioned and filtered air from the outside, as it contains pollutants and bioeffluents.

Take, for example, a hospital in winter; for the sake of simplicity, only fresh outside air is used - air that is very cold and humid (point A, temperature = -5°C, relative humidity = 80%). This is then heated by a coil in one of the hospital’s air handling units, undergoing a transformation whereby its temperature increases and its specific humidity (absolute moisture content in a given quantity of air) remains constant. The new temperature-humidity conditions of the air flow are now represented by point B, i.e. temperature = 22°C and relative humidity = 12%.

As a consequence of heating the air, the relative humidity - which expresses the relative moisture content in reference to the maximum that the air can hold at a given temperature before the moisture condenses - drops dramatically. The relative humidity has in fact fallen from the initial 80% of the outside air to about 12% in indoor air conditions, however without removing any moisture!

This is because the air, as it is heated, has increased the amount of water droplets in suspension (humidity) it can now "support".

Heating fresh outside air plotted on the Carrier psychrometric chart

The air flow will thus be delivered into the building in the conditions described by point B and, when mixing with the air already present inside the rooms, will bring about a gradual lowering of the indoor relative humidity.

From this example it is clear that the need for air change to ensure adequate IAQ has a direct relationship with the need to control humidity in the same environment. Even with low air change rates, the indoor environment will tend to become dry, while if adopting a ventilation criterion based on IAQ requirements or, as explained in the previous chapters, in order to minimises the risk of spreading infections, the air will become even drier, therefore representing a risk not only for the comfort of occupants but also for their health.

Given the importance of maintaining an adequate relative humidity level, a humidification system is needed to bring relative humidity back into the correct range. The humidification process can be implemented using an adiabatic system (line 1), by spraying very fine droplets of water into the air, or using an isothermal system (line 2), by boiling water to produce steam, which is absorbed by the air.

Heating and humidification of fresh outside air plotted on the Carrier psychrometric chart

Regardless of the humidification technology used, the system will work mainly during the winter, when the heating system lowers the relative humidity and makes the air very dry.

In conclusion, heating cold, humid outdoor air to 22°C can slash relative humidity from about 80% to just 12%, without removing any moisture, leaving winter air uncomfortably dry. 

To preserve comfort and health (reducing mucosal dryness, irritation and pathogen transmission), a proper humidification system is essential alongside ventilation. Whether adiabatic (atomisation) or isothermal (steam injection), the right technology will restore RH to the ideal 40–60% range, creating healthier, more energy-efficient indoor environments.

The contents of this blog post regarding IAQ can be examined more in depth by reading the white paper
“Indoor air quality - Guaranteeing health and comfort in buildings”

Download the white paper