Air Humidity and Indoor Air Quality

As an introduction let’s take a look at a commonly used design tool: the Psychometric chart

From USD College Auditorum website

I know it looks really complicated and we are not gonna explain everything about this graph. I’m using it to show two main points:

  •  You can clearly see that there is a given comfort zone in bright green
  •  And this comfort zone is characterized by the dry bulb temperature and the air humidity

For a room to be considered as comfortable for people, it must have a relative humidity in a specific range (here betweeen 40% and 60%).

The requirements  can be set according to the international standard ISO-177772.

The standard categorizes environmental parameters to “high”, “medium”, “moderate”, and “low”, the application of each category is the following:

  • High (Category I): should be selected for occupants with special needs (children, elderly, handicapped)
  •  Medium (Category II): normal level used for design and operation (typically used)
  • Moderate (Category III): can still provide an acceptable environment with some risk of reduced performance of occupants

Low (Category IV): should only be used for a short time of the year or in spaces with a very short time of occupancy

From ISO_17772

We’ve seen that the relative humidity (air humidity) is one of the key factors for the comfort of a building, but “why ?” you may asked, and I’ll try to give you the reasons.

But first let’s give a a clear definition of the relative humidity, according to

Relative humidity is a way of describing how much humidity is present in the air, compared to how much there could be. Meteorologists often use the relative humidity as a measurement to describe the weather at various places. When the temperature is warm, more water vapor can be in the air than when it is cold.

Now that we got that out of the way, let’s start by what happens when you have low humidity in a room.

Low relative humidity

It affects us in two disctinct categories of air quality: the comfort and our health.

As for the comfort, low humidity causes dry nose, mouth, eyes and skin, wich have a huge impact on people comfort and will lead to a lot of complaints from the users.

Concerning health, low humidity causes tissue lining nasal passages to dry, crack and become more susceptible to penetration of Rhinovirus cold viruses and influenza, it is a common cause of nosebleeds, and may also aggravate allergies in some individuals.

This is due to a smaller decay rate of virus and the enhanced suspension of the infectious particles in dry air.

The following figure shows the influence of the telative humidity on the decay rate of different virus.

Without going in much details about decay rate, let’s just simply explain the meaning of the figure:

  • A higher decay rate means that the virus dies faster
  • Hence the higher it is, the better it is for our health

As you can see, high relative humidity leads also to a decrease of the decay rate.

High relative humidity

Fisrtly, we just saw that there was an impact on decay rate: a decrease meaning a more infectious air.

High humidity reduces the evaporating cooling rate and also encourages the formation of sweat wich is widly considered as discomfortable.

And the most known of all impacts: mold growth !

Which is not only damaging the building but is a real health hazard.

MOLD can cause health problems that range from itching eyes, sneezing and coughing to serious allergic reactions, asthma attacks and even permanent lung damage.

From New York Times


I hope that you understand clearly now the link between Air Humidity and Indoor Air Quality (being the comfort and health impact) and that you will be able to prevent those impacts in the future, for you or the people you live with.