What is good indoor air quality?

What do we mean when we say that indoor air quality is good or bad?

It has been known for a long time that we have difficulties to express what we mean with good indoor environment. There are many parameters that affects when trying to express this.

Several of our senses give inputs to what we try to express. Meaning the sources that affects us are and can be light, draught, pollutions, temperature, humidity, emissions, etc…

Background

We know that in the same room, one person can feel comfortable and the other feel discomfort. This is due to each individual having our own ideal temperature.

Temperature is often the major reason to that we feel discomfort. In addition to the above mentioned factors also clothing and surface temperature can affect our experience.

There are several investigations and experiments done on the subject and it shows that no matter how good the environment is in the room, about 5% of the people will always be dissatisfied.
Below you can see the predicted percentage dissatisfied when temperature varies from average ideal temperature.



PPD (Predicted Percentage Dissatisfied) as a function of experienced thermal comfort PMV (Predicted Mean Vote).

How we perform

There is a big deviation in how we perform when we have an ideal temperature, and when it’s getting warmer or colder from the ideal point. This is individual as well as room temperature. This shows how important it is that we can control the temperature in the room.

Air Quality

Air quality is very difficult to measure. The most common way to do it is with CO2 concentration, but also VOC (volatile organic compound) measurements are being done in different ways. VOCs are organic chemicals that have a high vapor pressure at ordinary room temperature. Their high vapor pressure results from a low boiling point, which causes large numbers of molecules to evaporate or sublimate from the liquid or solid form of the compound and enter the surrounding air, a trait known as volatility. VOCs are numerous, varied, and ubiquitous. They include both human-made and naturally occurring chemical compounds. Most scents or odors are of VOCs. VOCs play an important role in communication between plants and messages from plants to animals.

Air Velocity

You often read the throw length defined from a diffuser at 0.2 m/s in terminal velocity. This is more a standard for manufactures to have as a common base for showing the result of a product. In the occupied zone (0.1 – 1.8 m above floor), the most common velocity requirements are 0.25 m/s in winter and 0.15 m/s in summer.
Here we also have two other factors called CLO (clothing) and MET (metabolism).

CLO is the insulation ability of clothes, at a given condition, to feel good comfort. In wintertime we normally set CLO-value to 1. In summer it is set to 0.8 and being naked means 0.
MET is depending on the activity level the person has and can be found in tables for different type of work or activities.

What we haven’t touched is the sound and light pollution or emissions in the daily environment affecting the person. What you can expect here is depending on the environment the person is in. If the person is for instance in a shopping mall he/she can probably accept a higher sound level than in one’s bed room. Range can be between 30 to 40 dB(A).

Humidity can have a major impact on occupant’s health if it is too low or too high. The “ideal” humidity conditions for humans can be considered between 40 - 60% (which equally depends on indoor temperature). Various geographic locations can have high or low humidity, which can be considered as comfortable or uncomfortable condition. In general if indoor air has a low humidity or “too dry”, this affects human mucous system negatively. Having too high humidity indoor can result in a risk of mold in the indoor environment.

If you have a chiller in your ventilation system, it will take care for some of the dehumidification of an outdoor air with high humidity.

Summary

To summarize what is good indoor climate, we have to consider all parameters that can affect our senses meaning air quality as well as thermal, acoustic and visual climate. All these together form an environment to work in and are all important to keep on the right level for creating a good climate. But maybe the most important issue that we are facing is to realize people’s different needs and what they feel is a good climate. To have an opportunity to influence your own condition in your room or part of room, is very essential.

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