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What is humidity?

A summary of humidity in its different forms and its effect on the human body

Indoor Air Quality

At the height of the summer months, it is not uncommon to hear people say, "the humidity is very high!" referring to a certain level of discomfort in addition to heat. This is a widespread occurrence in coastal regions. Despite being a commonly used term, few have a clear and concise understanding of humidity and the many forms in which it takes.

Classes of humidity

In short, humidity refers to water vapour suspended in the air. In the HVAC industry, there are three separate humidity measurements and terminologies to express different concepts, which are summarised below:

Absolute humidity

denotes the mass of water vapour per unit volume of air. It is measured with the aid of devices called hygrometers. Absolute humidity is a value that changes with climate and geography. It decreases as one moves from the equator to the poles and from coastal regions to inland and highland areas.

Specific humidity

is a simple ratio of the mass of water vapour to the total mass of air. It is generally used to indicate how many grams of water vapour is contained in 1 kg of air. In the psychrometric chart shown below, the figures on the left indicate specific humidity.

Relative humidity

is a ratio, expressed in per cent, of the amount of atmospheric moisture present relative to the amount that the air could hold at a temperature. 100% relative humidity means that the air is fully saturated with water and cannot have any more water vapour, meaning any more water vapour added to the air condenses into a liquid form or is suspended in the air in the form of fog. Based on this, it can also be said that the odds of rainfall is higher when relative humidity is high. The value that is sometimes expressed as humidity in weather forecasts is relative humidity. In contrast with specific and absolute humidity, people can sense relative humidity, which directly influences their comfort.

Psychrometric chart

In the psychrometric chart above, the figures on the bottom represent dry bulb temperature, while the figures on the left represent specific humidity and the lines relative humidity. As demonstrated in the chart, keeping specific humidity (the mass of water vapour in the air) constant will lead to a drop in relative humidity and an increase in temperature. As mentioned above, the amount of vapour that can be held by air increases with increasing temperature.

The figure above also shows the same quantity of water vapour produces different relative humidity values within the same room at varying temperatures.


100% relative humidity


52% relative humidity


28% relative humidity


The effect of humidity on the human body

Both high and low humidity have an unfavourable effect on human wellbeing. It is widely accepted that ideal conditions for people's health and comfort feature temperatures between 20-25 degrees C and relative humidity between 40-60%— conditions outside this range would be harmful.

The human body is capable of reflexive heat rejection through sweating as the temperature rises. Sweat expelled from the pores in the body evaporates and mixes with the air. It pulls heat from the human body as it evaporates, cooling us down. During the summer months, especially in coastal areas, since relative humidity is high, the air already holds enough water vapour that it becomes challenging to hold anymore. The sweat issued by the human body to cool down evaporates more slowly than is needed or does not evaporate at all. The heat that cannot be expelled increases the temperature sensed by the human body.

Aside from contributing to general discomfort, high humidity is known to trigger illnesses such as asthma, heart failure, loss of sodium, among other ailments. It is important to remember that low humidity also has unfavourable effects on human life. The low moisture levels held by the air, specific humidity, and cold weather can lead to dry atmospheric conditions. Dry air is known to cause dryness in the skin, respiratory illnesses, nosebleeds, and many other conditions.

Be aware

Given this, Systemair urges everyone to be aware of humidity control and its critical role in creating a healthy and comfortable indoor climate.