Condensation on the window of the Apollo 12 Lunar Module, November 1969

Condensation on the window of the Apollo 12 Lunar Module, November 1969 (NASA, Flickr)

Humidity on Earth and on the ISS

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Humidity is an important part of life on Earth and life in space. This backgrounder explains what humidity is, how it is measured and how it affects us both mentally and physically.

Humidity on Earth

Most of the air around you has water in it. But the water is invisible. So how do we know it is there? We can feel it! Humidity is a measurement of how much water vapour is in the air. Water vapour is water in its gas state. We can see the water in its solid state (ice) and its liquid state. But we cannot see water vapour. 

Misconception Alert

Many people think steam coming out of a kettle is water vapour. They’re wrong! Steam is actually tiny droplets of condensed liquid water. 

There is always water vapour in the air, even when the sky is clear and blue! 

Hygrometer (Source: bong hyunjung via iStockphoto).

How do we measure humidity?

Humidity is measured using a device called a hygrometer (high-grow-meter). The most common measurement of humidity is called relative humidity (RH). Relative humidity is the ratio of water vapor in the air to the maximum amount of water vapor that the air can hold at a specific temperature.

Relative humidity measurements are given as a percentage (%). For example, imagine the air can potentially hold 50g of water vapour at a certain temperature. However, there is only 25 g of water vapour in the air. In that case, the relative humidity is 25 g/50 g or 50% RH.

High humidity is over 50% RH. Low humidity is under 30% RH.

What role does water vapour play in the water cycle?

Water vapour plays a big role in the water cycle. Even in a clear, blue sky, there is still water vapour in the air. When water vapour in the air changes to liquid water, it’s called condensation. A cloud is an example of condensation. When a cloud can no longer hold all of its condensed liquid water, precipitation occurs. 

The opposite of condensation is evaporation. Evaporation occurs when water changes from a liquid to a vapour. Heat is what makes evaporation happen. That is why it often seems that warmer air can "hold" more water. In reality, when the air temperature is higher, it is more likely that water will evaporate and become vapour. This is why warm air feels like it is more humid than cold air. 

How does humidity affect our physical and mental health?

People tend to be very sensitive to humidity. We sweat to keep our bodies from overheating. When we sweat, water from the sweat evaporates from our skin. As it evaporates, the water transfers heat away from our skin. The result? Our skin feels cooler.

When the relative humidity is high, we do not cool down very well. That is because there is less evaporation from our skin. At times like these, it is very important to stay hydrated. If your body cannot sweat to cool itself down in high temperatures, being well-hydrated will help prevent your body from overheating. Overheating can make you feel sick. High humidity can also cause microorganisms like bacteria and mold (a type of fungus) to multiply and spread.

During the winter months, when the air is cooler, the relative humidity can go way down. Very low relative humidity can lead to dry, itchy skin, lips and hair, scratchy throats and noses, and long-lasting colds and flu.

So, what is the optimal or ‘best’ range for relative humidity in a classroom? It depends on the season. In the winter months, the optimal relative humidity for an indoor environment is around 30% RH. For the spring and summer months, the optimal relative humidity for an indoor environment is between 30% and 50% RH.

Humidity on the International Space Station

The International Space Station (ISS) is a closed ecological system. All of the water on board the ISS was originally brought up from Earth. On the ISS, almost all of the humidity in the air comes from the astronauts. The humidity comes either from respiration (breathing) or from perspiration (sweating). Controlling the humidity on the ISS is important to protect the astronauts’ health, the equipment on board the ISS, and the ISS itself.

How is humidity controlled on board the ISS?

Humidity on the ISS is measured and monitored by a special subsystem of the Environmental Control and Life Support System (ECLSS). This subsystem is called the Temperature and Humidity Control (THC) subsystem.

Water Reclamation Subsystem
The Water Reclamation Subsystem for the International Space Station (Source: NASA).

Humidity on board the ISS is kept at around 60% RH. The THC makes sure that the air circulates throughout the station. That way, moisture does not build up anywhere. Humidity is controlled on board the ISS by the heat exchangers that are part of the Active Thermal Control System (ACTS). (See the Temperature on Earth and on the ISS Backgrounder for more information on the ACTS). As air passes through the heat exchangers and is cooled, the water in the air condenses . The water is then collected so that it can be reused.

Recycling water on board the ISS is very important. Approximately 93% of the water the astronauts produce is reprocessed to be usable water. That includes perspiration, respiration and urine. This keeps the humidity on the station down. It also helps maintain the station’s water supply.

If the relative humidity gets to 70% or higher, there can be problems for both the astronauts and the ISS. The biggest problem is that warm, humid conditions can lead to the growth of microorganisms. Astronauts can get sick if they breathe in these microorganisms. 

Microorganisms may also be bad for the station itself. Bacteria that are linked to corrosion on Earth have been found aboard the ISS. However, scientists do not yet know if these bacteria will have the same impact in space. Microorganisms can also make glass hard to see through, make rubber seals brittle, and clog air and water filters.

These dust mites were found floating in a globule of water on board the Mir Space Station
These dust mites were found floating in a globule of water on board the Mir Space Station (Source: NASA).

High humidity can also lead to condensation inside the ISS. If water condenses and collects on electronics inside the station, the equipment could short-circuit and possibly start a fire. 

Russia’s Mir Space Station is an example of a space station that had some of these problems. Astronauts lived on Mir from 1986 to 2000. But as it got older, Mir had a lot of problems with microorganisms and condensation. In 1998, astronauts found a floating globule of water the size of a basketball in one of Mir’s modules. The globule was full of microorganisms! Mir was decommissioned in 2000. By that time, scientists had found over 234 different types of microorganisms living on Mir

As you have learned, high humidity and microorganisms can pose some health hazards. Aside from these health hazards, astronauts on board Mir found another problem. Thanks to the high humidity and microorganisms, the station smelled really bad! 

Learn More

The Challenges of Building Closed Ecological Systems (2015)

A short article by David Russell Schilling outlining the challenges of building closed ecological systems, including a short video about Biosphere 2.

Water Recycling on the ISS (2013)

Canadian Space Agency (1:52 min.) Commander Chris Hadfield explains the water recycling process on the ISS.

What Space Smells Like (2012)

A short article by Megan Garber chronicling the ways astronauts describe the “smell” of space, and a look into the way NASA is trying to recreate that “smell”.


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