Absolute humidity of the air (φ)

Definition: absolute humidity of the air

The gaseous water content in the air is commonly known as air humidity. Gaseous water—water vapor—is invisible, as it is not the wisps of vapor formed by tiny droplets that make up fog, which is clearly visible. The amount of water vapor that the air can hold depends largely on the local temperature.

To accurately determine the humidity in a room, it is necessary to distinguish between relative, maximum, and absolute humidity. Absolute humidity describes the density of water vapor—that is, the mass of water vapor contained in a given volume of air. It is typically expressed in g/m³ and ranges from zero to the maximum amount of water vapor that the air can hold at a specific temperature and volume. In contrast, relative humidity describes the ratio of water vapor density to the maximum water vapor content of the air.

In this context, the dew point also plays an important role. It refers to the temperature below which water vapor condenses into dew or fog. Since the air can hold less water vapor at lower temperatures, the excess turns into liquid once the dew point is reached.

Limit Values for Absolute Air Humidity

With regard to absolute air humidity, no limit values have yet been established for indoor spaces.

The greatest danger in rooms with excessively high humidity is likely the growth of mold. The threshold for this is a relative humidity of 60%. In a room with a temperature of 20 °C, this corresponds to an absolute humidity of 10.4 g/m³.

Fluctuating temperatures within a room are particularly problematic. Depending on the insulation, even a relative humidity of just 50% can be enough—as it drops below the dew point near cooler exterior walls—to contribute to mold growth. The building materials used also play an important role here. These significantly influence how quickly water vapor diffuses.

Consequences of Air Humidity That Is Too High or Too Low

Health problems related to the water vapor content of the air depend largely on the relative humidity. If it is too low, the immune system is weakened. In addition, it causes dryness of the mucous membranes, skin irritation, and reduced respiratory capacity. The risk of excessively dry air increases especially in winter. Cold outdoor air can absorb only a small amount of water vapor. When this air enters indoor spaces and is heated there—for example, by heating systems—the relative humidity decreases while the absolute humidity remains constant.

Excessively high humidity not only causes discomfort and puts a strain on the circulatory system, but also reduces productivity. It also increases the likelihood of mold growth indoors. If mold spores enter the body through the respiratory tract, they can cause symptoms such as conjunctivitis, gastrointestinal disorders, joint pain, asthma, or migraines.

In addition, house dust mites multiply more rapidly when the relative humidity is 60% or higher, which can be particularly problematic for people with allergies.

Source of Humidity in the Air

At the surface of water, individual water molecules evaporate into the air due to their thermal energy. Water molecules present in the air also return to the water’s surface if they come into contact with it. The more water molecules there are in the air, the more frequently this occurs. At 100% relative humidity, the number of molecules returning to the water’s surface equals the number leaving it. The air is saturated. If the air is heated, this equilibrium is only achieved with a greater number of water molecules in the air. Its absorption capacity has increased.

Since relative humidity depends largely on temperature—because the air’s capacity to absorb water depends on temperature—it is difficult to determine whether active humidification is effective. If, for example, a damp cloth is placed over a heater, the room temperature drops because the heating output is reduced. This drop in temperature alone causes an increase in relative humidity. Absolute humidity, on the other hand, does not change solely due to a change in temperature, because if the air does not absorb moisture, the mass of water per cubic meter of air does not increase. Only when absolute humidity increases can it be said with certainty that the damp cloth contributes to raising the humidity of the air.

Sensor used:

The absolute humidity of the air is measured in the air-⁠Q using a very high-precision relative humidity sensor. The absolute humidity of the air is derived from the measured value. The air-⁠Q reacts slightly more slowly to changes in air humidity than a fully exposed humidity sensor.

Measuring Air Humidity

The air-⁠Q, as an air analyzer and measuring device, also features a sensor capable of measuring the absolute humidity of the air and other indoor air parameters in real time. Absolute humidity is also known, by the way, as water vapor density.

Here you can find a lot of information about air-⁠Q technology. You can order the air-⁠Q from the online store.