Intro to Humidity
Humidity is simply a measure of how much water vapor is in the air. It’s an underused resource for indoor air quality, but needs to be used carefully.
The amount of moisture that air can absorb changes based on temperature. Warm air can hold much more moisture than cold air. Relative humidity (RH) is a measure of how much water vapor is in the air compared to the maximum amount it could hold at a given temperature, expressed as a percentage.
RH = Water moisture/maximum water moisture possible at that temperature
Health and comfort are related to relative humidity and not absolute humidity. Relative humidity can determine the risk of mold growth and building damage from condensation, so we generally use RH instead of absolute humidity.
The Psychrometric Chart
This is technical. You can skip to the next section if you like.
The psychrometric chart shows the relationship between temperature and humidity.
The x-axis is dry bulb temperature (what you measure on a thermometer). As you move right, the air temperature is increasing. The y-axis is the amount of water in the air, called the humidity ratio. The thin red lines that curve to the right and up are the relative humidity lines. As you increase in temperature, the amount of water the air can absorb increases, so the absolute humidity required to maintain a certain relative humidity increases.
The thick blue horizontal line shows what happens when you heat cool air without increasing absolute humidity. Let’s take the example where the air starts at 0 °C and 70% RH (blue 1). When the air is heated to 20 °C (blue 2), the absolute humidity remains the same, but since air at 20 °C can hold much more water, the relative humidity drops to just below 20% RH.
The thick red horizontal line shows what happens when you cool air at 30 °C and 60% RH (red 1). When it is cooled to 25 °C (red 2), the air cannot hold as much moisture, so the relative humidity will rise to 80% RH.
These are simple examples how you can start at normal humidity levels and then heating or cooling can create extreme RH values.
Health Effects
The Sterling chart summarizes how keeping humidity between desired levels can improve indoor air quality and health.
Low humidity has been associated with increased risk of airborne disease transmission and this has been known for a long time. Low humidity has also been shown to increase COVID transmission.
There are 3 main effects that cause airborne viruses to spread better with low humidity.
Innate Immune System
Your body has natural defense mechanisms to prevent you from getting infected with airborne pathogens. Those mechanisms can become impaired when exposed to air at low humidity.
Reduced Deposition
One of the ways infectious airborne particles are removed from the air is they land on a surface. This is called deposition. Once a particle has landed on a surface, it cannot infect you through inhalation, unless it is resuspended. How long a particle remains in the air depends on its size. In humid areas, more respiratory particles that leave your mouth or nose will land on the floor in a shorter period of time. When there is low humidity, the water in those particles can evaporate faster and they become smaller and will remain airborne for a longer time.
Decay
Viruses do not live long. How quickly they decay depends on the environment, including temperature and humidity. At low and high humilities, viruses have a longer half-life, so remain a hazard for a longer period of time. Between 40–60% relative humidity, viruses decay the fastest. Once a virus has decayed, it no longer poses a threat of infection. This is the decay of SARS-CoV-2 based on humidity.
Nevertheless, despite known benefits of proper relative humidity on reducing the risk of airborne disease transmission, there appears to have been a consensus formed that humidity should not be considered a primary tool to mitigate airborne disease transmission. The focus should be on removing the virus from the air through ventilation, filtration or UV light. The CDC recommends that humidity should generally not be considered as a tool to reduce the risk of COVID transmission (here FAQ 10). Recently published public review for ASHRAE Standard 241 Control of Infectious Aerosols only states to control temperature and humidity setpoint (9.1.8) and does not require any specific humidity level.
Dangers of high humidity
When humidity levels get too high, water moisture in the air can condense and collect on surfaces. This can ultimately lead to mold growth. Condensation can be visible or concealed. Generally, the first visible problems occur on windows. Outdoor air temperature and the quality of the windows determine at what relative humidity condensation will start.
Concealed condensation can occur when air at a high relative humidity leaks through the building envelope and condenses on surfaces inside the walls. This can occur before visible condensation occurs, which means caution is warranted when humidifying.
Humidity Standards
ASHRAE 62.1, the standard for ventilation and indoor air quality, does not require any humidification but if there is dehumidification, the dewpoint should remain below 15 °C. That corresponds to a RH below 55%–65% in most indoor environments (23–25 °C).
Advice from the Public Health Agency of Canada recommends 30–50% RH.
Various publications have recommended 40–60% RH.
The CDC recommends 30–60% RH in many healthcare settings.
We know there is harm at low humidity (like 20% RH). I personally feel more comfortable around 40% RH in the winter, but condensation issues might begin around those RH levels, depending on the building envelope, window type and outdoor air temperature. However, I do not think there is currently strong evidence of significant harm when dropping from 40% RH to 30% RH. Therefore setting a low limit of 30% RH in the winter makes sense. For dehumidification in the summer, a limit of 60% RH also makes sense. So, in accordance with the CDC recommendations, the final conclusion is:
In the winter, humidify up to 30% RH.
In the summer, dehumidify down to 60% RH.
Types of Humidifiers
There are many different types of humidifiers and they can be categorized in various ways.
Ultrasonic or Impeller
Ultrasonic humidifiers use a metal diaphragm vibrating at high frequency to spray mist in the air. Impeller humidifiers use a rotating disk to spray mist into the air. When it comes to their effects, they are the same. These humidifiers pose two main risks. If you use tap water for humidification, particles that are dissolved in the water will get sprayed in the air significantly increasing the amount of particulate matter in the air. Mold growth can occur (and often does) in the basin and mold or bacteria can then be sprayed into the air.
In order to mitigate the concern about particulate matter, never use tap water with these types of humidifiers. They require distilled water. To avoid biofouling, ensure to clean and disinfect these humidifiers often.
Evaporative
Evaporative humidifiers use a fan and a wick. The wick is like a metal sponge. Water is drawn over the wick. When air is pulled through the wick by the fan, it will absorb the water from the wick.
A typical furnace humidifier is evaporative, where air from the supply is bypassed back to the return where it passes through a wetted wick.
Because droplets are not sprayed into the air, but instead the water evaporates into the air, there is a reduced risk of particulate matter or biological matter in the air from this method.
Atomizer
High pressure water or compressed gas is added to the water to create a fine spray. This isn’t used in residential settings.
Cool Mist
Cool mist is not a type of humidifier, but a category. It generally refers to ultrasonic or impeller humidifiers. Sometimes it also includes evaporative humidifiers.
Adiabatic
Adiabatic humidifiers are a category of humidifiers that do not heat the water in order to turn it into a vapor, but rely on evaporation. All the previously mentioned technologies (ultrasonic, impeller, evaporative and atomizer) are types of adiabatic humidifiers.
Hot Mist/Steam/Vaporization/Isothermal
These are all different names given to humidifiers that create steam by boiling water. These are effective and pose a low risk of biological fouling or particulate matter generation. Portable steam humidifiers do pose a burning risk around children. Higher quality humidifiers for furnaces generally produce steam.
