The feeling of dry skin, a scratchy throat, and static electricity during the colder months is a common experience, directly linked to the lack of moisture in the air. Humidity is simply the concentration of water vapor suspended in the atmosphere. While winter weather is often associated with rain or snow, the air itself is typically very dry. This dryness is a consequence of atmospheric physics, which dictates how much water the air can actually hold at different temperatures.
Why Cold Air Holds Less Moisture
The amount of water vapor air can contain is governed by its temperature, a principle explained by the concept of the saturation point. Warm air molecules are more energetic and spread farther apart, meaning they can suspend a significantly greater number of water molecules before reaching 100% saturation, or the dew point. For instance, a volume of air at 30°C can hold more than three times the moisture than the same volume of air at 10°C.
Cold air, conversely, has a much lower saturation point. When outdoor temperatures plummet, any existing water vapor quickly condenses out as dew, fog, rain, or snow, effectively wringing the moisture out of the air. The low absolute humidity, which is the actual mass of water vapor present in a given volume of air, is the reason outdoor winter air is naturally so dry.
How Heating Makes Indoor Air Drier
The extreme dryness experienced indoors stems from taking that already low-moisture, cold outdoor air and heating it dramatically. When outside air with a low absolute humidity infiltrates a home and is subsequently heated to a comfortable indoor temperature, its capacity to hold moisture rises sharply. Since no new water vapor is added during the heating process, the absolute amount of moisture in the air remains the same.
However, the relative humidity—the ratio of the actual moisture to the maximum possible moisture at that new, higher temperature—drops precipitously. This is why an indoor space heated from 0°C to 20°C can see its relative humidity plunge to desert-like levels, sometimes as low as 10% to 20%. Forced-air heating systems commonly used in homes exacerbate this issue by continuously circulating and warming the air, creating a cycle of very low relative humidity.
Health Effects of Low Humidity
Exposure to air with relative humidity levels consistently below the recommended range of 30% to 50% can have noticeable impacts on human health and comfort. One of the most common effects is the rapid evaporation of moisture from the skin barrier, leading to dry, itchy, and flaky skin, and often exacerbating conditions like eczema.
Low humidity also irritates the delicate mucous membranes lining the respiratory tract. When these membranes dry out, their ability to filter airborne pathogens and particles is compromised, making the body more susceptible to respiratory infections, colds, and flu. Individuals may experience symptoms such as a scratchy throat, nasal congestion, and even nosebleeds as the nasal passages become irritated. Dry air can worsen asthma and allergy symptoms and cause eye irritation, leading to dryness and a gritty sensation.
Practical Ways to Add Moisture Back
Counteracting the effects of dry winter air involves intentionally introducing moisture back into the indoor environment. The most effective method is using a humidifier, which releases water vapor directly into the air. Humidifiers come in various types, such as evaporative models that use a fan to blow air across a wet wick, and ultrasonic models that create a cool mist using high-frequency vibration. Regular cleaning is necessary for all types to prevent the growth of mold or bacteria in the water reservoir.
Simple household actions can also contribute to raising indoor humidity levels. Placing containers of water near heat sources, like radiators or heating vents, allows the water to slowly evaporate into the room. Leaving the bathroom door open while showering allows the steam to circulate outside the room, or air-dry laundry inside instead of using a dryer. Introducing houseplants, which release moisture through a process called transpiration, provides a natural solution to gently increase the air’s moisture content.