The Effect of Humidification on The Indoor Environment

Adding humidity to the indoor environment is an essential part of maintaining good indoor air quality (IAQ) during the cold months. However, the way it's done—and the systems used—can have a major impact on a building's health and its occupants.

Why We Need to Add Humidity

In the winter, the cold outdoor air contains very little moisture. When that air is brought inside and heated, its relative humidity (rH) drops drastically. This creates an extremely dry indoor environment that can cause several problems:

• Health Issues: Dry air can lead to a range of health problems. It dries out the sensitive linings of the nose and throat, which can cause frequent nosebleeds and make us more vulnerable to infections. It also makes it easier for airborne viruses to spread.

• Static Charges: The lack of moisture in the air prevents the dissipation (the process of scattering or dispersing) of static electricity. This can lead to uncomfortable static shocks and cause issues with electronics.

• Comfort: Beyond the health risks, dry air can make a room feel colder than it is and can cause skin and lips to become dry and chapped.

By adding moisture to the air with a humidifier, we can counteract these negative effects and maintain a more comfortable and healthier indoor environment.

Maintaining relative humidity (RH) above 40% significantly reduces the transmission of airborne viruses like influenza and coronaviruses. When an infected person coughs or sneezes, they release droplets containing viruses. The size and travel of these droplets are heavily influenced by the humidity of the surrounding air. The "sweet spot" for humidity, often cited by building scientists, is between 40% and 60% RH. 

Mechanisms of Viral Deactivation

Humidity affects viral transmission through a few key mechanisms:

• Droplet Size and Desiccation: In very dry air (below 40% RH), the water in respiratory droplets evaporates quickly. This causes the droplets to shrink into tiny, lightweight particles called aerosols. These aerosols can remain suspended in the air for hours, travel long distances, and be easily inhaled deep into the lungs. This increases the likelihood of viral transmission.

• Viral Viability: Viruses are sensitive to the chemical makeup of the droplets they are in. At very low humidity, the rapid evaporation can lead to the crystallization of salts and other compounds within the droplet, which can protect the virus and prolong its life. In contrast, at a moderate humidity (40-60% RH), the evaporation is slow enough to concentrate the salts and other components to a level that is toxic to the virus, but the droplet doesn't dry out completely. This process can quickly inactivate the virus, reducing its ability to infect.

• Human Immune Response: Low humidity also has a negative effect on our bodies. Dry air can irritate the respiratory tract, impairing the natural defenses of our nasal passages and airways, making it easier for viruses to take hold. Maintaining proper humidity helps keep our mucous membranes moist and our immune systems functioning optimally.

By keeping indoor air humidity in the 40-60% range, we can change the physics and chemistry of airborne droplets to our advantage, making them less likely to spread viruses and reducing the viruses' ability to survive.

The Dangers of HVAC-Connected Humidifiers

While humidification is necessary, many common humidifiers, particularly those integrated into a home's HVAC system, can introduce their own set of serious IAQ problems if they aren't properly maintained and understood.

• Misuse and Mold Growth: A primary issue is that these systems are often misunderstood by homeowners. They are sometimes left on during the cooling season when the air is already humid. This can push the indoor relative humidity above 60%, which is a critical threshold for mold. Excess moisture provides the ideal conditions for mold spores to grow on surfaces and inside the HVAC system itself, potentially spreading throughout the property.

• The Wrong Water Source: Many HVAC-connected humidifiers are plumbed directly into the building's domestic water supply. This is a problem because domestic water contains dissolved solids (minerals and other substances, like calcium and magnesium, that are dissolved in the water). As the humidifier evaporates the water, these dissolved solids are left behind. They can build up on the humidifier's components and, more importantly, can be released into the airstream as fine, inhalable particles. This is in direct contrast to using distilled water, which has had these solids removed.

• Poor Maintenance: These humidifiers are often out of sight, and homeowners may not know they require regular cleaning and maintenance. A neglected humidifier can become a breeding ground for bacteria, mold, and other contaminants. When the unit turns on, it can release these pollutants directly into the building's air supply, causing serious IAQ issues.