How Does High Humidity Affect Indoor Air Quality?


We often focus on how toxic and polluted our air is, but humidity can be just as dangerous if left unchecked.

So, how does high humidity affect indoor air quality (IAQ)? A relative humidity level above 70% is conducive to mold, bacteria, virus, and dust mite growth. The increased water vapor in the air facilitates off-gassing of some volatile organic compounds (VOCs) like formaldehyde. Our body’s inability to sweat at high humidity levels can also raise the body’s internal temperature, increasing the risk of cardiovascular disease and heat stress.

As you can tell, high humidity poses a danger to both our health and the quality of air we breathe.

Let’s go into more detail and discuss each of those effects mentioned.


How Humidity Is Measured

Humidity is measured as relative humidity, or RH. In simple terms, it’s how much water the air can hold relative to the conditions of the surroundings. Ideally, the interior of a home should stay within 30 to 60% RH. For the best health, it’s even better to stay in the 30% to 50% range whenever possible.

Anything above 60% is considered higher than normal, and anything above 70% can have a major impact on IAQ.

It will be relatively easy to keep RH in check during colder seasons and in the living and sleeping areas of the home, but much harder during the summer months and in basements, kitchens, and bathrooms.


How High Humidity Impacts IAQ and Its Effects on Your Health


The greatest risk of high humidity is mold growth. Mold is very limited in spaces where RH is below 50%. It has a slightly better possibility of growth at RH between 50% and 60%, and even up to 70%. But at RH above 70%, it can spread easily after it first appears in the home.

One study claims that most mold species cannot grow at RH below 60%, with its ideal growth environment being at 80% RH.

But just how dangerous is mold anyway?

Generally speaking, the degree of danger that an air pollutant poses will depend on its size. Pollutants with a small particle size tend to be more dangerous to health, while larger pollutants are generally safer.

Mold falls in the middle.

It’s smaller than pollen and dust, so the body’s reaction to mold could be more serious than, say, the sneezing and runny eyes you’d get from those other pollutants.

At the same time, mold is by far larger in particle size than volatile organic compounds (VOCs), bacteria and tobacco smoke, so you don’t have to worry about serious illnesses like cancer or being poisoned by it.

It mostly causes symptoms like those found in respiratory allergies and asthma – wheezing, red eyes, coughing, shortness of breath, and headache. Mild infections are rare and tend to happen in immunocomprimised people mostly.


Dust Mite Allergies

Dust mites create waste that triggers an allergic reaction in the human body that’s appropriately called a dust mite allergy.

Dust mites thrive at room temperature (68 to 78 °F). They also thrive at relative humidity levels between 70% and 80%.

Naturally, humidity levels are low during the winter, so dust mites are virtually non-existent in the average home during the winter season. As the calendar approaches the spring and summer seasons, you’ll have to monitor RH levels and keep them below 70% to thwart the level of dust mite growth.

Besides limiting the humidity levels, you’ll have to limit the organic materials these microscopic bugs thrive on. Wash your bed sheets often, as they are full of your skin flakes. Switch out fabric curtains for hard, natural blinds and carpet for hardwood floors if possible. Air conditioning filters and air purifier filters that have been rated to trap allergens will typically be able to trap dust mites as well.


Heat Stress

At comfortable humidity levels, our body is able to control its own metabolic temperature by sweating. High humidity interferes with this process, especially when it’s also hot. Your body stops sweating and can’t cool you down. We may know this as heat stress in extreme cases, or hot, sticky air in the middle of summer when our air conditioning suddenly stops working.

But did you know high humidity and high temperature have been linked to other adverse health effects as well?

This inability to control one’s own metabolic temperature under high humidity and temperature can even contribute to cardiovascular disease. In fact, the risk of a heart attack is increased. Also, the inability for the body to sweat at high humidity levels interferes with its electrolyte balance, causing fatigue and muscle cramps.


Bacterial and Viral Infections

Just like mold, most bacteria and viruses thrive at high humidity, but they can do the same at low humidity as well.

Some viruses like the influenza virus, human rotovirus, and rhinovirus can spread much easier at humidity levels that are too low. That’s because at low humidity levels, our body is less able to transport and clear pollutants via its mucous membranes. It’s also because airborne viruses can settle to the ground much faster when there is heavier, human air in a home. Lightweight pollutants tend to stay suspended in dry air longer, on the other hand.

A study states that viruses made of nucleic acids and proteins tend to thrive at high humidity levels, while viruses made of lipids prefer low humidity levels.

If this sounds too complicated, understand this – viruses and bacteria are severely inhibited when the RH is between 40% and 50%.

So keep your RH in the recommended range to keep infections at bay.


More Off-Gassing

Volatile organic compounds (VOCs) are toxic gases that are among the smallest particles found in indoor air. They can be things like tobacco smoke, or chemicals coming from furniture, rugs, building materials, flooring, wall insulation, laundry detergent, and wall paint (the list goes on and on).

When these VOCs “leave” those surfaces and become suspended in the air they are said to be off-gassing (more on off-gassing in this post). And the scary part is that many of them are carcinogens, meaning that they could cause cancer.

Because VOCs are small particles like bacteria and viruses, they too can stay suspended in the air when humidity is low.

Formaldehyde is a carcinogen found in so many things in our home. It’s water soluble, so in a high humidity environment, it’s more likely to off-gas and be suspended in the air. Other chemicals that act similarly under high humidity are sulfur dioxide and nitrogen dioxide. Contrary to some belief, ozone levels are reduced at high humidity levels.