What are the harmful effects of humidity on your health?

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Comfortable temperature? Check.

Fresh air? Check.

Still not feeling comfortable? You may need to check your indoor humidity levels.

The indoor humidity level is one parameter you cannot afford to overlook when trying to build your perfect home. Maintaining a good humidity level in your home is critical for comfort and neglecting it may cause some problems that can damage your home and spoil your health. This article will talk about the ideal indoor humidity that you should aim for in the house and the harmful health effects caused by both low and high indoor humidity. 

How Humid is my house?

If you suspect you have a humidity problem at home but do not have a humidity meter, take our Home Humidity Check to have a general idea if your home is too humid, too dry or just nice.

1. Do you usually feel hotter at home than the reported temperature outside?


2. How long does your laundry take to dry if left to hang indoor?


3. Do you see any mold spots in your kitchen or bathroom?


4. Do you frequently get dry skin and dry eyes?


5. Do you have any problems with wood rot on your wooden furniture?


Question 1 of 5

What Is Humidity & What is Relative Humidity?

First, let us get the terminology out of the way.

Humidity is the amount of water vapor in the air. 

Relative humidity is the ratio of the current amount of water vapor in the air to the maximum amount of water vapor the air can hold at a particular temperature. When the air has reached the maximum amount of water vapor, the relative humidity is at 100% and that is known as dew point. Dew point is when condensation starts and water droplets start to form on windows and walls.

What is the ideal indoor humidity?

A good humidity level is one in which we can get the most amount of comfort and where it is most conducive to our health. There is a wide range of ideal indoor humidity reported by studies and online sources but most seems to agree that a comfortable humidity level is between 40% to 60%. 

In terms of health, this level of humidity is also considered to be the most optimum for health. The survival and infectivity of airborne transmitted bacteria and viruses are minimized by exposure to relative humidity of 40% to 60%. This greatly reduces the risk of disease transmission from these pathogens.

ideal humidity infographics

Humidity and Your House

When indoor humidity levels get too high (air is being saturated with water vapour), condensation begins and water droplets start to form on your windows and walls. Moisture and houses generally do not mix well. With prolonged exposure to excess moisture in the house, the structural integrity of the house may be compromised as wood rot and corroding beams start to set in. You may also notice damp spots or a musty smell in your home due to the mold that is starting to grow in the corners of the house. 

It is important to recognize these signs early as continued damage to the structures of the house will result in irreversible damage to the structures of the house and weaken it’s foundation. Also, repairs to these vital structures will generally be extremely costly and troublesome. Therefore, it is always best to diagnose the issues early and treat it as soon as possible.

infographics of humidity effects on house

Humidity and Your Health

The inappropriate indoor humidity levels can have both direct and indirect effects on your health and comfort. The direct effects are the result of the humidity on your body systems and workings while indirect effects result from the impact of humidity on viruses, bacteria, fungi and chemicals.

Direct Health Effects

These are probably effects that most of us have experienced before. When high humidity is combined with high temperature, our sweat finds it very difficult to evaporate as the air is already saturated with water vapor. Therefore, we generally end up feeling very hot and sticky resulting in visible physical discomfort. 

Conversely, in areas with very low indoor humidity (dry air), we tend to suffer from irritation of the eyes, nose, throat and skin. This is due to the drying of the surface membrane that covers these areas. Some common symptoms that may arise are nose bleeding and eczema. 

Indirect Health Effects

Many studies have found that humidity can increase the risk of allergies and infections in the respiratory system. This effect is likely caused by the growth of bacteria, dust mites, viruses and fungi caused by inappropriate humidity levels. In addition to that, relative humidity may increase the concentration of harmful chemicals in the air when water vapor reacts with chemicals in the air.  

infographics on humidity effects on health

Biological Irritants


In this study comparing culturable bacteria in a typical urban area, they found that relative humidity had a positive correlation with bacterial aerosol concentration and this was statistically significant. Their results show that the average concentration of culturable bacteria was 4 times higher in spring than during winter. 

In other studies, Serratia marcescens, a bacteria known to cause respiratory infection is found to be least active during exposure to 50% relative humidity while becoming the most active above 80% relative humidity. Other bacteria like Brucella suis and Staphylococcus albus also favor high humidity levels of 70% to 80%.

However, there are also some bacteria that like a dry environment. One such bacteria is Mycoplasma pneumoniae. It is an airborne bacteria that can cause pneumonia or other severe respiratory disease. Tests on it shows that it can survive longer when exposed to low or high relative humidity levels.

Generally speaking, bacteria survive longer in very dry or very wet environments. Keep them away by maintaining your home humidity levels between 30% – 70% and keep your family safe from the disease they cause.

drawing of bacteria


Fungi has been proven to be a cause of asthma and other allergic reactions. Some species of fungi like Aspergillus are especially allergenic and can cause hypersensitivity reactions in people that do not normally suffer from allergies. 

And guess what, fungi love moist environments. They literally thrive in high humidity levels. Most species of fungi will only grow in relative humidity more than 75%. Therefore, they are usually seen in areas like kitchen, bathrooms and window frames subjected to frequent condensation.

In this study done in Cincinnati, 35 homes were investigated and they concluded that high humidity levels had a positive correlation with the richness (number of species) of the fungi population in the house. In objective terms, there were 10 times more fungi species in homes with high relative humidity compared to homes with low relative humidity. It was worse if the home is older and this is likely because they are likely to have more water damage. 

In summary, fungi loves moisture. If you want to avoid fungi, keep your home dry.

fungi growing on petri dish

Dust Mites

Mites are the biggest culprits of house allergies. Like fungi, the common house mite, Dermatophagoides pteronyssinus, requires humidity for optimum growth. They reach their maximum size during exposure to 80% relative humidity. A number of other studies also demonstrated that dust mites are significantly associated with high relative humidity levels. In this study of mites that lasted 2 years, they found that the number of mites per gram of dust varied from 400 to 1100 when the relative humidity was at 70% but it dropped to 50 when relative humidity is at 40%. 

In Chicago, a study across 5 homes found a statistically significant relationship between high relative humidity and dust mites allergens. They also showed that mites impermeable covers may eliminate it from the bed but it will still be present at other areas of the house. Other areas that may be potential hotbeds for mites are carpets, upholstered furniture, soft toys and clothing. 

If you are suffering from allergies without any obvious cause, it might be worth your time investigating your humidity levels to see if you mite have a problem.


The events of COVID 19 have given us a stark reminder of how deadly viruses can be. However, way before the pandemic, many countries have already been struggling with the effects of influenza. In the United States, about 12000 to 61000 people die from influenza annually since 2010.  

The available data on virus survival shows that their infectivity and survival are the lowest at relative humidity between 40% to 70%. They demonstrate the greatest activity at a low relative humidity level of 20% which decreases rapidly when the humidity level is increased to 60%. Their activity then increased again after exposure to humidity levels of 70% to 80%. However, the survival and infectivity at 80% is lower than at 20% relative humidity. 

To minimize virus transmitted disease, a mid-range relative humidity level is preferred.

stuffed bear wearing mask

Chemical Irritants

Chemicals that are found in indoor air can combine with water vapour to form respiratory irritants. Examples of such chemicals are formaldehyde and ozone. Health effects caused by chemical interaction with humidity are probably less common than those caused by biological irritants. However, it is still important to recognize the potential harmful effects on health they can cause to prevent and avoid them.


Formaldehyde may sound like an industrial chemical that is rarely found in homes but you couldn’t be further from the truth. Formaldehyde may be present in common household items like urea-formaldehyde foam insulation, plywood, wood products, carpets and textiles. 

The characteristic that makes formaldehyde so dangerous is that it is water soluble and this promotes the off-gassing of formaldehyde from the items mentioned above. A study in 20 homes found a statistically significant correlation between indoor relative humidity and formaldehyde concentration in the air. Prolonged exposure to formaldehyde can cause adverse health effects like irritation to the skin, eye and throat. It may also cause aggravation of respiratory disease and allergies. 


Ozone is another chemical irritant that is closer to us than we think. When we speak of ozone, we may think of the ozone layer in space but actually ozone can be present in urban areas as well. Ozone pollution is especially elevated in cities like Denver, COlorado, Houston and Texas. If you live in these cities and occasionally smell a chlorine smell when there shouldnt be chlorine around, you may be exposed to ozone pollution. 

Ozone is a powerful respiratory hazard and has been shown to cause irritation to the eyes and membrane of our respiratory tract. Contrary to formaldehyde, they are enhanced by low humidity levels and inhibited by high humidity levels. 

Humidity Control Is Essential For Health

Data doesn’t lie. There are numerous studies showing that the optimum relative humidity level for human health is between 40% to 70%. Inappropriate humidity levels promote the growth of airborne pathogens and chemical irritants that are not compatible with health. Harmful health conditions like asthma, allergies, chronic obtrusive pulmonary disorders (COPD), respiratory illness are all shown to be increased in places where humidity is not controlled properly. 

man sneezing into tissue

How To Measure Humidity

The easiest way to measure the relative humidity in your home is to use a hygrometer which is a humidity sensor or humidity monitor. Digital hygrometers are very readily available now and they determine the humidity by using a sensor to monitor an electric current that is affected by moisture levels. 

These digital hygrometers are often integrated into a climate monitor which measures multiple other parameters like temperature, air quality and noise levels.


Where To Place The Hygrometer

Avoid places like the kitchen or bathroom where there is definitely going to be excess moisture in the air as this will overestimate the level of humidity in your house

Likewise, do not place it right under the window or right next to the door where the ventilation is the best as this will underestimate the humidity level.

Also, take note not to place the hygrometer near any heat sources like the fireplace or a heavily used appliance as this can also disrupt the accuracy of the humidity reading. 

It is best to place in the central area of your living room as this is also the place where your family would spend the most amount of time. It makes more sense to get an accurate reading of this area so that the proper humidity level can be maintained to achieve the most comfort for your family. 

Questions and Answers (Q&A)

Q: Is a humidifier the same as a diffuser?

A: No. A humidifier is usually a larger device meant for the purpose for increasing the water vapor content in the air. A diffuser is usually a smaller personal device used to diffuse essential oils for aromatherapy. There are some devices that can act as both.

Q: Is a humidifier good for asthma?

A: Yes and no. Asthma due to allergies may improve as the indoor humidity is maintained at the ideal level of 40% to 60%. However, a humidifier may also breed mold and fungi if it is not cleaned regularly. A dirty humidifier may release more allergens into the air and aggravate asthma.

Q: Is it advisable to run a humidifier and an air purifier together in the same room?

A: No. The moisture from the mist coupled with the residues in the filter may become a breeding ground for mold and fungi if the filter is not maintained regularly. You can read this guide on humidifier placement to learn about other do’s and dont’s of humidifier usage.

Q: How do I get rid of musty smell at home?

A: The main culprit behind this is mold growth and the volatile organic compound it produces. In order to tackle mold, you need to reduce humidity and increase ventilation. Vent your house often by opening windows and using fans. Depending on your home humidity levels, you can reduce it using natural method or using a dehumidifier. Read our guide on how to reduce home humidity level naturally here.

Take Action Now

You are the last line of defense in your fight against these harmful effects of poor humidity control. You can take the following steps now to give you and your family the most ideal humidity level for comfort and health

  • Check the humidity level of you house regularly with a hygrometer
  • If your house has more than 60% humidity (too high)
    • Improve ventilation and air circulation by opening the windows
    • Use extraction fans in bathrooms and laundries
    • Use a dehumidifier
  • If your house has less than 40% humidity (too low/dry)
    • Use a humidifier

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