On average, people spend up to 90% of their time indoors, and indoor air can be two and a half times as polluted as outdoor air.

Indoor Air Quality is of particular concern for those with asthma and allergies, but a healthier home is of broader benefit to all.

The asthma & allergy friendly® Certification Program seeks to assist people to identify products which will make a genuine difference to their indoor environment. It develops certification standards for relevant categories of products, and all certified products undergo testing to those standards. In this way, the consumer can make an informed choice about materials like air cleaners, insulation, flooring products, etc. that they bring into their home.

In the case of dehumidifiers, customers want to be confident that their product can control humidity without dust or allergen being kicked up into the breathing zone. It is important that the dehumidifier can work well even when loaded with dust, and that after cleaning it also works well. Our standard addresses these issues.

Our dehumidifier standard is one of over 50 standards in the asthma & allergy friendly® Certification Program, covering appliances, bedding products, air filters, building products, and services that can have an impact on the indoor air environment. You can explore the dehumidifier standard in more detail using the menu on the left, or use the links above to explore other standards.



We’ll have a case study up here soon, but in the meantime please check out some of our other case studies

ASP:11-01/102 Dehumidifier Certification Standard

The Certification Standard utilizes an algorithm of proprietary and recognized scientific techniques to assess Dehumidifiers for their contribution to the goal of thermal comfort and indoor air quality. Products that pass Certification testing are granted a Certificate stating that the Dehumidifier meets the requirements for the asthma & allergy friendly® ASP:11-01 Certification Standard.

Part 1: Control of Relative Humidity (RH)

Optimization of RH levels is likely to contribute to favorable thermal comfort and may reduce adverse allergen triggering effects in the home associated with high RH levels over extended periods of time. Certified dehumidifiers must be capable of achieving and maintaining RH levels within defined specifications.


Pass/Fail Criteria

Pass/Fail Criteria

Reduction of RH levels

From 90% to 50%

Demonstrable over 4 hours at temperature of 20-25ºC

Maintenance of RH levels

50% RH +/- 5%

Demonstrable over 8 hours in presence of a humidification source

Maintenance of RH levels

50% RH within +/- 5%

Demonstrable over 8 hours within the thermal comfort range (i.e. 50%)

Part 2: Allergen Disturbance

While operating, the dehumidifier must contribute to maintaining low airborne allergen and particles levels.


Pass/Fail Criteria

Pass/Fail Criteria

Airborne Allergen Levels

>1000pg Fel d 1/m 3

Must not increase by criterion levels

Total Particle Counts

>170,000 counts/m 3

Must not increase by criterion levels

Part 3: Maintenance

During sequential dust loading and following maintenance of the dehumidifier, the potential exposure of the operator to airborne allergen and particles is assessed:


Pass/Fail Criteria

Pass/Fail Criteria

Airborne Allergen Levels

>1000pg Fel d 1/m 3

Must remain below criterion levels

Total Particle Counts

>170,000 counts/m 3

Must not increase by criterion levels

Part 4: Ozone

The dehumidifier unit must comply with the Code of Federal Regulations (CFR) standard for ozone limit levels. Third party testing data must be provided by the manufacturer showing compliance with this Principle.

All Certified asthma & allergy friendly® Dehumidifiers carry a unique Certification code.

Why and how we certify dehumidifiers – a Q&A

There is ever-increasing awareness of the chemicals and allergens we come into contact with every day, particularly if you or a member of your family has asthma or allergies. We created the asthma & allergy friendly® Certification Program to help you improve your indoor environment by identifying products and services that can help to reduce allergens and other triggers, and create a healthier home environment.

But what is it that makes some dehumidifiers better for the indoor environment than others, and where do we draw the line to decide to certify a dehumidifier as asthma & allergy friendly® ? We hope that the questions below will clarify this. Let us know if you have more questions!

Why do you certify dehumidifiers?

Our goal in the asthma & allergy friendly® Certification Program is to create a healthier indoor environment for you and your family, as well as to reduce allergens in the home. So we look at all elements of the indoor air environment.

The feature of the air in your home that is most affected by a dehumidifier is the Relative Humidity (RH). This a percentage that reflects the amount of water vapour in the air compared to the amount of water vapor that could be held by the air at that temperature. A high RH means the air is very humid and has a lot of water vapor suspended in it.

When RH is less than 30%, eyes and skin can become dry. When RH goes over 80%, it can encourage molds and mites to grow. So the humidity of the air is a key feature of creating a comfortable and healthy indoor environment. An optimal level of humidity for thermal comfort would be somewhere between 40-60%. Most types of mold grow when RH is over 70%, and so if you live in an area with high humidity, this could have a substantial impact on the quality of your indoor air environment.

And even though dehumidifiers generally do not filter allergen, they can help to reduce levels of allergen in your home by removing the moisture that helps molds and dust mites to thrive.

When we test dehumidifiers, we look to recreate what happens in the home, so that we are testing the appliance in a way that is relevant for you. We make sure that is can do its job in terms of reducing Relative Humidity. Also, dehumidifiers create a flow of air around them, and this movement of air can result in allergen and dust being picked up into the breathing zone. It’s important that a dehumidifier does not cause an increase in airborne particles that could be uncomfortable to breathe, and we test that.

When should I use a dehumidifier in my home?

It depends on where you live, and what your climate is like. If you live in a very dry zone, then a dehumidifier will not help you! But if you live somewhere that is quite humid, and this is causing you discomfort, or making your asthma or allergies worse, then maybe you could consider a dehumidifier.

If you do live in a very dry zone, perhaps a humidifier is something that could be helpful. Have a look at our page on humidifiers for more information.

How do you test dehumidifiers?

There are four elements to our dehumidifier testing. These are designed to test the relative humidity controls of the product, and to make sure the dehumidifier does not cause allergen and other particles levels to increase in the air because of the air flow it creates. We make sure that the product still performs well when it is loaded with dust and after it is cleaned. We also ensure the dehumidifiers meet strict ozone emission limits. You can find more details about each part of testing below.

Relative Humidity

We use a controlled environmental chamber to test dehumidifiers. We set up the chamber so that the temperature is 21-25°C and the relative humidity is greater than 90%. Then we switch on the dehumidifier, set it to a target of 50% relative humidity, and let it run for 12 hours. We run a second test where after the humidity reaches 50% we then introduce more moisture to the air to quickly bring the humidity back up to 90%, and again monitor the RH for 12 hours to make sure the dehumidifier can bring the RH back to 50% and maintain that level.

Allergen Disturbance

The operation of the dehumidifier has an impact on the air in the room. However it should not cause an increase in the levels of airborne allergen or particles. We test this by pumping allergen containing dust (ATD) into the chamber and measuring levels throughout the test. We make sure that this does not cause particle and allergen levels in the room to go above certification limits.


Changing of the filter (if present), and emptying of the water receptacle, can also be a cause of disturbance of the air and recirculate allergens and particles. This is assessed by contaminating the dehumidifier with ATD and then cleaning it following manufacturing instructions. When we bring it back into the chamber we run the dehumidifier again and make sure that it does not cause allergen levels and particle counts to increase.


The dehumidifier unit must comply with the Code of Federal Regulations (CFR) standard for ozone limit levels. Third party testing data must be provided by the manufacturer showing compliance with this Principle.

What else do you certify?

We have over 50 different asthma & allergy friendly® certification standards for products and services, addressing all areas of the indoor air environment. Some of these relate to products which remove allergens and dust from the indoor environment, like vacuum cleaners, air cleaners, humidifiers, and washing machines. Some of them relate to products where it is important not to provide an easy home for allergens and that it is possible to remove allergen from them – like bedding, toys, and flooring. And some of them relate to household products that should make as little an impact on the indoor environment as possible – like flooring and paint.

You can find out which products are certified asthma & allergy friendly® and read more here:

Humidifiers and Dehumidifiers – which one do I need?

What is humidity?

This is a very short and summarised friendly tutorial of sorts on all you need to know about humidity! Simply put, humidity is the amount of water vapor in the air.

Humidity is high when there is a lot of water vapor in the air. The higher the humidity, the wetter the environment feels and that’s what gives us that hot and sticky feeling that is so uncomfortable. When humidity is high, it’s hard for us to cool down because when we sweat, the sweat cannot evaporate effectively from our skin as the air is too wet. This makes it feel hotter than what it actually is.

Conversely, when humidity is low, we can feel much cooler than the actual temperature. This is because our sweat evaporates easily into the dry air.

We often hear humidity being referred to as relative humidity. Relative humidity is the amount of water vapor actually in the air, expressed as a percentage of the maximum amount of water vapor the air can hold at that temperature. For example at 14 degrees Fahrenheit the air can hold 2.2 grams of water per cubic meter. So if there are 2.2 grams of water per cubic meter in the air when its 14 degrees Fahrenheit outside, that means it is 100% relative humidity. If there was 1.1 grams of water in the air, its 50% relative humidity. The warmer the air is, the more water it can hold.

A comfortable indoor humidity is between 30-50% and this is what the Environmental Protection Agency (EPA) recommends. The human body is sensitive to humidity so anything lower than 30% feels too dry and anything over 50% too humid. At temperatures usually found indoors, this humidity level makes the air feel approximately what the temperature actually indicates. Controlling the indoor humidity within these parameters contributes to better indoor air quality.

A hygrometer is a device used to measure humidity in the air and can be mounted on the wall or placed on table tops or shelves. There are very affordable, so it’s not a bad idea to invest in a number of them for different areas of your home – you may find that your basement has a very different humidity than your bedroom.


Low Humidity

Low humidity not only makes us feel colder but the dry air dries out our skin and mucous membranes, leading to dry, itchy skin, chapped lips and a dry, sore throat first thing in the morning. Our airways can dry out all the way down to our lungs, precipitating a cough, even in those people with a completely healthy respiratory system. Very dry air can make eczema worse.

Very dry air increases the chance of catching airborne viruses like a cold or the flu – possibly because they can survive longer in dry, cool conditions but also because low humidity lowers our immune system. Studies exposing mice to influenza virus have shown that at low humidity, nasal cilia (the microscopic hairs in our nose) become less effective at removing viral particles and mucous. Airway cells are less able to able to repair damage caused by viruses and, finally, interferons- signalling proteins that tell neighbouring cells that there is trouble ahead- are impacted.   

Low humidity also effects our homes- it dries out furniture and plants. Paint on our walls can crack and flake. Wood floors and furniture can split and crack. It increases static electricity, causing sparks when you touch off something or someone.


So, how can I increase the humidity in my home?

Well, you can choose the old low tech, low cost options and can put a pan of water on the stove or on a radiator, or you can hang wet towels near a heater duct, but a more efficient way is to invest in a humidifier.


What is a humidifier?

A humidifier increases the vapour in the air. There are, in general, 4 different types:

Evaporative humidifier: This is the most common type of humidifier. A reservoir holds cold water and dispenses it into a basin. A moistened filter absorbs the water from the basin. A fan then blows air through the moistened filter. As the air passes through the filter, it evaporates some of the water there, pushing the humidity into the room. The higher the relative humidity, the harder it is to evaporate water from the filter, which makes this humidifier somewhat self-regulating — as humidity increases, the humidifier’s water-vapor output naturally decreases. Some humidifiers monitor the relative humidity of the air and will turn on and off as appropriate to maintain a pre-set level. 

Sometimes an evaporative humidifier will be hooked up to the heating and air conditioning system of a house and therefore can control the humidity level throughout your entire house. This is the most expensive option, but it’s also the most effective. Cost is offset in the long-term though, as simply by bringing the humidity up to 70%, you can make it feel 5 degrees Fahrenheit warmer in your home. Since it costs a lot less to humidify the air than to heat it, a humidifier can save you a lot of money.

Steam humidifier or vaporiser: Often referred to as a “vaporizer,” a steam humidifier boils water and releases the warm steam into the room. This is the simplest, and therefore the least expensive, technology for adding moisture to the air. They are easily moved from room to room. Another advantage of this technology is that you can use a medicated inhalant with the unit to help reduce coughs. On the down side, they have high energy costs and potentially could be dangerous around children because they could cause burns.

Impeller: In this humidifier, discs rotating at high speed fling water at a diffuser.The diffuser breaks the water into fine droplets that float into the air. These droplets are seen as a cool mist exiting the humidifier. Impeller humidifiers have low energy costs.

 Ultrasonic: An ultrasonic humidifier also produces a cool mist. A metal diaphragm vibrating at an ultrasonic frequency creates water droplets and this is pushed into the room. Energy costs for this type of humidifier are also low.


What are the benefits of having a humidifier?

By maintaining humidity in your home at 30-50%, a humidifier will make your home environment more comfortable to live in. In terms of health, use of a humidifier can relieve runny nose, nose bleeds, dry throat and sinus congestion. Dry air is an asthma trigger as it dries out our airways and this can cause bad flare-ups, so by keeping humidity at the ideal of 30-50% the risk of an asthma attack is reduced. The dry itchy skin and exacerbation of eczema seen with dry air is also relieved.

Importantly, by keeping the humidity at 30-50% the risk of catching airborne viruses is reduced.


Maintenance of your humidifier

Filter: A central humidifier will have a filter that needs changing as per manufacturer’s instructions.

Check the humidity: If you have a model that doesn’t measure humidity, then it’s a good idea to monitor it yourself. Too much vapor can cause problems too. Check out the issues with high humidity below.

Refresh the water: Stagnant water easily promotes the growth of bacteria and mold so if the water isn’t refreshed, you risk spraying infected water into your home. Not the result you were hoping for. Some high end models have antibacterial features built in to help reduce this risk.


High Humidity

High humidity makes us feel uncomfortable and hotter than what the temperature actually reads. Rooms feel unpleasantly stuffy and it can force us to crank up the air conditioning to lower the temperature, thereby increasing our energy costs.

High humidity can be a problem for people with asthma. When the air is wet, it can feel harder to breathe and this can make our body temperature rise, causing sweating. Subsequently, this leads to dehydration, which can make you breathe faster. All of this combined can trigger asthma symptoms.

But this isn’t all. High humidity levels provide an environment for two common asthma and allergy triggers: dust mites and mold. Dust mites are one of the most common allergen triggers for asthma, especially in humid and coastal areas. They live in soft furnishings such as beds, bedding, carpets, upholstered furniture, soft toys and clothing and, rather disgustingly, it is their faeces that is the actual trigger for asthma and allergies. They thrive in humidity levels at and above 70%.

Moist, damp air – anything above 50% but typically 60-70% humidity – is the ideal environment for mold to flourish. Mold is a fungus that can be detrimental to our health and the toxins, called mycotoxins, cause eye and throat irritation, skin rashes, sneezing, itchy throat and coughing, not to mention can contribute to asthma attacks. It can be seen growing on walls, ceilings, or around toilets and showers but for mold to affect your breathing it doesn’t even have to be visible– it just has to be in your home. Small mold patches can quickly spread – it actually only takes  a period of 24-48hours for mold to take hold. They usually appear in clusters of small black spots but can also be grey brown or green in color. Bathrooms and basements are prime areas for mold to grow as these tend to be the dampest rooms in the home.

High humidity and mold growth leads to that musty smell that is reminiscent of unloved old homes. Mold produces gasses called microbial volatile organic compounds and if you can smell it, you’re probably also inhaling mold spores. To make the whole situation worse, moisture in the air tends to hold and trap odors.

Over time, moisture can affect the wood in your home, including the support beams, causing them to warp, crack or even split. When there’s excess humidity in the air, wood will retain the excess moisture and start to rot. When this happens, it will start to attract bugs such as termites.

High humidity will cause condensation on your windows. Condensation occurs when warm air collides with cold surfaces, or when there’s too much humidity indoors. If you notice the glass of your windows to have beads of water or fog on the glass it’s a sign of too much moisture in that room. This condensation drips onto window sills and can deteriorate the wood framing around windows. Condensation can break down plaster and other building materials. Metal fittings may start to rust.

Home electronics and music systems and ocular equipment (cameras and binoculars) can be damaged by rust or short-circuiting. Documents suffer from too much water and water vapor in clothing, sofas and curtains can make them turn moldy and smelly.


So, how can I decrease the humidity in my home?

You can open windows and doors to increase ventilation and help the air dry out, especially in rooms where there is a lot of moisture -your bathroom and kitchen for instance.

Keep the bathroom fan on for longer after showering. Showers produce a lot of excess steam, so a shorter, cooler (or cold!) shower will go a long way towards reducing water vapor in the air.

Always use your exhaust fan in the kitchen, as stove top cooking will generate lots of steam. Keep boiling pots covered as much as possible.

Hanging wet clothes indoors will increase indoor humidity levels, especially in rooms where ventilation is bad. The best option to reduce indoor humidity is to hang clothes to dry outdoors, especially during humid seasons. If that is not an option, then use a clothes dryer that is vented to the outdoors.

Don’t over water your house plants and in humid months place your plants outdoors as plants themselves contribute to moisture production.

Fix any leaks- check the obvious places like under sinks, in bathrooms, laundry rooms and basements, seals around windows. Walls that have cracks or holes can also be responsible for letting water get indoors. Warm, moist outside air can travel indoors through cracks and holes during warm, humid weather. Regularly check your home’s external walls to ensure there are no cracks and fix them properly.

Turning on your air conditioning not only cools down the room, it will also help reduce indoor humidity especially during humid weather.

If all this doesn’t work, your best bet is a dehumidifier. A dehumidifier removes water from the air in your home until relative humidity is reduced to the level you choose. Once it has reached this level a good dehumidifier should automatically maintain that comfortable level, leaving you with less to worry about!


What is a dehumidifier?

Dehumidifiers draw excess moisture from the air. There are two main types of dehumidifier to choose from – refrigerant (also known as compressor) and desiccant. They work in different ways and may be better suited to different environments.

Refrigerant (compressor) dehumidifiers: A fan sucks in moisture heavy air through a filter and over freezing cold coils, through which a coolant circulates. As the air cools, the moisture condenses into liquid water onto the coils and this then drips down into a water tank. Now free of moisture, the air passes over a hot compressor/condenser unit to warm it back up and this warm dry air blows back into the room.

Desiccant dehumidifiers:  These use an adsorbent material- water holding material- to extract water from the air, not unlike a mop. The desiccant dehumidifiers pass the air over a wheel coated with a water absorbing material (called desiccant). The water is removed from the desiccant by passing a stream of warm air through it so it is dried and then re-generated to collect more moisture on its next pass. The material is then heated so that the moisture drips into the water tank. Desiccant dehumidifiers are designed to work more effectively than refrigerants in lower temperatures – the sort of environment you might expect in your garage maybe or a conservatory or an unheated basement. It’s often claimed that desiccant dehumidifiers tend to use more energy than refrigerant dehumidifiers because of the way they use heat to warm the adsorbent material. .

What are the benefits of having a dehumidifier?

Dehumidifiers take excessive moisture out of the air and this can have a huge impact on the environment in your home during the summer, a rainy spell or anytime with high humidity. The most common place to install one is in the basement, which tends to be one of the dampest places in the house, but they can be beneficial anywhere.

Dehumidifiers help keep the overall humidity in your house at an optimal level, therefore helping eliminate the environment in which dust mites survive. While you are never likely to be completely rid of these tiny mites, creating an unfavorable environment for them to live in will go a long way to preventing health-related problems like asthma and allergy.

High humidity levels can destroy your home’s structural integrity. A dehumidifier is an easy way to prevent your home from slowly deteriorating, while simultaneously creating a healthier living environment for your family.

When moisture is present in your space, temperatures may feel warmer than they actually are. By removing moisture from the air, you will feel more comfortable without having to crank up your air conditioner. This is especially true throughout the warmest times of the year.

The quality of air in your home is not only measured in the amount of toxins that you may or may not be breathing in, but also by the humidity levels of the air itself. It is impossible to keep the quality of air in your home at a healthy level if there is too much humidity. A dehumidifier plays an important role in improving your indoor air quality. 


Maintenance of your dehumidifier

Set it up right: It’s important to place your humidifier on a level surface with no obstructions to its air intake, otherwise it may overheat and become a fire hazard. Incidentally, close windows and doors so that the unit doesn’t have to work overtime against incoming humid air.

Check water levels:  This should be done every day. Most units will automatically shut off if the water tank is full so do this every day if you want your dehumidifier to run continuously. On the first day running in a very damp room, it may even need to be emptied twice. A full reservoir containing stagnant water will promote the growth of mold – another reason to be vigilant about checking. It’s also a good idea to clean the bucket regularly to get ahead of any mold. Warm water and soap will do the job if you don’t have disinfectant.

Change any filter/ wash it: Another regular job is to change any filter/ wash it according to manufacturer’s instructions. A filter removes harmful airborne particles such as dust and other allergens and it protects the internal components of the humidifier. Your dehumidifier will not run efficiently if it’s got a clogged filter.

Don’t turn your humidifier off and back on immediately: Turning the dehumidifier off and on too quickly can cause the compressor to overheat and could trip your circuit-breaker so give it about 10 min before turning it back on. To prevent this short-cycling, look for dehumidifiers that feature an automatic delayed start function to protect the compressor from short-cycle damage.

Have a look at the coils in your unit: About once a season, have a look at the coils in your unit. If they are dusty or see any debris, use the vacuum cleaner or a soft bristle brush to gently clean them. The coils shouldn’t ever be wet or icing up. Any ice generally means that the room temperature is too cold for the unit to function properly. Units with a automatic defrost function avoid this problem.

Storage: It’s the norm for a humidifier to be used for only the humid months of the year, so before storing make sure to empty and thoroughly clean the water tank.


Let’s sum up and eek out the key points from this short read. We know that humidity is important to our health and wellbeing and is a main contributing factor to indoor air quality. The recognised healthy humidity is between 30 and 50% but depending on the climate and season, the humidity in your home may stray outside this ideal and contribute to asthma, allergies, skin conditions, infections and general discomfort. It can lead to higher energy costs as it can force you to over-use the air conditioning/heating. Finally, it can cause electronics to malfunction and can interfere with the structural integrity of your home. However, all is not doom and gloom. Some basic tips and tricks can help, combined with a little love and care to your home to prevent exacerbation of the problem. And thankfully there is a low cost, simple, low maintenance fix available – investment in a humidifier or dehumidifier.

How Does Humidity Affect the Indoor Air Environment?

Relative humidity is the term used to describe the amount of moisture retained in the air, relative to temperature. It is important because of its impact on the body and on the surrounding environment. A Relative Humidity (RH) of 30-60% is a comfortable physiological range and is recommended by ASHRAE1. The indoor air environment is commonly a reflection of the outdoor air environment. For example, in the absence of significant intervention, in regions with high humidity, the indoor air is also likely to have a high humidity level.

Low relative humidity

Low relative humidity

If the RH starts to drop below 30% the air can start to feel dry and quite cool, however it can also cause your skin to dry out as the moisture is evaporated from the body; very dry air can result in worsening of eczema and irritation of the airways. Low RH can also have other negative impacts on a range of physiological functions. Some viruses can survive for longer in dry, cool conditions and lower RH levels can result in drying out of the eyes and mucous membranes. These two impacts combined can cause a greater risk of infection2.

High relative humidity

As RH increases, the air starts to feel warmer than it actually is, this is due to the higher moisture content. It is more difficult for sweat to evaporate as the moisture content of the air is too high. The body works harder to keep the body cool and this can result in the body overheating and also loss of water, salt and chemicals the body needs. Dust mites and mold require elevated levels of relative humidity (RH) in order to grow. While 30-60% RH is recommended for physiological comfort, an RH of above 50% provides conditions that will support the growth of dust mites3 and greater than 60% will support the growth of mold4.

Impact of relative humidity

RH levels that are too low, or too high, can have a negative impact on both comfort and health. RH levels in the home should ideally be maintained at a range of 40-50% to avoid getting too close to either of these extremes. If you live in an area with high RH, an effective dehumidifier can be an excellent way to control humidity, particularly in the bedroom or kitchen.

As RH levels increase above 50%, the potential detrimental impact is more from an environmental perspective. As RH increases, it creates a more suitable environment for dust mites and mold to grow. Dust mites thrive at higher RH levels and controlling RH to below 50% is essential to prevent their proliferation. Generally, ideal mold growth conditions are above 70%, however any RH level above 60% will allow mold to grow. For those with asthma and allergies, controlling exposure to allergens is crucial and ensuring unfavourable conditions for mold and dust mites is therefore essential.

Impact of relative humidity

The allergenic impact from dust mites is from their body parts and faeces, the more dust mite present, the greater the load of allergenic particles5. In terms of mold, it is primarily the spores that give rise to any allergic response. Mold spores are microscopic particles that mold can release when they are disturbed6. Not all mold release spores, and some are more dangerous than others. However you should avoid any exposure to mold spores if you suffer from asthma and allergies. Greater asthma morbidity – specifically a larger number of hospitalizations, wheezing episodes and night symptoms due to asthma – have been associated with the presence of moisture, mildew and cockroach allergen in homes7.

Humidifiers and dehumidifiers can both play supporting roles in the maintenance of a healthy indoor air environment. Humidifiers introduce moisture to the indoor air environment and dehumidifiers remove it. In general, a home will utilise either one or the other, based on the outdoor environment and indoor activities.

How Humidifiers work

Humidifiers introduce moisture into the air using water vapour or steam. Humidifiers can be built into the home heating and air conditioning systems, or they can be mobile units that can moved around the home as necessary. Some types of humidifiers include:

  • Wick/Evaporative – dry air is pulled into the system and passed through a saturated wick filter. As it passes through the filter, the air absorbs the moisture and is then delivered to the room environment.
  • Steam – water is gently brought to boiling point in this approach, producing a fine mist which is then released, cooled, to the home environment.
  • Ultrasonic – a metal diaphragm vibrates at ultrasonic speed to break down the water into very fine particles. These particles are then introduced to the room as a fine mist.
  • Impeller – a rotating disc forces water towards a comb-like diffuser. The diffuser breaks up the water droplets into a fine mist which is released to the room environment.

How Dehumidifiers work

Dehumidifiers remove moisture from the air, and generally domestic dehumidifiers have one mode of action for this, which is based on condensation. As air is drawn into the dehumidifier, it crosses over the dehumidifier’s refrigerant cooled coils. This causes the moisture in the air to condense and releases the air back into the room environment. The water is generally collected in a water receptacle which will need to be emptied on a regular basis, dependent upon the environmental RH conditions.

Selection, Operation and Precautions

Use of humidifiers and dehumidifiers can be an excellent way to maintain and improve indoor air quality. It is very important however to select the right machine, and once in use to maintain and operate in an appropriate way that ensures effective functionality that is also safe for the user. If you or your family has asthma or allergies, you should consult your doctor before use of a humidifier due to the problems that high RH levels can cause for those with asthma and allergies.

The humidifier or dehumidifier should be capable of controlling RH in the room in which it is placed – it is important to select a machine that is appropriate for the size of the room that it will be operated in. Where possible, you should determine what type of testing has been performed on the selected machine to ensure that they work effectively, and also that they are suitable for those with asthma and allergies.

Both types of machine are built around water usage; humidifiers have a water receptacle to introduce water to the air and dehumidifiers have a receptacle to collect water that they remove from the air. In both circumstances there is the potential for mold growth which must be carefully monitored.  When water receptacles are being removed, they should be done so carefully so as to reduce the potential for spore release from any mold present. As the dehumidifier operates it will collect water, and it should also not expose the user to allergens or fungal spores during filter and receptacle change procedures. You should ensure that the model that you choose has well designed maintenance procedures so that your exposure to any potential mold growth is minimised. Finally you should select a machine that automatically adjusts to your desired RH; this is really the only way to ensure that the RH stays between 40-50%.

Certification of Humidifiers and Dehumidifiers

In order to support consumers in the selection of a machine that is most suitable for them, there are a number of certification programs.

Certification of Humidifiers and Dehumidifiers

The Association of Home Appliance Manufacturers (AHAM), established more than 50 years ago, represents the appliance manufacturing industry by through leadership, education and advocacy. Part of this role involves setting standards for the performance of these appliances, including dehumidifiers and humidifiers. These AHAM standards measures specific characteristics that dehumidifiers and humidifiers should achieve, mostly based around maintaining relative humidity in a standard room sized chamber.

Energy Star

Energy Star – Energy Star is a globally accepted measure of the energy efficiency of a range of consumer electrical appliances. Consumers trust this logo as being representative of a product that has been tested to be energy efficient. Dehumidifiers and Humidifiers can be quite energy intensive and choosing an appropriate machine is therefore very important.


asthma & allergy friendly

asthma & allergy friendly® – The asthma & allergy friendly®  Certification Program is operated by Allergy Standards Ltd in collaboration with the Asthma and Allergy Foundation of America (AAFA), Asthma Canada and is also operated internationally through a global certification mark. Humidifier and dehumidifier certification is based on the impact on the indoor environment for those with asthma and allergies. The standard measures for the performance of these machines in a room sized chamber and addresses control of relative humidity, ozone production and general operation such that those with asthma and allergies should be appropriately protected.

Dehumidifiers and humidifiers can play an essential role in an allergy management plan. However selection of the right machine, with effective relative humidity control is essential. Machine maintenance and operation are also essential to consider and prior to purchase of a humidifier or dehumidifier for your home, it is important that you educate yourself as much as possible in order to make the right choice.



  1. ASHRAE Handbook—HVAC Systems and Equipment, 2016; Chapter 22.
  2. Wolkoff, P and Kjaergaard, SK. The dichotomy of relative humidity on indoor air quality. Env Int 2007; 33 (6): 850-857.
  3. Arlian, LG, Neal, JS, Morgan, MS, Vyszenski-Moher, DL, Rapp, CM and Alexander, AK. Reducing relative humidity is a practical way to control dust mites and their allergens in homes in temperate climates. J Allergy Clin Immunol 2001; 107 (1): 99-104.
  4. Environmental Protection Agency [online] [accessed July 2020]
  5. Asthma and Allergy Foundation of America [online] [accessed July 2020].
  6. Asthma and Allergy Foundation of America [online] [accessed July 2020].
  7. Bonner S, Matte TD, Fagan J, Andreopoulos E, Evans D. Self-reported moisture or mildew in the homes of Head Start children with asthma is associated with greater asthma morbidity. J Urban Health. 2006; 83:129-3
ASL Helps Shape the Future of Healthy Buildings at NeoCon and Design Days 2023

16 June 2023

ASL had a hugely successful few days at NeoCon and Design Days 2023 in Chicago

ASL to Share IAQ Expertise with the Design Industry at NeoCon and Design Days 2023

2 June 2023

We are looking forward to bringing our expertise in indoor air quality to NeoCon this month to help shape the built environment today and into the future.

ASL to Share New Research on Textile Flooring at EAACI Congress 2023

1 June 2023

ASL is excited to share the results of our new research on textile flooring at the EAACI Congress 2023

Dr. John Ryan delivered an impactful keynote speech at Pulire 2023

17 May 2023

Dr. John Ryan, CSO at ASL, addressed the Importance of indoor air quality and ESG needs in his impactful keynote speech at Pulire 2023

Dr. John Ryan at ISSA Pulire 2023, Addressing the Importance of Indoor Air Quality and ESG Needs

5 May 2023

Dr. John Ryan will speak at ISSA Pulire 2023 with a talk entitled 'Cleaning For Purpose Not Just Appearance.'

ASL Bring Indoor Air Expertise to BAU 2023

25 April 2023

BAU 2023 was a great opportunity for ASL to bring their expertise in indoor air quality to Europe, meet new clients, and catch up with Tarkett at their busy booth

You’ll Start Tracking Home Health Like You Track Heart Health; Dr. John McKeon tells Well+Good Why

16 December 2021

Dr. John McKeon, CEO of Allergy Standards, gives expert advice on tracking home health and why it makes perfect sense

Why regular laundry washing is important to maintain a healthy home environment

28 October 2021

Many people can be allergic to dust mites, pet dander, and other allergens that can build up in bedding, clothes, and other washable items at home. Taking care of these items by washing them regularly is a fundamental part of maintaining a healthy home environment.

NBC News Consults Dr. John McKeon

14 October 2021

In this NBC News article, digital editorial intern for Select, Zoe Malin seeks expert advice from ASL CEO, Dr. John McKeon, on indoor air quality and air cleaners.

Dr. John McKeon on Healthy Indoors Live

23 August 2021

Watch our CEO Dr John McKeon and President & CEO of the Asthma and Allergy Foundation of America, Kenneth Mendez, on Healthy Indoors Live with Bob Krell for a very interesting dive into the significant role a healthy indoor environment plays in our overall health & well-being.