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Air purifiers vs humidifiers: What's the difference?

Filters inside air purifiers remove certain size particles from the air.

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If you're intrigued by the debate between air purifiers vs humidifiers, then you're likely wondering how both affect air quality. Going beyond that, you might also not know exactly how each of them works. In this in-depth article, we explore the differences between air purifiers and humidifiers, understanding how the science stacks up.

Straight out of the box, air purifiers and humidifiers look very similar, but they're pretty different inside. One works to sanitize allergens in the air, the other to add extra moisture to soothe irritation caused by dry air.

Understanding the differences between the best air purifiers and humidifiers is crucial to helping you make the right choice for you, your family, and your home. If you want a deeper dive, we have an investigation on how air purifiers work, and we also look at the differences between air purifiers vs ionizers.

Air purifiers vs humidifiers

When it comes to exploring the topic of air purifiers vs humidifiers, it's important to get down to how each of them works; beginning with air purifiers, which are devices that trap and remove contaminants from the air. Most air purifiers work by sucking air through the machine, where it passes through a series of filters that effectively trap any airborne irritants.

There are other types of air purifiers on the market, including those using negative or bipolar ionization, carbon filtration, and ultraviolet (UV) light to filter the air. However, the jury is out on the efficacy of such devices, so we've focused here on filter-based humidifiers.

The majority of commercial filter-based air purifiers have a high efficiency particulate air (HEPA) filter. The HEPA filter standard was developed in the 1940s for facilities containing nuclear materials before finding a commercial market in the 1960s.

HEPA filters are built from a mesh of plastic and fiberglass threads woven together to stop contaminants from creeping through. They're capable of capturing particles as small as 0.3 microns and can remove a huge range of particles, including irritants and allergens such as dust, bacteria, mold, and more. HEPA filters don't remove viruses completely, if you're wondering, but they do have an effect on them.

Humidifiers work by adding moisture to the air in a room. The machine draws water from a reservoir at the bottom of the device, which it pulls through and transforms into a fine mist that is expelled from the machine. Systems can boil water and transform it into steam, use a fan to evaporate the air, or even use ultrasonic technology to vibrate water droplets. The fine water particles spread through the air, raising the relative humidity level. Once it reaches the correct level, the machine will shut off.

The primary purpose of humidifiers is to raise the humidity to a steady level, typically between 30% and 50%. If rooms become too dry or too humid, it can be linked to health issues, particularly among those people with existing health conditions such as asthma or allergies.

Related: Are air purifiers safe for pets?

Air purifiers vs humidifers: what's the difference? Image of woman coughing — (Image credit: Getty Images)

Air purifiers vs humidifiers: how do they affect air quality?

Besides investigating how they work, the most important thing to establish with air purifiers vs humidifiers is how each device affects the air you're breathing. Starting with purifiers, where the obvious purpose is to improve air quality, which they do by filtering out particles.

Research suggests that air purifiers can have a clinical benefit, but – as with anything – you get what you pay for. An evidence review from Current Allergy and Asthma reports that "Inexpensive, low-efficiency HVAC filters offer no better particle removal than no filter".

The lesson? Invest in a better-quality machine with a HEPA filter. HEPA filters are used in clinical settings such as hospitals and work at incredible levels of efficiency, trapping up to 99.97% of particles, according to the Environmental Protection Agency.

The emergence of COVID-19 has raised awareness of the potential for airborne threats. While still cautious of a purifier's ability to stop the spread of colds, flu, and coronaviruses,SAGE has "medium confidence" that air-cleaning devices such as purifiers offer health benefits in spaces that aren't well ventilated.

Interested in what a HEPA filter captures? Researchers from Nature.com compared the particles in household dust and those found in a HEPA filter and concluded that the filter provided a new environment for mold and bacteria to proliferate. In some cases, these organisms can collect, multiply, and be released back into the environment, so you must check the filter according to the manufacturer's guidelines.

Humidifiers are used to add moisture to the air, making the room feel more comfortable, particularly for those with health conditions, such as dry skin, irritations, and allergies. Air purifiers can help with allergies too.

One study from the Respiratory Medical journal claimed that low humidity poses a real health risk, including respiratory tract infections. Low humidity can also exacerbate asthma symptoms, with the air irritating and drying out the mucous membranes that line the respiratory system and lungs.

It's essential that the water used in a humidifier is good quality and free from pollutants, or it could be causing you harm, one recent study found.

Humidifiers must be kept clean and free from mold, particularly if they're used around children. Dust, and more worryingly, mold can be expelled by the humidifier into the air, causing more harm than good. The advice from the experts is to regularly clean and change the filter in any humidifier to keep it safe.

Related: Sick building syndrome: what to look out for

Air purifiers vs humidifers: what's the difference? Image of woman putting on face mask — (Image credit: Getty Images)

Air purifiers vs humidifiers: which is right for you?

We've seen the science in the debate between air purifiers vs humidifiers but which is right for you?

Air purifiers are used to sanitize the air of pollutants. They're ideal for those with health conditions such as asthma and allergies that could be aggravated by poor air quality.

When selecting an air purifier, always choose one with a HEPA filter. Portable air filters will work effectively in one room but won't be powerful enough to cleanse the air in the entire house.

If humidity levels are low, a humidifier can help increase the moisture in a room, restoring it while reducing the negative impacts of low humidity. Chapped lips, a dry throat, and constantly feeling thirsty are some of the warning signs, but you'll need a humidity sensor to be sure. As with any health issue, you should always seek medical advice for an official diagnosis.

When buying a humidifier, check that its size is suitable for your home. Whichever humidifier you choose, it's crucial that you keep it clean, including removing and checking the filters and use clean water.

We've established that air purifiers and humidifiers do different things, but can they work together? Yes. It's safe to operate both an air purifier and a humidifier at the same time.

Read more: Do indoor plants purify air?

Lawrie is a journalist and editor who writes about healthcare, science, technology, engineering and design. He's worked for some of the UK and Europe's leading titles, making complex science as simple as possible. He also owns content agency 42group where he helps household brands, academic institutions and charities to grow through engaging, informative and inspirational words.