Can Activated Carbon Remove Formaldehyde?

Can activated carbon remove formaldehyde? In earlier tests, Smart Air found that carbon can reduce VOCs from cigarette smoke. But formaldehyde is a particular concern for many people, so we put it to the test by ordering formaldehyde straight from the factory and testing it against carbon filters.


Formaldehyde is a big deal in China. I’ve found most of my Chinese friends know the word “formaldehyde.” But this is not because Chinese people are all science nerds. At dinner with Chinese colleagues, my friend David once used the word “carbohydrate” (碳水化合物), His friends instantly made fun of him for using a “science word” in casual conversation. Why the double standard? Because in China, just like carbs in America, formaldehyde is an everyday health concern.


What’s the big deal?

Why are they so concerned? Formaldehyde is a volatile organic compound (VOC) that causes scary health problems like ulcers and cancer. Yet it’s common in construction materials and new furniture. The people who should worry the most are people in new or recently renovated homes. Tests of new and renovated homes routinely find high levels of formaldehyde.


Why formaldehyde is so hard to remove

Formaldehyde is a tricky problem because – just like all VOCs – it escapes from materials as a gas. That’s tricky because the most common filters are HEPA filters, but they aren’t made to capture gases. I’ve heard companies claim that activated carbon can remove formaldehyde, but I’ve also seen companies selling sprays that claim to clear formaldehyde from your home. That sounds a lot like snake oil to me.


Formaldehyde spray China


Since there’s a profit motivation for companies to claim they can get rid of formaldehyde, I wanted to empirically test whether carbon actually works.


Formaldehyde Tests

In my quest for an answer, the first stumbling block was detecting formaldehyde. Many companies on Taobao will sell you a “formaldehyde detector”, but they’re actually general VOC detectors. (There are lots of volatile organic compounds; formaldehyde is one type.) So even if the machine tells you it’s detecting formaldehyde, you have no way of knowing whether it’s formaldehyde or some other VOC. These “formaldehyde” detectors even react to oranges.

Thus, to be absolutely sure we were detecting formaldehyde, Smart Air co-founder Anna bought bottles of liquid formaldehyde. We’re risking our health for science!


Formaldehyde solution China


To spread it in the room, we put it in a rice cooker along with 250 ml of water. When the rice cooker heats up, it releases formaldehyde as a gas into the air.We ran the cooker in a 4.14m2 porch (volume 10.35m3).


Formaldehyde activated carbon charcoal filter test


The Industrial Scientific MX6 detects different types of VOCs, not formaldehyde specifically. But because we released formaldehyde in the room, we can be sure that that VOC is formaldehyde.


Ibrid MX6 Industrial Scientific Review Test VOCs


To attack the formaldehyde, Anna put a composite activated carbon filter on the Cannon.


Smart Air Cannon purifier DIY carbon


Anna turned the cooker and the fan on at the same time and let them run until the VOC level fell back to zero. We also ran a control test with a fan only. We ran a total of three carbon tests and two fan-only tests.



Here’s what one of the carbon tests looked like, starting from the peak formaldehyde level:


Formaldehyde activated carbon charcoal filter test


Great, it looks like carbon can remove formaldehyde! But we need to be sure that’s the effect of the carbon, not just the formaldehyde dispersing over time. To do that, we need to compare those results to the fan-only condition. Here’s what the two tests look like side by side:


Formaldehyde activated carbon charcoal filter test


The formaldehyde levels remained high in the fan-only condition. But in the carbon condition, formaldehyde dropped much quicker.

I averaged across all three carbon tests and compared the average reduction compared to the fan-only condition. On average, the carbon reduced formaldehyde levels to 50% within 15 minutes of the peak formaldehyde levels. By 25 minutes, formaldehyde was down to 0%.


Formaldehyde activated carbon charcoal filter test



These composite activated carbon filters removed formaldehyde from the air. Earlier tests show that these carbon filters remove other types of VOCs too.


Does everyone need carbon?

Studies have found that formaldehyde is much more common in new and recently remodeled homes. That means people in new or remodeled homes probably need carbon. However, my Ibrid MX6 detector found zero VOCs in ALL apartments I tested, except for places that had recent renovation or smoking,


Now, there could be VOCs at levels lower than the MX6 can detect, so I’m not confident to say there are ZERO VOCs in most houses. But I think it’s reasonable to say that carbon filters are not mandatory for homes without obvious sources of formaldehyde or other VOCs.


Do I still need a HEPA?

Activated carbon is made to get smells and gases like VOCs. It is not designed to get particulate in general. In fact, activated carbon is made to be as porous as possible to get as much air into contact with the carbon. And my tests with a carbon filter alone show that it is does not remove high amounts of small particles. Thus, we still need a HEPA.


As always, I’m including the raw data and more details on the testing for fellow nerds.


The raw data is a large file, so I’m making it available as a download. Here is the summary data:





Check out my earlier VOC and carbon tests for more details on the MX6 detector, placement of the detector, and the fan-only control condition. Conditions were identical except for the pollution source.


How do we know the detector was actually detecting formaldehyde?

I wanted to test whether the MX6 was detecting the formaldehyde and not the heat and humidity coming from the electric cooker. To test that, I also ran a condition where I filled the rice cooker with water, but no formaldehyde. In that condition, the MX6 read zero:



That tells us that the MX6 wasn’t mistaking heat or water for formaldehyde.



Similar to my earlier VOC tests, one limitation is that the formaldehyde here was not naturally occurring. It would be ideal to find a house that was recently remodeled and already has formaldehyde in it. That setup would more closely approximate how most people face formaldehyde.


However, one difficulty of that sort of test is that the VOC detector can’t tell us if we’re detecting formaldehyde or other VOCs. Thus, we can’t be 100% sure whether the carbon is actually removing formaldehyde or other VOCs. In one sense, that’s not important–we want to get rid of all VOCs. But it would also be interesting to know if we’re getting formaldehyde specifically. To do that, we would need to take gas samples and have them sent to a lab.

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What is your suggestion for a filter system to put in our new manufactured home that warns of formaldehyde?

Measuring formaldehyde is not easy. See this article we wrote on how you can detect formaldehyde in your home. Short answer: your nose does a very good job of detecting it, and may well be more reliable than other devices out there.


My son makes large quantities of slime in our enclosed garage. I am concerned about the fumes from glue, fragrances and borax that he uses in bulk. My question is would the carbon filter be a solution at would you be able to recommend a purifier for our situation. Thank you.

Hi Jill, great question! Since smells are a type of VOC, then the carbon filter will be able to capture the smells and fumes from the slime! However, as we can’t be sure what VOCs the slime gives off it’s difficult to know just how effective the carbon filter will be at removing the VOCs. I suggest you pick up a carbon filter (they should be quite inexpensive) and make yourself a DIY carbon purifier. If after running it there’s a noticeable reduction in smells then you know that the carbon filter is working!


Love your site and the work you’ve done.
But –
1. Carbon is inefficient at removing formaldehyde compared to other VOCs.
2. Most PID meters that use a 10.6ev blub will not register formaldehyde effectively. The energy needed to knock off an electron of formaldehyde is higher
3. The PID meter you are using is rather low resolution at 100ppb. You would be much better off with a meter with 1ppb resolution. See the ppbRAE3000. That’s what I use in testing.

Hi Randy, Paddy here from our Beijing lab. Great points on carbon and the MX iBrid6. Carbon filters are not a perfect solution like you say, since there are many types of VOCs and it’s difficult to have a ‘one filter suits all’ kind of situation. That’s why the best way is still to solve the problem at its source, or to increase ventilation in your apartment (for example by opening the windows when PM2.5 levels are low). I’d not heard of the ppbRAE3000 before, but it looks like an expensive piece of kit! If you have any data comparing… Read more »


Carbon filters adsorb VOCs only for a rather short time of several weeks, then they saturate. In case your carbon filter really could collected a higher amount of formaldehyde, then the carbon filter is now loaded with dangerous gas molecules. If sun heats up your carbon filter now, it will out-gas all HCHO in a short time and you will face suddenly much higher formaldehyde concentrations in your room. Therefore carbon filters are actually pretty dangerous, if not monitored well for regeneration or replacement. A carbon filter acts only as a low pass filter or swamp, it does not remove… Read more »

That’s exactly right Prad. Carbon filters don’t actually break down the VOCs, just capture them. That’s why replacing carbon filters regularly and keeping an eye on when they are saturated is important. The concept of off-gasing can also be used to increase the useful life of the filter, by allowing the filter to off-gas in an outdoor environment. However, it’s difficult to have an accurate idea of how much this impacts the useful life and is therefore not recommended.

Noah Flesher

Does the activated carbon filter get attached first or after the hepa filter? Your site is hard to navigate and find the information about the products and how to use them. I’ve been using the Cannon for 4 years w/ the hepa filters. Just got some charcoal ones to add… wish it was easier to find things .. Thanks!

Hey Noah, Paddy here in Beijing. You’re right it’s pretty hard to navigate the website, we’re working on trying to streamline everything, but these things take a while. To answer your question: we generally suggest putting the carbon filter on the outside, as that ensures a better seal between the fan and the HEPA filter (the pre-filter acts as a good seal!). Here’s an image explaining it, but I’ve also added it to the cannon page. Thanks for your suggestion!

Activated carbon and cannon purifier filter stacking order