In a previous blog post, I wrote about how carbon can reduce VOCs. I tested activated carbon’s effectiveness at filtering out VOCs from cigarette smoke, and I also wrote about who actually needs activated carbon to reduce VOCs (hint: not everyone).
One type of VOC that many people are concerned about and want to reduce – especially in China – is “Formaldehyde”. It’s not because Chinese people are all nerds that they know the word formaldehyde. At dinner with Chinese colleagues, my friend David once used the word “carbohydrate” (碳水化合物) and was instantly made fun of 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 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, and HEPA filters 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.
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.
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.
Thus, to be absolutely sure we were detecting formaldehyde, my collaborator Anna bought bottles of liquid formaldehyde–risking our health for science!
To spread it in the room, we put it in a rice cooker along with 250 ml of water in a 4.14m2 porch (volume 10.35m3). When the rice cooker heats up, it releases formaldehyde as a gas into the air.
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 VOC is formaldehyde.
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:
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:
The formaldehyde levels stayed higher for longer in the fan-only condition, but the levels dropped much quicker when we used carbon.
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 and then down 0% by 25 minutes.
These composite activated carbon filters removed formaldehyde from the air. My earlier tests show that these carbon filters remove other of VOCs too.
Does everyone need carbon?
Studies have found that formaldehyde is much more common in new and recently remodeled homes, so people in new or remodeled homes probably need carbon. However, my MX6 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 because that would be a closer approximation of how most people would use it.
However, one difficulty of that sort of test is that the VOC detector wouldn’t be able to tell us if we’re detecting formaldehyde or other VOCs – hence we wouldn’t be able to detect 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 (although if anyone knows of an easier way to detect formaldehyde and only formaldehyde, let me know).
Thomas is a new Assistant Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.