Carbon or charcoal has been used to filter since at least 3,750 BC, when the ancient Egyptians used it to remove smells. In the ensuing five and a half millennia it was used by seafarers to keep water clean on long voyages. Today, most air filters have what is called activated carbon, which is carbon or charcoal that has been specially treated to have extensive networks of nano-sized tunnels to trap the maximum amount of contamination. This type of air filter is very effective when removing indoor air pollutants like VOCs and ozone.
Activated carbon works because contaminant molecules stick to it, so after time all of the possible sticky spots are used up and it needs to be discarded and replaced. Most manufacturers of activated carbon filters offer a duration for usage, but every indoor space is different with different sources of pollution and different ventilation with the outside. A recent paper in the journal Chemosphere* by a combined research team from the University of South Florida and Florida Polytechnic took a very close look at carbon filters and how long it might take them to be saturated both during normal conditions and during extreme air quality events like wildfires. What they found might give you a better idea of how long your carbon air filter may last and when it may stop being useful.
Sources of VOCs in the home
VOCs are volatile organic compounds, which typically enter the home as liquids in bottles or sealed containers but become airborne at room temperature. Indoor sources of VOCs both contribute to poor air quality and are very common, including any electronic products, personal care products, building materials, furniture, printers, cooking, and many others. Because we spend most of our lives at home and VOCs cause the biggest problems when concentrated indoors, it’s important to know how they impact your air.
You will usually be able to tell when there is a high concentration of VOCs in the room because there will be a strong smell, though some VOCs are odorless. The overpowering smells when use of scented cleaning products, essential oil diffusers, or applying lacquers all indicate very high levels of VOCs.
Some VOCs are toxic, such as components of gasoline or formaldehyde from cigarette smoke. VOCs like essential oils aren’t toxic right out of the bottle, but when they react in the air with bleach, ozone, or other oxidizing agents they can form very small particles that may be responsible for more disease than outdoor particles. One study said that cleaning while using bleach is like running a car in your home.
It is more rare that an outdoor source of VOCs impacts indoor air quality and usually only happens in certain places or during specific events. Wildfires or burning of crops are events that can raise outdoor VOCs to be a problem. Homes near refineries or other industrial operations are also at risk for VOC exposure.
What are acceptable indoor levels of VOCs?
Total VOC levels over about 500 parts per billion or ppb are where research seems to indicate they could start to affect air quality, and over 3,000 ppb will make the air quality bad. Without a total VOC monitor, however, it will be impossible to know the ppb levels in a room. In addition, because different VOCs have different toxicity levels, spilled gasoline and spilled wine might result in similar VOC levels though gasoline is much worse to inhale.
Most studies indicate that VOCs are always present but in widely varying amounts. Homes in America have been found with an average baseline of 150 micrograms of VOCs per cubic meter, which is anywhere from 25 to 75 ppb depending on the mix of VOCs. Some homes were as high as 2,200 micrograms per cubic meter, which is 500 to 1200 ppb. Short-term spikes can be even higher than this. Wildfires can raise VOCs by many times, with affected regions hitting 2,400 to 12,000 ppb.
In order to be sure to cover the average levels but also spikes from events or unique household conditions, the Florida team exposed activated carbon to several different VOC concentrations from very low levels to the equivalent of a major wildfire. Most data about activated carbon has been on extremely high concentrations only found in manufacturing, finding out how they work for wildfires or household products can help with indoor air quality.
Activated carbon removes VOCs
Activated carbon air filters are made from plant material like coconut shells that have been specially prepared to be full of tiny holes, as you can see in the image here.
Every nook and cranny in the individual carbon grains is a place where VOCs accumulate and technically turn back into a liquid state. The Florida research team compared 45 combinations of different activated carbon media and VOCs to investigate how accumulation changes over both time and temperature. Over time the carbon can become saturated, and changes in temperature, humidity, or in the composition of the air can dislodge captured VOCs. It’s important to change carbon filters for this reason.
Different VOCs also interact with carbon in very different ways. The team selected three groups of VOCs and tested them against different types of activated carbon air filters.
Group A has VOCs in the aliphatic hydrocarbon family, which includes a lot of gasses we use for heat, like propane and acetylene. These gasses don’t have much direct impact on our health, but in large concentrations can displace oxygen, all of them are explosive, and leakage from natural gas fixtures contributes to climate change.
Group B has some aromatic hydrocarbons, chlorinated hydrocarbons, and a few others. Many of these gasses have a handful of neurotoxic and carcinogenic properties.
Group C contains some of the same hydrocarbon families as the other groups, but also several long-chain hydrocarbons and plant-derived terpenes.
Let’s take a look at what the team found out about when to do that and if extreme air quality events should change the schedule.
How much activated carbon is needed for VOC removal
As expected, the results varied widely across different VOCs and different types of activated carbon, but the team was able to narrow down a median amount of carbon. To paraphrase the authors, the median amount of activated carbon needed at typical indoor levels (0.1 to 1.0 micromoles per cubic meter) is 2.7–3.7 kg for Group B and 10–60 g for Group C. At elevated levels (10 micromoles per cubic meter) would require a median of 6.4 kg for Group B and 285 g for Group C. Group A was much more difficult to capture and mostly required hundreds of pounds of carbon.
The first interesting aspect of this study is to note that VOC efficacy does increase linearly. In this experiment, reducing the VOC concentration by 90% or even 99% does not increase the lifetime of the carbon appreciably.
Another interesting aspect of the data is that different low levels of VOCs did not seem to impact the removal ability of the carbon. The team looked at three basic groups of VOCs, and for the most part, at low levels the concentration had little effect on how long the carbon was able to last.
Average kilograms of activated carbon needed for typical indoor concentrations (0.1 to 1.0) or extreme conditions (10 to 10,000).
This graph shows the average amount of carbon needed across each group. The authors point out that for different combinations of VOCs, particularly at concentrations found during wildfires, the amount of carbon needed could increase by an order of magnitude, or around ten times.
The authors go on to say that an air filter with 15 kilograms of carbon might be good enough for 30 days during wildfires, but that it would have to be an impractically huge device.
At Molekule we are always investigating air purification technology. Keep an eye on this blog for more on the latest in air purification research.
*Jaspreet Dhau is the Vice President of Research and Development at Molekule and was a co-author on this peer-reviewed paper.
Header photo by Anete Lusina.
