Coconut or coal-based carbon: which is better for VOCs

Why the type of carbon matters, not just the amount

Activated carbon traps gases by adsorption: molecules stick to the walls of millions of microscopic pores. The amount of carbon decides how much it can hold before becoming saturated; the type of carbon decides which molecules it holds best, because that depends on the size of its pores. We have already covered how VOCs are removed in our article VOCs at home: what they are and how to really remove them: here we focus on the material.

Micropores and mesopores, in simple terms

Coconut shell and coal-based carbon compared

Air and water filters mainly use two materials: carbon made from coconut shells and carbon made from bituminous coal. According to a technical article published in the trade magazine Water Conditioning & Purification, coconut shell carbon has the most micropores, about 50% more than coal-based carbon, which instead has more mesopores and macropores.

In the same magazine, a laboratory test in water measured how much benzene one gram of carbon holds before saturating: about 11 mg for coconut shell carbon versus about 6 mg for coal-based carbon. It is a useful indication of the trend, but it concerns water, not air, so it should not be read as the performance of an air purifier.

What the air studies say

University of Texas (2013): it depends on concentration

A study by the University of Texas at Arlington compared two commercial carbons, one coconut-based and one coal-based, in adsorbing several gases (xylenes, ethylbenzene and methyl ethyl ketone). With methyl ethyl ketone the two carbons performed similarly. With xylenes and ethylbenzene, however, the coconut carbon held more at low concentrations, the coal-based carbon at high concentrations. The authors' conclusion: both work, but at different concentration levels.

Polytechnic University of Catalonia (2013): coconut carbon on real air

A team at the Polytechnic University of Catalonia tested a commercial coconut shell carbon filter on real air, taken inside and outside an ordinary building, with VOC concentrations much lower than those usually used in the laboratory. In a single pass, the filter removed on average about 65% of indoor-air VOCs and 62% of outdoor-air VOCs, and 100% of ozone. Very low humidity reduced its efficiency.

Why low concentration matters for home air

At home, VOCs are almost always present in very small amounts: traces of formaldehyde from furniture, solvents from paints, fragrances, cleaning products, exhaust fumes coming in through windows. This is exactly where the micropores of coconut shell carbon perform best. Coal-based carbon, with its wider pores, is better suited to the high loads typical of industrial settings.

There is no best carbon in absolute terms

Anyone claiming that one type of carbon is always superior is oversimplifying. For a home purifier, the type of carbon is one factor, alongside others that are just as important:

The carbon in Austin Air filters

Austin Air filters use coconut shell activated carbon, together with zeolite, in amounts of up to 6.8 kg in standard models and 2.7 kg in Junior models. Thanks to the 360° intake, air passes slowly through the carbon layer surrounding the HEPA filter, with a longer contact time.

• HealthMate: non-impregnated coconut shell carbon and zeolite, for odors and common VOCs
• HealthMate Plus: with part of the carbon impregnated with potassium iodide, more effective on formaldehyde and aggressive chemicals
• Immunity Machine: carbon, potassium iodide-impregnated carbon, natural mineral filtration and a mineral with a proprietary impregnation, plus HEGA cloth

To choose the right model you can use our buying guide or the model comparison, or write to us.

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Useful links

• VOCs at home: what they are and how to really remove them
• Formaldehyde at home
• Replacement filters guide
• All Austin Air purifiers

Sources: K. Schaeffer, R. Potwora, Coconut shell versus bituminous coal activated carbon, Water Conditioning & Purification; Advantages of water treatment with coconut shell carbons, Water Conditioning & Purification; A. Athappan, M. Sattler, A comparison of bituminous coal-based and coconut shell-based activated carbon for removal of trace hazardous air pollutants in landfill gas, University of Texas at Arlington (2013); E. Gallego et al., Experimental evaluation of VOC removal efficiency of a coconut shell activated carbon filter for indoor air quality enhancement, Building and Environment 67 (2013); official Austin Air Systems technical data.

Note: study data refer to specific test conditions and do not describe the performance of any single purifier. Performance may vary with conditions of use and maintenance. Air purifiers are not medical devices.