Up close: Activated carbon

The first people to notice the power of activated carbon were the ancient Egyptians, who used the material as medicine. In the modern era, we’ve adopted a host of other uses for the porous material. In World War I, for instance, it was a key ingredient in some of the first gas masks, and today we use it to do everything from filtering air to cleaning up oil spills.

The material’s filtration superpower lies in its porous structure. By opening up carbon’s pores (i.e. activating it), chemists expand the material’s surface area, which creates more sites to bond with, or capture, pollutants. In fact, a pound of activated carbon is so porous that its surface area could be four million square feet if flattened — that’s nearly 70 football fields.

The process of making activated carbon begins with heating raw materials, such as coconut husk and coal, to turn them into charcoal. From there, steaming — the most-popular activation method — stimulates a chemical reaction that removes carbon from the pore walls and enlarges them. During this process, chemists can alter the size of the pores to suit particular needs; water purification, for example, works best with more-open pores, while whitening sugar works better when pores are less open. When finished, the carbon can bond naturally to many pollutants.

In home air filters, in particular, activated carbon is helpful in trapping tiny smoke and odor particles and volatile organic compounds that can evade HEPA interception. As air flows into a filter, such as Airmega’s Max2, the carbon pores grab gases and vapors, while the HEPA filter works to trap larger particles like dust and dander.

Activated carbon filters do require upkeep. Because adsorption works by binding molecules to the carbon, the material can become “full.” At this stage, the overloaded pores allow chemicals to pass through and can even off-gas some of what they’ve captured, which is why it’s important to replace filters once they’ve spent.