The most common method of filtration found in home water treatment products: carbon cartridges. These are reliable, effective tools for improving the quality of water that’s already certified safe to drink.

But, carbon filtering isn’t a cure-all when it comes to the most common contaminants and parasites that affect the taste and safety of drinking water. Let’s take a deep dive into carbon filters, and find out exactly what they are, how they work, and when or when not to use them.

What’s the carbon in a carbon filter?

It’s charcoal! Charcoal is a high-carbon compound created when organic materials such as wood are heated to high temperatures (around 1000 degrees) without oxygen. Anybody can make charcoal—it simply requires burning wood in a sealed box, usually made from steel or clay. This method is intended to remove non-carbonic elements from the material, leaving behind charcoal and ash.

Not all charcoal sources are made equal, however. Though wood is probably the first thought, other natural materials are capable of producing charcoal with a higher carbon density or purity, which in turn ups the potential surface for the ‘activation’ process—which is all-important for carbon filtration. Nutshells (often coconut shells) are the most common material for producing a carbon water filter.

How is carbon ‘activated’?

Activated carbon is highly porous charcoal that has been ground down to a fine powder and treated with a controlled oxygenating burn, which opens up countless micro-pockets or pores between the carbon atoms. The same ‘activating’ effect can also be achieved by adding a positive charge to carbon, which is intended to enhance carbon’s ability to attract and bind contaminants carrying an opposing negative charge.

Just like the standard production of charcoal described above, the idea behind ‘activating’ carbon is to create as pure a product as possible, removing almost all of the non-carbon parts of the material (be it coconut, wood, or other organic compounds). Activated carbon also has a reduced structure compared to the non-activated stuff, creating that the all-important high surface area. ‘Activity’ in this sense refers to the total amount of exposed surface area of the carbon after it’s been treated.

Most pitcher filters use activated carbon. Here are our most-recommended water pitcher filters.

Why ‘activate’ it?

Here’s where carbon’s filtering potential comes in. Once the surface has been increased with these millions of micro-pockets, activated carbon possesses an absolutely huge surface area. Developing this unusually large surface means that, when introduced to a contaminated water source, there’s an unusually high amount of contact between the carbon and the contaminants, greatly increasing the chances of contaminants binding—or to be specific, adsorbing—to the active sites on the filter.

In fact, one gram of activated carbon has a surface area in excess of 3,000 meters squared, say the Annals of Emergency Medicine. In other words, that’s 32,000 sq ft, or about three-quarters of an acre! That’s a whole lot of surface for contaminated water to pass through.

In essence, this theory of filtering is not so far away from a natural process of filtering through sedimentary rock. Water filtration in nature occurs when water is pushed over long periods of time through several layers of rock and sediment—resulting in spring water. Carbon filters achieve a similar process in miniature (and boosted through the principle of adsorption), by using a material with a crazy weight-to-surface-area ratio.

What is ‘adsorption’?

When a material adsorbs another material, it’s attaching to the surface of the adsorber through a chemical attraction. Adsorbing is something that activated carbon is really, really, good at. Thanks to its immense area and purity as a compound, activated carbon has countless bonding sites, maximizing opportunities for contaminants to attach and become trapped as water passes through the filter.

A standard carbon filter is capable of trapping molecules from 0.5 to 50 micrometers or microns (the same unit) from a water source inside its endless pore structure. Exactly how powerful a filter adsorptive ability is will be listed on an individual filter in terms of micron filtering size.

Likewise, how well and how long a carbon filter will remain adsorptive depends on the quality and amount of activated carbon used, plus a couple of other factors. For example, the slower water runs through a filter, the more time it spends in contact with the carbon, and the more chances there are for adsorption.

What solutes can be adsorbed by activated carbon?

Activated charcoal is great at trapping contaminants that, like itself, have a carbon base. These impurities are organic chemicals or compounds and are often referred to as ‘volatile organic compounds’ or ‘VOCs.’

(VOC is a term you’ll see a lot in materials related to carbon filters, as it’s really the bread and butter of carbon filtering.)

VOCs or carbon-containing chemicals include common water contaminants such as chlorine, pesticides, and herbicides. VOCs are widely used as ingredients in thousands of household products, many of which can eventually find their way into water sources. Examples include:

“paints and lacquers, paint strippers, cleaning supplies, pesticides, building materials, and furnishings, office equipment such as copiers and printers, correction fluids and carbonless copy paper, graphics and craft materials including glues and adhesives, permanent markers, and photographic solutions.”

Not all of the VOCs derived from the above sources are necessarily harmful, especially in the trace amounts in which they’re usually found in water. But, VOCs often contain a handful of ingredients that do pose legitimate health concerns. Here’s an in-exhaustive list from Livestrong of potentially harmful organic base contaminants, and their associated risks. (Again, most carbon filters are more than capable of removing these).

Side note: Carbon’s adsorptive ability also makes it a great air purifier, as well as being capable of removing toxins in the body, which is why you’ll find plenty of content on activated carbon in the medical literature. In fact, charcoal has been used since the earliest medicine practices as a gastrointestinal decontaminant. The Encyclopedia of Toxicology describes how charcoal is “neither absorbed in the gastrointestinal tract nor metabolized.” As a result, charcoal will help to remove toxins from the body by preventing them from absorbing into the bloodstream.

What cannot be adsorbed by activated carbon?

Carbon does boast some impressive filtering credentials. However, it needs to be pointed out that most of carbon’s claims when it comes to filtering are related to organic compounds only. The majority of inorganic chemicals are simply not attracted to carbon at all, meaning that they pass straight through any carbon filter stage. Therefore, carbon filters are generally not effective at removing minerals, salts, or other dissolved, inorganic substances.

Whatmore, these categories of contaminants contain some of the most harmful and regularly occurring materials found in tap water. Examples of inorganic materials include:

  • Lead (most sources are (inorganic)

  • Copper

  • Sodium

  • Nitrates

  • Silver

  • Mercury

  • Fluoride

Carbon water filter

Many viruses and microorganisms are also non-absorptive, or too small to be filtered by activated carbon. To rid a water source of these, the Water Quality Association recommends chemical or other kinds of non-carbon filtration. As any camper knows, halogens such as chlorine and iodine will easily deal with protozoa, while common home filter choices are ultraviolet (UV) light and reverse osmosis mechanisms.

As a general rule, if it’s an organic compound, it contains carbon. Therefore, it’s potentially filterable by activated carbon (depending on the compound’s size, and the filter’s ability). If the material is inorganic or non-carbon-based, then a carbon filter will not remove it through adsorption—but, the may filter still work, simply as a result of its ability to strain out particulates.

But my carbon filter says it’s certified for lead/non-organic compounds?

Some manufacturers do use various blends and add-ons to turn activated carbon into a material capable of advanced contaminant reduction. For example, some carbon block filters can also be engineered to remove lead. NSF-International certifications are the marker to look for in these cases. (Basically, it comes down to the size of microns able to get through the filter itself).

There’s a bit of a grey area here, as any decent filter will remove some amount of most solubles. This is partly because, as described, not all activated carbon is made equal, with literally dozens of filters with different densities on the market. As a general rule, however, the removal of inorganics can only be assured by a multistage filtering process, such as reverse osmosis or distilling.

Again, Always look to NSF international certifications for the final word on what an individual filter is capable of doing.

Which filters use activated carbon?

The short answer: almost all of them. Unless it’s a heavy-duty under-sink or point-of-entry filter, nearly all models will use a carbon cartridge. Even those more industrial models will utilize carbon as one stage of their filtering process.

Pitcher filters like the Brita filter are the most well-known carbon filters, but there are other designs and options, too. Faucet-mounted filters operate in a similar way, except they use the power of the tap to pass water through the carbon, instead of relying on gravity. There are also countertop models, and most if not all types of integrated refrigerator filters use their own activated carbon cartridges.

Read: Our in-depth guide comparing the different types of water filter.

How long does a carbon filter cartridge last?

Because it’s a physical process, adsorption as a filtering principle is time-limited. Once all of the binding sites in a piece of activated carbon are used up, the filter won’t be efficient anymore. This is why all carbon filters are designed with carbon cartridges, which can be replaced periodically.

In general, activated carbon water filters can be expected to last between 2 and 6 months, depending on their size. There are a few common sense things that can affect the lifespan of a carbon cartridge—how often you filter water being the most obvious. Poorer quality water, which requires greater filtering, will also have a significant effect. In other words, the more a filter does, the shorter its lifespan.

Summary

Carbon filters are the most ubiquitous home water filtering method, and for good reason. When carbon is activated, it’s huge potential surface area and adsorptive properties are unleashed, giving water contaminants plenty of opportunities to bind to the carbon’s surface as they pass through the filter.

However, be sure to know what you’re filtering for, as the majority of carbon filters only deal with organic compounds or Volatile Organic Chemicals. That means materials such as chlorine, with a carbon base. Non-organic compounds and protozoa will mostly pass straight through a carbon filter—though, seeing as carbon filters are only intended to be used on pre-treated drinking water, you shouldn’t really need to worry about these contaminants.