EWG's Tap Water Database — 2021 UPDATE

water filter

Water filter technology: A primer


August 2020

There are hundreds of home water filters on the market, and choosing one can feel overwhelming.

The good news is that most filter products on the market use one or a combination of three primary technologies: carbon filtration, reverse osmosis and ion exchange. Once you understand how these technologies work, selecting the best one is much easier.

When you're weighing your options, keep in mind these considerations:

About the technologies

Carbon and activated carbon

Activated carbon chemically bonds with and removes contaminants as water flows through the filter. Activated carbon filters' effectiveness varies significantly. Some are certified only to reduce chlorine and improve taste and odor. Others can reduce the levels of contaminants such as asbestos, lead, mercury and volatile organic compounds, or VOCs. However, activated carbon does not remove common inorganic pollutants such as arsenic, fluoride, hexavalent chromium, nitrate and perchlorate.

In general, carbon filters come in two forms: carbon block and granulated activated carbon.

Reverse osmosis

Reverse osmosis systems sold for home installation typically include one or more activated carbon and sediment filters, allowing such systems to reduce or remove a large number of contaminants. The initial activated carbon treatment captures and removes chlorine, trihalomethanes and VOCs. Next, during the reverse osmosis filtration, tap water passes through a semipermeable membrane that blocks any particles larger than water molecules. As a result, reverse osmosis systems effectively remove many contaminants, such as arsenic, fluoride, hexavalent chromium, nitrates and perchlorate.

Wasted water is the primary drawback of reverse osmosis systems. These systems typically take in five times more water than they produce for use, and the unused “reject” water is flushed down the drain.

Additionally, reverse osmosis treatment removes minerals that improve the taste of water and are essential for health, such as iron, calcium and magnesium. Manufacturers of reverse osmosis systems may offer different options to address this problem, such as the addition of mineral drops for remineralization.

Ion exchange and water softeners

Water softeners typically use an ion-exchange process to reduce the levels of calcium and magnesium, which can build up in plumbing and fixtures, as well as barium and radium, which can be found in tap water. The levels of other contaminants usually remain unchanged.

Since water softeners replace calcium and magnesium with sodium, doctors may advise people with certain health conditions, such as those who wish to maintain a low-sodium diet, to avoid softened water. Softened water is not recommended for watering plants and gardens.


This technology heats your water enough to vaporize it and then condenses the steam back into water. The process removes minerals, many bacteria and viruses, and some chemicals that have a higher boiling point than water. But it does not remove chlorine, trihalomethanes or VOCs from water. Home distillation systems are less common than other types of available water filters available.

Whole house filters

Whole house filters are installed at the point where water enters your house, so that all your taps and appliances receive filtered water. This type of system is expensive compared to point-of-use filtration and is not necessary in most cases. Since whole-house systems also remove chlorine, they may introduce the additional risk of harmful bacterial growth in plumbing.

Whole house reverse osmosis filtration systems waste a large amount of water. For this reason, EWG recommends a point-of-use filter and suggests that reverse osmosis-treated water be used for drinking and cooking only.