Filtered water is any water that has been passed through a device or material designed to remove impurities, chemicals, or particles. The term is broad: it covers everything from a basic pitcher filter on your kitchen counter to a multi-stage system installed under your sink. What all filtered water has in common is that something unwanted has been physically or chemically separated from the water before you drink it.
How Water Filtration Works
At its simplest, a water filter forces water through a material that traps or absorbs contaminants while letting clean water pass through. The specific method determines what gets removed. The three most common technologies in home filtration each work differently.
Activated carbon filters are what you’ll find in most pitcher filters and refrigerator filters. The carbon is manufactured to be extremely porous, creating a massive internal surface area. As water flows through, chemicals and organic compounds stick to (adsorb onto) the surface of those tiny pores. Carbon filters are effective at removing chlorine, improving taste and odor, and reducing some heavy metals like lead. They’re the most affordable and accessible option.
Reverse osmosis (RO) systems push water at high pressure through a semi-permeable membrane that blocks contaminants based on their molecular size. Water molecules pass through; larger molecules containing lead, copper, chromium, sodium, and many other dissolved substances get rejected and flushed away. RO systems are typically installed under the sink and involve three to five filtration stages, often including a carbon pre-filter.
Ion exchange filters use tiny resin beads that act like chemical magnets, attracting and holding specific contaminants as water passes through. These are particularly useful for targeting specific dissolved substances that carbon alone can’t catch.
What Filtration Actually Removes
The CDC notes that most home water filters, like pitchers and fridge filters, are not designed to remove germs. They primarily use carbon to improve taste and reduce heavy metals like lead. If you need to remove volatile organic chemicals from all your household water, a whole-home filtration system is more appropriate.
Reverse osmosis and under-sink two-stage systems (combining mechanical and activated carbon filters) can remove a much wider range of contaminants. Research published in 2020 found these systems are capable of removing nearly all types of PFAS, the “forever chemicals” linked to cancer, liver damage, and reduced fertility. The EPA confirmed that granular activated carbon, ion exchange, and reverse osmosis systems can all greatly reduce PFAS levels in drinking water.
One important tradeoff: filters remove both harmful and beneficial substances. A carbon filter that strips out chlorine is also removing the chemical that kills germs in your water supply. Some filters reduce fluoride, which helps prevent cavities. Knowing what your specific filter targets helps you weigh the benefits.
Filtered vs. Purified vs. Distilled Water
These terms overlap but aren’t interchangeable. Filtered water is the broadest category, covering any method that removes some impurities. Purified water has gone through a multi-stage process (often reverse osmosis) that removes nearly all contaminants while typically retaining some beneficial minerals. Distilled water is made by boiling water and capturing the steam, which leaves behind virtually everything, minerals included. It’s the purest form but also the flattest tasting, and it’s more commonly used in medical or industrial settings than for everyday drinking.
For cooking and drinking, purified water through reverse osmosis is generally considered the best balance of safety and taste. Basic filtered water offers improved flavor and clarity but has limited effectiveness against many contaminants. Distilled water, while extremely clean, lacks the minerals that contribute to both taste and nutrition.
The Mineral Question
Reverse osmosis is powerful, but that power comes at a cost. RO membranes remove 92 to 99% of beneficial minerals along with contaminants. Calcium and magnesium, both important for bone and dental health, drop by 94 to 98%. In countries that rely heavily on RO-treated water, calcium content in drinking water falls below 6 mg per liter, virtually eliminating it as a dietary source.
If you use an RO system, you can add minerals back. Options include a remineralization filter stage (many RO systems offer one), an alkaline water pitcher, or trace mineral drops. These are simple additions that restore the calcium, magnesium, and other minerals that make water taste fuller and contribute to your daily mineral intake. Basic carbon pitcher filters, by contrast, leave most minerals intact since they primarily target chemicals rather than dissolved minerals.
How to Know a Filter Does What It Claims
Not all filters are tested equally. The certification system to look for is NSF/ANSI, which sets independent performance standards. Three standards matter most for home use:
- NSF/ANSI 42 covers aesthetic improvements: chlorine, taste, odor, and particulates. If a filter is certified to this standard, it will make your water taste and smell better.
- NSF/ANSI 53 covers health-related contaminants. This standard offers over 50 specific reduction claims, including lead, the parasite Cryptosporidium, volatile organic chemicals, and chromium. For PFAS reduction, look for certification under this standard.
- NSF/ANSI 401 addresses emerging contaminants, including pharmaceutical traces and other compounds increasingly detected in water supplies at low levels.
For reverse osmosis systems specifically, NSF/ANSI 58 is the relevant certification. The EPA recommends looking for either NSF/ANSI 53 or NSF/ANSI 58 on product labels when your goal is reducing PFAS. These certifications appear on the product packaging or the manufacturer’s website, and you can verify them through NSF’s online database.
Why People Filter Their Water
Municipal tap water in the U.S. is regulated and generally safe, but “safe” and “ideal” aren’t the same thing. Aging infrastructure introduces contaminants after water leaves the treatment plant. Lead from old pipes is a common concern. One London physician reports regularly seeing lead toxicity from old water pipes, where home filters can make a meaningful difference.
Trace amounts of pharmaceuticals, including hormones from contraceptives and certain psychiatric medications, have been detected in water systems. While these levels are extremely low, some people prefer an extra layer of protection. PFAS contamination is a growing concern in many communities, and home filtration is one of the few practical steps individuals can take to reduce exposure.
In settings where water safety is less assured, the impact of filtration is dramatic. A trial that installed filters in homes across 16 villages in the Dominican Republic saw diarrheal illness drop from 25.6% to under 10%.
Then there’s taste. Chlorine is added to municipal water to kill pathogens, but many people find the smell and flavor off-putting. A basic carbon filter solves this immediately, which is why pitcher filters remain the most popular entry point into home filtration.
Keeping Your Filter Effective
A filter that isn’t replaced on schedule can become less effective or, in some cases, release trapped contaminants back into your water. General replacement timelines vary by type:
- Carbon filters (pitchers, fridge filters, faucet-mount units): every 6 to 12 months.
- Sediment filters: every 6 to 9 months for typical home use, or every 3 to 6 months in high-demand settings.
- Reverse osmosis systems: pre-filters every 6 to 18 months, and the RO membrane itself every 2 years.
Your actual replacement schedule depends on how much water you use and the quality of your incoming water. Harder water or water with more sediment will exhaust filters faster. Many modern systems include indicators that signal when it’s time to swap a cartridge, but tracking the installation date yourself is a reliable backup.