18 Things Reverse Osmosis Removes – And The 2 Things It Won’t

Reverse osmosis has been used to purify drinking water in the U.S. since the 1970’s. In fact, reverse osmosis is just about the best water purification method available, and can remove up to 98% of contaminants.

Reverse osmosis can remove common contaminants from water, including mineral ions, such as calcium and fluoride, and heavy metals, such as lead. Reverse osmosis is also highly effective at removing viruses and parasites from water.

Keep reading to discover ALL the contaminants that reverse osmosis removes, and importantly what it can’t remove and why.

18 Contaminants Reverse Osmosis Removes

1. Calcium and magnesium

Have you ever heard of hard water?

Hard water is water that has a high concentration of calcium or magnesium in it – and most of the U.S. has ‘hard’ or ‘very hard’ water.

Don’t get me wrong hard water isn’t actually bad for us – in fact, calcium and magnesium are actually minerals that are essential for our good health.

But, these minerals cause fouling and scaling.

That means if you live in an area with hard water, then calcium and magnesium end up in your household pipes.

Once in your pipes these minerals build-up and damage your pipes and also any connected appliances – like dishwashers and washing machines!

Don’t think that a reverse osmosis system is immune to scaling and fouling either. In fact, calcium and magnesium can build up on the semi-permeable membrane, essentially clogging it up.

But that’s easy to fix – just replace the membrane regularly.

If you are concerned about missing out on those minerals in your drinking water then you can always add them back in using mineral drops

2. Lead and Copper

It’s really important that any water purifier you invest in is able to remove lead and copper.

Why? Because, according to the U.S. Environmental Protection Agency (EPA) the main source of copper and lead in our drinking water is from the

“… corrosion of household plumbing systems… “

u.s. environmental protection agency

That means your own pipes could be adding metals to your tap water – and you wouldn’t even know it.

Contamination of drinking water by lead and copper is so common that the EPA has a special rule for it – It’s actually called the “Lead and Copper Rule”.

The rule means that there is not only a limit on how much lead and copper can be in tap water, but more importantly that the community and public water systems that treat our water, have to test the tap water at residences in their supply area.

Unfortunately, they don’t have to test all of them – so a reverse osmosis system is the way to go to guarantee you are not drinking metals.

3. Metalloids

A metalloid is a chemical element that is somewhere between a non-metal and a metal. Many people have never heard of the term ‘metalloid’ before, but most people have usually heard of some of the different types, such as:

  • Arsenic
  • Boron
  • Silicon

Arsenic is probably the most infamous metalloid when it comes to drinking water – and is considered a toxic pollutant.

The EPA says arsenic, like most metalloids, get into our water supply from the natural erosion of rocks.

Arsenic is especially high in our groundwater, so if you are among the 115 million people that drink well water, then you probably want a reverse osmosis to remove it.

If you are concerned about how much arsenic is in your water then Amazon has a great range of kits that specifically test for it. But be aware, the cheap ones ($25-50) have a really poor detection limit and some of the more expensive ones (>$215) often need you to send the water away to a lab.

A reverse osmosis will remove the arsenic, and any other metalloids, as it pushes the water through the semi-permeable membrane.

4. Chlorine

Ok, so chlorine isn’t exactly a contaminant.

Water treatment plants actually use chlorine to disinfect our drinking water!

But have you ever noticed how sometimes your water smells like chemicals and doesn’t taste right?

Well, some chlorine can stay in the water after treatment, which can make it taste and smell bad.

The carbon filter in most reverse osmosis systems is effective at removing chlorine, and improving the taste and smell of the water at the same time.

But, not every reverse osmosis system has the correct carbon filter to remove chlorine.

Only reverse osmosis systems with National Sanitation Foundation certification 42 (NSF-42) are guaranteed to remove chlorine.

5. Fluoride

Like chlorine, fluoride really isn’t a contaminant.

In fact, fluoride is purposefully added to our water by the water treatment plants.

Fluoride is added to our water to make our teeth strong, and also to reduce tooth decay and cavities.

The semi-permeable membrane removes the fluoride ions from the water as it flows through the reverse osmosis system. Most of the fluoride is removed, though it is never possible to remove 100%.

In some ways it’s a shame that reverse osmosis is so effective at removing fluoride from our water, because fluoride does such a good job at protecting our teeth. But, you can always buy toothpaste that has fluoride in it – that way you won’t miss out on the benefits.

To read more about the benefits of fluoride, its history in our water supply and why it’s added to our tap water, click here.

6. Amoeba

There are a TON of nasty things in our water that are too small for us to see.

Amoeba are one of those things – and they can actually be fatal!

Fortunately, the semi-permeable membrane of a reverse osmosis system can remove amoeba from water.

But not every system is certified to remove amoeba. A reverse osmosis system must have NSF-53 or NSF-58 certification to remove amoeba.

If a system doesn’t have this certification but has an “absolute” pore size of 1 micron or smaller then it can also remove amoeba.

There is a lot to learn about amoeba in our drinking water, so if you want to know more then check out this post!

7. Parasites

While amoeba are nasty (and potentially deadly) they are just one type of parasite that contaminates our water supply. 

Many different parasites can live in our water and make us very sick if we ingest them. Cryptosporidium and Giardia are two of the most common parasites that can live in our tap water.

A reverse osmosis system is able to remove parasites through the semi-permeable membrane – but remember, the membrane has to be clean and replaced regularly to ensure that it is able to do its job.

As these parasites are removed from the water they can attach to the semi-permeable membrane, so when it comes time to clean your system and replace the filters make sure you follow the Centers for Disease Control and Prevention’s (CDC) advice and wear gloves!

8. Viruses

According to the CDC…

“Reverse Osmosis Systems have a very high effectiveness in removing viruses”.

centers for disease control and prevention

Viruses that can survive in water are a major problem, because water provides them with an almost perfect means of transmission – from place to place and to people.

Reverse osmosis can remove many viruses, including:

  • Enteric
  • Hepatitis A
  • Norovirus
  • Rotavirus

9. Cyanotoxins

Most likely, you have never even heard of cyanotoxins – poisonous substances that can cause liver, nerve and even brain damage(Ref 1)!

Cyanotoxins are toxins made by cyanobacteria, which are also known as blue-green algae.

Cyanobacteria breed in our water supply in the warmer months. Often causing harmful algal blooms.

During these algal blooms it is almost impossible for our water treatment plants to guarantee our drinking water is truly clean.

But, according to the EPA, reverse osmosis is effective at removing both cyanobacteria and their cyanotoxins.

10. Pesticides and Herbicides

While reverse osmosis is effective at removing most contaminants, not all reverse osmosis systems are the same.

So, while some reverse osmosis systems can remove pesticides and herbicides, there are some that can’t.

After a lab test of different reverse osmosis systems, the EPA found that some systems could remove >99% of pesticides and herbicides.

But, the EPA also found some systems performed really badly and only removed 23%!!

It turns out that only reverse osmosis systems that have a semi-permeable membrane made of thin film composite are able to remove pesticides and herbicides effectively.

Semi-permeable membranes that are made of thin film composite may also appear on the packaging as:

  • NS-100 (cross-linked polyethylenimine membrane),
  • FT-30 (cross-linked polyamide that contains carboxylate group),
  • DSI (modified polyalkene on a polysulfone base with non-woven polyester backing)

However, the really important thing to know is that if the semi-permeable membrane is made from cellulose acetate (CA) or polyamide then the reverse osmosis CAN’T remove pesticides or herbicides. Even though sometimes they might claim that they can.

11. Total Dissolved Solids

Total Dissolved Solids (TDS) is just a fancy way of saying ‘contaminants in water that are dissolved’ – which means we can’t see them.

I say ‘contaminants’ but in reality most are just naturally occurring minerals – But TDS can also be heavy metals.

Reverse osmosis systems can remove between 90% and 95% of TDS in your drinking water.

But, if you don’t already have a reverse osmosis system and you’re worried about the TDS in your water then you can check out your TDS level yourself.

You can pick up a TDS monitor from Amazon that are super easy to use.

12. Sediment

Unlike TDS, sediment is definitely visible to the human eye.

Sediment can be any type of particle, but it’s often things like sand, silt, mud and dirt.

The pre-sediment filter on a reverse osmosis system does the grunt work here, removing the sediment from the water long before it reaches the carbon filter and semi-permeable membrane.

13. Radionuclides

I wouldn’t blame you if you hadn’t heard of a radionuclide before – but they are certainly something that you should be aware of.

Radionuclides are a type of radioactive element, such as:

  • Radium
  • Uranium
  • Alpha particles
  • Beta particles
  • Photon emitters

Radionuclides can be in any water supply at any time, but are particularly problematic in domestic wells, because they aren’t treated by a commercial treatment plant.

However, according to the EPA…

“It (reverse osmosis) can remove up to 99 percent of these radionuclides”.

u.s. environmental protection agency

14. Nitrate

Nitrate is colorless, odorless and tasteless – so it is almost impossible to know if it is in your tap water.

Plus, nitrate is a serious problem in our water. In fact the Environmental Working Group (EWG) has it listed as one of the more concerning contaminants in our drinking water.

Nitrate can get into our water supply from:

  • Agriculture
  • Industrial areas
  • Septic systems

While reverse osmosis can remove nitrate it’s important to know that carbon filters and standard water softeners do not remove nitrate – so don’t try them.

And even more important, is to NOT boil water. Boiling water actually INCREASES nitrate concentrations!

15. Sulfate

Sulfate can make our drinking water taste and smell like rotten eggs. It can even have a laxative effect on us when we consume water with a high concentration of sulfate.

Sulfate can enter our water supply naturally, but can also come from contaminated rain – or acid rain, as it’s also known.

The semi-permeable membrane is able to remove around 99% of the sulfate as the water flows through the reverse osmosis system.

16. Trihalomethanes

Trihalomethanes are a by-product of chlorination.

Water treatment plants add chlorine to our water to disinfect it, but some chlorine can actually combine with organic materials to form this contaminant.

Trihalomethanes include contaminants such as chloroform, and are related to a range of health issues, includingRef2:

  • Cancer
  • Liver and kidney damage
  • Nervous system damage
  • Reproductive problems

So, it’s a good thing reverse osmosis can remove them!

17. Haloacetic acids

Just like trihalomethanes, haloacetic acids form from chlorine (or chloramine) bonding with organic materials, like leaves.

The effects of consuming haloacetic acids are similarRef2, and include:

  • Cancer
  • Developmental problems
  • Reproductive problems
  • Liver, kidney and spleen damage

Haloacetic acids are definitely not something I want in my drinking water.

18. PFAS

Per- and Polyfluorinated substances (PFAS) are man-made chemicals that once introduced into the environment, stay in the environment.

PFAS is so persistent, that if we consume any, then it stays in our body and accumulates.

PFAS is toxic and is currently estimated to be found in the drinking water of nearly 110 million Americans.

It has become an urgent problem all over the country, because of the use of PFAS in fire-fighting foam. Especially on and around military bases and airports.

According to the EPA, reverse osmosis is…

“…more than 90 percent effective at removing a wide range of PFAS”.

u.s. environmental protection agency

But remember, a reverse osmosis system will only be able to remove PFAS, and other contaminants, if it is clean and well maintained. Always keep up with the maintenance schedule and replace your filters and membrane regularly.

Now it’s time to talk about…

The 2 things that reverse osmosis CANNOT remove.

Carbon Dioxide

A reverse osmosis system is unable to remove carbon dioxide from water because it is a gaseous component.

Gases are able to pass unhindered through the semi-permeable membrane that stops so many other types of contaminants.

But don’t worry, any carbon dioxide that is present is completely harmless and wont even affect the taste or smell of your water – so drink up!


Radon is a radioactive element, just like uranium.

But, while reverse osmosis is able to remove other radioactive elements, including uranium, it is unable to remove radon from water.

That’s because radon is actually a gas, a colorless, odorless, tasteless, radioactive gas – and as we discussed earlier, reverse osmosis can’t remove gases.

According to the CDC,

“30 to 1,800 deaths per year are attributed to radon from household water”.

centers for disease control and prevention

Unfortunately radon can only be removed by:

  • Aeration treatment
  • Granular Activated carbon

And special handling by a professional may even be required!


1B C Hitzfeld, S J Höger, and D R Dietrich. Cyanobacterial toxins: removal during drinking water treatment, and human risk assessment.Environ Health Perspect. 2000 Mar; 108 (Suppl 1): 113–122.

2Zazouli, M. A., & Kalankesh, L. R. (2017). Removal of precursors and disinfection by-products (DBPs) by membrane filtration from water; a reviewJournal of Environmental Health Science and Engineering15(1), 25.

Theresa Orr

Theresa Orr is an Earth Scientist who specializes in determining past climates from rocks using geochemistry. Her passion for clean water drives her to breakdown the science to provide easy to understand information that everyone can read.

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