Hydrogen Peroxide turned the #RioOlympics diving pool Green

August 17, 2016

This article is a follow-up to our last article about the Rio Olympic diving pool turning green overnight. Regardless of what happened in Rio, we at Orenda looked at the situation as a great opportunity to share knowledge, and apply it to a current event. What we have discussed are known, established practices in the aquatics industry. Our objective has been to simplify the conversation for everyone reading our website. It's a lot to absorb, and we encourage the discussion.

For the mystery of the green Olympic diving pool in Rio, officials announced that a maintenance person mistakenly dumped over 160 liters of hydrogen peroxide in the pool, thinking it was the right thing to do. It remains unclear if they thought it was a different chemical, or if they genuinely believed hydrogen peroxide belonged in a chlorine pool. It is also unclear if hydrogen peroxide in the pool is the only reason for it turning green, given the myriad of other reasons given already; reasons that also could have contributed to the problem.

Let’s put this new information in context, so we can try to make sense of what happened in Rio. If this latest information given to the public is true, one thing is for certain: those pools were not safe for people to be using.

What is Hydrogen Peroxide?


hydrogen peroxide

Hydrogen peroxide is a chemical compound—H2O2—which is a strong oxidizer, and weak sanitizer. It is typically diluted so it is safe for human contact, because pure hydrogen peroxide is harsh enough to burn skin. While it is most commonly used around the household for first aid, such as cleaning cuts and disinfecting wounds, such household products are heavily diluted (usually about 3-6% in solution). When you put hydrogen peroxide in the pool, however, the most common concentration is 35%.

Around the home, hydrogen peroxide is actually somewhat of a miracle substance. It has many different uses. Read this to learn more about 60+ uses of hydrogen peroxide.

For use in pools, hydrogen peroxide is an alternative sanitizer and oxidizer to chlorine. It also effectively wipes out free chlorine. They are conflicting chemicals, so adding hydrogen peroxide to a chlorine pool is a bad idea.

Chemical incompatibility

When chemicals conflict, the reaction and results are what we need to pay attention to. In the case of putting hydrogen peroxide in a chlorine pool, the reaction is:

Cl2 + H2O2 → O2 + 2HCl

Translation: hydrogen peroxide neutralizes chlorine. Without chlorine (or active hydrogen peroxide acting as a sanitizer), bacteria and other contaminants can pollute a pool in short order. The initial response from the Rio event staff was 'algae', which is starting to sound more plausible now. It's really hard for algae to grow so fast with proper levels of chlorine. But without a sanitizer, algae can reproduce every 3-6 hours.

This is not the only example of conflicting chemicals in the pool business. Here are some others to look out for:

Bromine and chlorine. Both of them are effective sanitizers for pools, but when mixed together, the pool can turn brown under certain circumstances. You may have heard that it is possible to convert a chlorine pool into a bromine pool, but not vice versa. This is because adding chlorine to a bromine pool converts the used-up bromine (bromide) back into free bromine. When you add bromine to a chlorine pool, on the other hand, the bromine does not go away but builds up over time, creating a chlorine demand. Therefore, when you shock the pool with chlorine, the residual bromine will convert the chlorine into, you guessed it, bromine. So when professionals test the water, it will indicate that there’s no chlorine, when there’s actually bromine.

Acid directly in chlorine. Pouring acid into liquid chlorine can cause a dangerous reaction. That said, when properly introduced, chlorine can be stabilized with cyanuric acid. Just don't mix them directly in high concentrations.

Trichlor and cal-hypo or bleach. This can cause a dangerous reaction. Yes, they are all forms of chlorine, but they are incompatible to use together. Never mix any types of chlorine together.

Acid and sodium bicarbonate. When adjusting alkalinity with sodium bicarb, do so before adjusting the pH with acid. Acid and sodium bicarb accomplish opposite goals. While sodium bicarb raises alkalinity and pH, acid drives them down.  If you pour them both in the same location, acid can destroy alkalinity and defeat your chemical goal.

Sodium Bicarb/Soda Ash and Calcium Chloride. If you have ever added a pH increasing chemical (like bicarb or soda ash) around the same time as calcium chloride, you know that it clouds up completely. This is because of how calcium interacts with carbonate ions, carbon dioxide and Hydrogen. It makes a frothy, white reaction that can cloud up your pool for hours or more.

Undiluted muriatic acid. Before adding muriatic acid to the water, dilute it. Dilution raises the pH of the acid, which reduces its alkalinity-lowering impact. Dilute muriatic acid to preserve your alkalinity, which is a buffer for pH.

Stain and metal sequestering agents and phosphate remover. Most stain and metal sequests are phosphonic acid (phosphate-based). Sometimes they are even if the label doesn't say so, because a phosphate test kit will not show orthophosphates. It is counterproductive to use these metal controlling products with phosphate removers. The phosphate remover them out, which will release whatever metals and calcium they were sequestering.

It is important to know that these side reactions are not regulated by the EPA or NSF. There is currently no requirement to label products in a way that discloses such interactive risks or byproducts.

Lessons Learned

It appears the staff in Rio allowed a mistake to get out of control. By adding hydrogen peroxide, they wiped out their sanitizer, and as a result, the pool turned green. The lessons here are pretty obvious.

  • Educate yourself before you pour something in your water, and learn how it reacts with everything else before you add it.
  • If you aren't sure, ask. The pool community is readily available for advice and there are plenty of forums online to learn more.
  • If things go really wrong (like a suddenly green pool), close it down until you figure out what's going on. Letting people swim in a questionable body of water is irresponsible, and subjects swimmers to unknown bacteria and contaminants.
  • Continue to learn. The moment you think you 'know it all' is the moment your guard goes down. With multiple operators, it's virtually impossible to control all variables in a pool. So pay attention and speak up if there's an issue.


All information provided is intended for educational purposes and is not implied to replace consultation with a qualified pool professional. It is recommended that all information from this or any other source is to be performed assuming individuals performing these functions will consult local state and federal requirements before you act upon it in any way. While this site attempts to provide information that may be relevant to you, no guarantees are made that some relevant information will not be missed. We recommend you consult a local pool professional before acting. 

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