Summary
Ozone disinfection is safer than chlorine or bromine, because it degrades into pure oxygen without producing harmful byproducts.
Most ozone generators built for hot tubs don't work -- both because the generators themselves are poor quality and because hot water destroys ozone.
The arc discharge ozone generator in the Morozko ice bath has been tested using both an oxidation-reduction potential (ORP) meter in the water (for efficacy), and an ozone gas sensor above the water (for safety).
Ozone is beneficial when used on your skin, but is harmful when breathed into your lungs. Whenever you smell ozone in the air above your Morozko, you know the disinfection is complete and it is safe to turn off your filtration.
Ozone disinfection protects your ice bath
The most powerful and safe disinfecting agent available for water treatment systems is ozone -- more powerful than chlorine and bromine, and safer because it consists only of pure oxygen. Ozone is a chemical rearrangement of O2, the usual form of oxygen in the atmosphere, into a less stable, more powerful combination of three oxygen atoms, O3. The third oxygen atom is what makes ozone such a strong oxidizing agent. It is the oxidizing power of ozone that destroys pathogenic bacteria and viruses, and cleans the water by attacking other organic compounds like algae, and old skin cells. Ozone even destroys pollutants like benzene, toluene, xylenes (BTX), methyltertbutylether (MTBE), and volatile organochlorinated compounds. It even destroys the deadly ebola virus (Davies 2015).
In short, ozone is terrific for cleaning water, which is why it is used in the most advanced drinking water treatment plants in the world. The USEPA recognizes that “ozone treatment has the ability to achieve higher levels of disinfection than either chlorine or UV” for destruction of water-borne pathogens (USEPA 1999). In fact, the USEPA recommends ozone for multiple benefits:
Ozone sanitizes even the most virulent pathogens on contact.
Ozone removes foul tastes, odor, and color from water.
Ozone oxidizes natural organic matter, clearing water of contaminants.
Ozone enhances filtration by promoting particle coagulation.
For water disinfection, ozone is twice as potent as chlorine and over 3000 times faster. - Tripathi & Hussain 2022.
So why isn't ozone used in swimming pools and in your municipal drinking water distribution system? Why do our drinking water engineers and pool health standards till poison us with chlorine and the byproducts of its use (like toxic trihalomethanes)?
Ozone is highly reactive and unstable
The problem with ozone has a very short lifetime in warm water. It can be so reactive that it degrade into oxygen and hydroxide ion before it has a chance to disinfect or destroy anything. That's why ozone cannot be stored in a tank, like chlorine. Ozone must be generated at the point of use, so it can treat the water before it degrades. For example, even where ozone is employed for drinking water disinfection, municipal treatment works will still add chlorine to the water before it leaves the treatment plant, so that the there is always residual disinfectant in the distribution pipes.
That's also why ozone isn't used in swimming pools, because unless you produce ozone 24 hours a day, 7 days a week, most pools would lose all their ozone to natural degradation less than an hour after the pool pump turns off. Chlorine and bromine are both more stable than ozone, and so it's easier (and cheaper) to maintain high concentrations in water for longer.
This is especially true for swimming pools during the warm summer months, when they are most popular. The high temperature will accelerate the degradation of the ozone, reducing effective concentrations of ozone in the water and inhibiting disinfection.
Temperature effects on disinfection & sanitation
Chlorine gets more effective as water temperature increases -- so much so that a massive amount of chlorine (e.g., 8ppm) can be required to meet disinfection standards in ice baths, even though the United States Center for Disease Control sets a safety threshold for chlorine levels in drinking water at only 4 ppm (CDC 2020). That's why chlorine is so problematic in cold water.
Ozone is the opposite of chlorine. Because ozone is more stable at lower temperatures, it is less prone to degrade in the ice bath (compared to hot tub) and will reach higher concentrations.
Warm water destroys ozone.
Cold water preserves it.
In that way, chlorine is better for warm water, while ozone is better for cold water. For example, in hot tubs, ozone is insufficient for disinfection because it degrades so quickly. That's why it is necessary to add chlorine (or bromine) disinfectant to hot tub water to ensure it does not carry dangerous levels of pathogenic bacteria or viruses.
High concentrations of ozone disinfect water very fast. It takes little more than seconds at ozone saturation levels to destroy even the strongest viruses. But in hot water, ozone has two disadvantages:
The saturation concentrations of ozone are much lower, because the hot water temperatures boil the ozone out of solution and into the air.
The heat speeds up the ozone degradation.
Because the Morozko is so cold, the disadvantages of ozone in hot tubs are advantages in the ice bath.
Your nose always knows
Your sense of smell is one of the first lines of defense against noxious fumes and airborne poisons. If you've ever visited a cheap motel pool, then you know what I'm talking about. They typically are over chlorinated to the extent that the odor of chlorine seems to permeate the walls of the pool room and contaminate the hallway outside. The odor is repulsive and your nose is trying to warn you that chlorine is poisonous and you should avoid it.
Ozone has a distinct smell that was once called "the smell of electricity." Most people have experienced the smell of ozone in Nature after a lightning storm. The giant spark of lightning ionizes the air and turns the oxygen in the atmosphere into ozone.
The ozone smell is partly responsible for the fresh scent most people notice after a thunder shower. However, my first memories of ozone are from when I was seven years old and I got an electric train set as a Christmas present. The small sparks from the train engine were enough to ionize some of the oxygen in the air around the Christmas tree, to create an odor for me of ozone and pine needles while I was sitting on the floor, operating my new toy.
It wasn't until graduate school that I found out the human nose can detect ozone concentrations in the air at concentrations between 1--50 parts per billion. In other words, your nose is an excellent ozone detector.
Although ozone in the water promotes health, ozone in the air is recognized by the United States Environmental Protection Agency (USEPA) as potentially dangerous pollutant because it is such a strong oxidant that in high enough concentrations it attacks the cilia in the throat and membranes in the lungs. Several regulatory agencies have established allowable thresholds for exposure to airborne ozone. For example, the Occupational Safety and Health Administration (OSHA) allows no more than 1 ppm for eight hours of workplace exposure. Exposure in excess of 5 ppm is considered an immediate threat to human health.
Ozone monitoring (air)
Although ozone in the upper reaches of the atmosphere is protects all life on Earth from the most harmful ultraviolet parts of the sunlight spectrum, ozone in our lungs is a dangerous pollutant. The big difference is that the upper atmosphere ozone, known as the ozone layer, occurs naturally and is essential, whereas ozone in city smog is artificial.
Because the Morozko system releases ozone bubbles into the ice bath water, the possibility exists that excess ozone will escape from the water into the air when these bubbles float to the surface and burst. That's when customers will likely detect an ozone smell that reassures them their ozone system is working.
Most people report the “smell of lightning” at concentrations of less than 0.01 ppm – one hundred times less than the limit allowed by OSHA. In its test facilities, Morozko monitors airborne ozone levels several inches above the water surface using a Forensics Detectors FD Ozone Detector with 0.1 ppm resolution. You may click thru the picture to buy one for yourself at Amazon if you want to monitor ozone levels at your business or in your home. At no time during the continuous operation of any Morozko ice bath have measured ozone levels exceeded 0.2 ppm.
Ozone monitoring (water)
The use of ozone in water is not dangerous or controversial. In fact, ozone therapy is beneficial for the skin. What's more, high doses of ozone have been used as an anti-ageing therapy by administering directly to the blood (which is not how a Morozko works).
Nonetheless, ozone levels in the water cannot be measured directly as the can be in water. Instead, we must measure something called oxidation-reduction potential (ORP) that measures the activity of electrons in the water. An ORP meter is a specialized piece of laboratory equipment that is easy to learn, but difficult to interpret. For example, I first used an ORP meter when I was a graduate student in environmental engineering studying water chemistry. Now I use a portable ORP meter called PINPOINT that is manufactured by American Marine in Connecticut, USA.
Once calibrated, the electrode can be held in place below the water line inside the tub by using suction cups to attach it to the side. Look for readings between 500-900 mV as an indication of sanitary conditions. Even though the real work of disinfection is taking place further upstream in an ozone contact chamber, where ORP can exceed 1000 mV, you're measuring the ORP in the tub itself, where the ozone will make contact with the bathers skin.
How much ozone for an ice bath?
In most home applications, it's not necessary to run the filtration in your Morozko for more than about 120 minutes a day. If you ever observe a problem with water clarity, then go ahead and run it overnight, but for the most part, you're going to be fine without running it all the time.
The Morozko Ice Bath, Ice Plunge, and Original Morozko Forge are equipped with 100 mg/L corona discharge ozone generation units manufactured especially for Morozko. However, the PRO and XL are equipped with dual ozone generation and injection systems to double the ozone dose to a maximum 200 mg/hr.
In commercial applications, we recommend keeping the ozone on whenever the Morozko is open for business and at least several hours every night. Given the higher bathing loads, it's sensible to use all the disinfection protection that the Morozko provides.
References
Center for Disease Control (CDC) 2020. https://www.cdc.gov/drinking-water/about/about-water-disinfection-with-chlorine-and-chloramine.html
Davies N. Technologies to fight Ebola. Engineering & Technology. 2015 Feb 1;10(1):76-9.
Tripathi S, Hussain T. Water and wastewater treatment through ozone-based technologies. In Development in Wastewater Treatment Research and Processes 2022:139-172.
USEPA. 1999. Wastewater Technology Fact Sheet: Ozone Disinfection. EPA 832-F-99-063
About the Author
Thomas P Seager, PhD is an Associate Professor in the School of Sustainable Engineering at Arizona State University. Seager co-founded the Morozko Forge ice bath company and is an expert in the use of ice baths for building metabolic and psychological resilience.