... and why sea salts are not good for your cold tub.
Epsom salts are the only approved additives for Morozko Forge ice baths. This article reviews the potential risks and benefits of popular sulfate salts in home ice baths, solely for the purpose of general education and entertainment. The information contained herein is not medical advice, nor a recommendation. Morozko Forge makes no warranty about the efficacy, safety, or results that may be obtained by use of sulfate salts in any brand of cold tub, cold plunge, or ice bath.
Epsom Salt Ice Bath Summary
Chloride salts are not for use in cold tubs with ozone, because they interact with ozone in ways that interfere with disinfection and may result in harmful byproducts.
Magnesium sulfate (Epsom salt) softens ice and may provide health benefits. However, magnesium is a also fertilizer that can promote a green algae bloom requiring extra filtration -- especially in stainless steel tubs that are deficient in zinc.
Zinc galvanized tubs already resist algae growth, because zinc from the tub coating enhances the anti-microbial properties of the ozone filtration. Also, zinc is an essential mineral for metabolic and immune system health. Adding zinc sulfate salts to stainless steel tubs may boost water clarity and provide some transdermal zinc benefit.
Copper is a powerful anti-microbial, anti-fungal, and anti-algae agent, as well as an essential trace mineral. Adding a copper sulfate to a zinc galvanized tub will dissolve some of the zinc coating, and deposit a black dusting of copper metal in its place (video). In a stainless steel tub, copper sulfate may enhance water clarity and provide some transdermal copper supplementation. However, in excess copper can be toxic to humans.
Potassium sulfate is an electrolyte that increases water conductivity, and lowers the freezing point. Transdermal absorption of potassium thru the skin during or following an ice bath likely provides some electrolyte supplement to the bather that supports cold thermogenesis.
Salt safety
When we first wrote about Epsom salts (magnesium sulfate) for ice baths in Salt Water Cold, the article zoomed to the top of Google rankings for people searching for good information on both health benefits and temperature effects.
Competing cold plunge companies noticed the attention we were getting, and soon came out with their own search-engine-optimized (SEO) ads touting the historic health benefits of Epsom salt cold water baths, even if they could make no claims about one of the most interesting aspects -- how adding salt can both soften and help build more ice.
Nonetheless, not all salts are suitable for, or safe, to use in your ice bath or your cold plunge. The two most popular types of bath salts are:
Epsom salt, which is pure magnesium (cation) and sulfate (anion), and
Sea salt, which contains a mixture different cations and anionic chloride.
We only recommend Epsom salt, because it provides you with trace quantities of essential magnesium, which is a critical benefit for cold thermogenesis. However, the chloride in sea salts can form chlorinated compounds that are known to cause cancer.
Keep sea salt out of your ice bath.
Never add chlorine, or chlorine bleach, or chloride salts of any type to your ice bath, because the results might be unhealthy.
Ice bath salts
There's more to management of your ice bath salt balance than just adding Epsom salt every time you top up your water. The three important considerations are: 1) water quality, 2) health, and 3) thermodynamics.
We're going to talk about each of them individually, and the relationship between them, in the paragraphs below.
Water Quality
Magnesium is not only an essential micronutrient for cold thermogenesis in people, it is also a powerful fertilizer for plants. For example, you can buy pure Epsom salt in the garden section of most hardware stores, to add to magnesium-deficient soils. And almost all modern soils are magnesium deficient.
Several customers, and our own experiences, have reported that when they first add several pounds of Espom salt to their Prism Forge, they notice a green tint to the water that appears overnight. The zinc galvanized coating in Filtered Forge tub helps suppress algal growth, but in the stainless steel tubs, that green tint may be the result of magnesium fertilization of algae spores that have been suspended in the water, invisible, since they first filled up the Forge with water.
Algae spores are ubiquitous in all water, and can even be carried between water bodies by the wind. The spores are harmless, albeit notoriously resistant to chemical attack, so they can be dormant in a water body for extended periods of time, like plant seeds waiting for the right conditions. However, when magnesium fertilization causes algae spores to germinate, they become visible, unsightly, and vulnerable to destruction by ozone.
Running the ozone filtration continuously for about 24 hours will kill active algae and clear the water, so you can enjoy magnesium in your Forge without allowing alga growth -- as long as you're running your filter longer than you did before adding Epsom salt.
There are other sulfate salts that I've experimented with in my Forge to research how different electrolytes might enhance protection against algae and other microbes. The first two are zinc and copper.
Both zinc and copper are essential human micronutrients (see Health below), and effective anti-microbial agents. That is, human beings thrive when they have the right amount of zinc and copper in their diets, but microorganisms like bacteria, fungus, and algae die.
The zinc galvanized coating in the Filtered Forge already provides some zinc to the water that is not present in the stainless steel Prism Forge. When we add a little zinc sulfate and a little copper sulfate to Prism Forge water, the result is a spectacularly clear water column that ozone cannot achieve alone. These two additional salts may help prevent biofilms from forming inside your filtration equipment (where the ozone is weakest), extend the time between required filter changes, and maintain extraordinary water clarity.
However, adding copper sulfate to the zinc galvanized tub in the Filtered Forge will create an ion substitution reaction that dissolves extra zinc into the water, and deposits a black dust of copper on the the surface of the tub. When I tested this in my Filtered Forge at home, I got results very similar to this video.
Health
Cold thermogenesis depletes the body of the electrolytes involved in energy metabolism. In one study, subjects were exposed to whole body cryotherapy at -70C for 3 min, while blood concentrations of critical electrolytes were monitored (Juravlyovaa et al. 2018). As expected, the cold stimulated release of magnesium stored in the bones, so that the mitochondria in brown fat had the magnesium they needed to drive non-shivering thermogenesis. To the extent that transdermal absorption can help meet magnesium demands for deliberate cold exposure, adding Epsom salt to your ice bath will help maintain healthy magnesium levels.
However, changes in potassium levels were even larger than magnesium -- perhaps because potassium gradients in the mitochondria regulate fat-burning metabolism (Garlid 1996). That is, while magnesium catalyzes energy production, potassium regulates energy transport.
The most common symptom of potassium deficiency is muscle cramping (Kardalas et al. 2018), suggesting that the addition of potassium sulfate salts to your cold tub may help relax muscles and maintain healthy potassium levels.
Both zinc and copper are micronutrients. For example, "over 300 enzymes require zinc for activation" and yet up to 2 billion people worldwide may be deficient (Prasad 2012). In particular, zinc is essential for cardiovascular health, including prevention of atherosclerosis (Choi et al. 2018). Moreover, zinc is essential for maintaining a strong immune system, preventing infection, and may aid in treatment and prevention of cancer (Chasapis et al. 2020).
Meanwhile, copper deficiency has been implicated in ischemic heart disease, accelerated mitochondrial ageing, and chronic inflammation. Copper is critical to mobilization and metabolism of fat, and higher levels are associated with less body fat (Collins 2021). Most people get too little copper in their diet:
Copper intakes have been declining and it appears that a large fraction of the population does not even consume the recommended daily allowance for copper (0.9 mg per day), let alone an optimal intake of copper (2.6 mg per day) -- DiNicolantonio et al. 2018).
While too much copper in the body can result in a syndrome called copper poisoning (Franchitto et al. 2008), adding zinc and copper sulfates to bath water in a stainless steel tub may suppress microbial activity and have the added benefit of ameliorating chronic dietary deficiencies.
Thermodynamics
It is well established that adding salt will lower the freezing point of water. The more salt, the lower the freezing point. This is why highway departments in the northern United States spread calcium chloride on the road to prevent ice build up.
The extent to which water can be cooled below the typical freezing point of 32F (0C) depends on a chemical property called ionic strength, which is a measure of the amount of salt dissolved. This is why ocean water freezes at a temperature closer to 29F, rather than 32F, like fresh water.
The more salt dissolved in the water, the greater the ionic strength, lower the freezing point, and softer the ice that eventually forms. For example, by adding 8-10lb of Epsom salt, it is possible to freeze the entire Forge into the slushy consistency of a Slurpie.
The danger in adding too much salt is the possibility of achieving water temperatures lower than 32F.
The freezing point of human blood was established back in 1930 as 31F (-0.55C). Thirty years later, two courageous British scientists sought to measure the freezing point of their own pinky fingers by applying a tourniquet to cut off blood flow while holding them in cold brine (28.6F, or -1.9C) for 7 straight minutes (Keatinge & Cannon 1960). They reported that each finger, when removed, "felt and sounded hard when tapped with a piece of wood."
While they didn't establish a reliable point estimate of the freezing point of skin cells, they likely succeeded in giving themselves a minor case of frostbite.
When adding sulfate salts to your ice bath, care must be taken to keep the temperature setting no lower than 32F, to avoid freezing the water inside your own skin cells and causing a frostbite injury. For example, we've measured temperatures as low as 31.7F in the Forge at our Studio in Phoenix AZ, and felt a big difference between that super cold temperature and the usual 34F we achieve without so much Epsom salt.
However, salt water ice chunks can be even colder than the surrounding water, creating the possibility that setting your temperature too low could subject the parts of your skin to temperatures that could cause the skin to freeze when in contact with the icy slush.
For example, two of our engineers tested themselves against 7 minutes in a salt water ice bath at 31F. Although they're both experienced ice bather, they experienced a mild form of frostbite called bath frostnip that feels a lot like sunburn. While frostnip is not expected to cause permanent skin damage, it causes soreness and red splotches on the skin that are uncomfortable, counter-productive to the purpose of cold water cryotherapy, and giving up one of the major safety advantages of an ice bath over frigid gas cryotherapy.
To minimize the risk of frostnip, maintain your ice bath at temperatures above 32F.
An additional note on aluminum...
The most popular chemical used in the treatment of water is called alum, which is aluminum sulfate. It works as a coagulant -- i.e., agglomerating tiny impurities into larger particles that can be removed by precipitation and filtration.
Zinc, copper, and magnesium sulfate also act as coagulants in water, although they are less than half as effective as aluminum sulfate. Still, part of the reason that the water stays so clear when we add these salts is because they make the filtration more efficient.
So why don't we recommend addition of aluminum to your Forge, to get the coagulation boost it has to offer?
Because aluminum is neither anti-microbial, nor a human micronutrient. Some studies have associated aluminum exposure with Alzheimer's, and when it comes to brain health, we're not taking any chances.
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.