... and why sea salts are not good for your cold tub.
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, LLC makes no warranty about the efficacy, safety, or health results that may be obtained by use of sulfate salts in any brand of cold tub, cold plunge, or ice bath.
Stainless steel Morozko Ice Baths are warrantied for use of Epsom salt (magnesium sulfate), zinc sulfate, potassium sulfate, and copper sulfate. However, the zinc-galvanized Original Forge will tolerate only magnesium, zinc, and potassium sulfate salts. It is not warrantied for use of copper sulfate.
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 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 I 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. However, competing cold plunge companies noticed the attention Morozko was 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.
I recommend Epsom salt only, because it provides trace quantities of essential magnesium, which is a critical benefit for cold thermogenesis without interacting with either the ozone or the metals in contact with water. By contrast, the chloride in sea salts contributes to decay of metal and 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
I add more than 4lb to my Morozko at home. There is almost no upper limit to how much can be added -- even up to more than 100lb -- because the solubility limit of magnesium sulfate in water is very high and all the Morozko components that come in contact with are salt tolerant. That is, I could turn my Morozko ice bath into a float tank if I chose, but for me it's not be practical.
Nevertheless, to maintain my salt concentration, I add additional Epsom salt whenever I top up the water, and there are three important reasons that I do that. They are: 1) water quality, 2) health, and 3) thermodynamics.
I will write 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 have reported that when they first add several pounds of Epsom salt to their stainless steel Morozko Ice Bath, they notice a green tint to the water that appears overnight. The zinc galvanized coating in Original 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 ice bath 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 ice bath without allowing algae 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 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 Original Forge already provides some zinc to the water that is not present in the stainless steel Morozko Ice Bath. When I add a little zinc sulfate and a little copper sulfate to a stainless steel ice bath, 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 Original 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 zinc-galvanized Original Forge at home, I got results very similar to this video.
Do not add copper sulfate to a zinc galvanized tub.
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.
In addition to supporting good water quality, 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 help remedy 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 bath 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, I've measured temperatures as low as 31.7F in the Morozko at our Studio in Phoenix AZ, and felt a big difference between that super cold temperature and the usual 34F achievable 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 the skin to temperatures that could cause it to freeze when in contact with the icy slush.
When two Morozko engineers tested themselves against 7 minutes in a salt water ice bath at 31F, they discovered a phenomenon called frostnip that feels a lot like sunburn. Although they're both experienced ice bathers, they weren't prepared to submerge themselves in icy slush for several minutes and they likely froze several out layers of their skin. When they called me the next day, both reporting symptoms of painful, red skin over every part of their body that had been submerged, we all realized that they'd overdone it.
To minimize the risk of frostnip, maintain your ice bath at temperatures above 32F.
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.
Ice Bath Salt Dosages
I purchase bulk sulfate salts from Amazon. These are the brands I've tested that have worked well for me:
Epsom salt https://amzn.to/3YM2gjp
Zinc sulfate https://amzn.to/3M64WRi
Potassium sulfate https://amzn.to/4coprn1
Copper sulfate https://amzn.to/4dDmqQQ
Although adding magnesium sulfate to your Morozko is very safe to do at very large dosages, and potassium sulfate need not be a cause for great concern, a word of caution is warranted with regard to copper and zinc.
Copper
USEPA drinking water standards limit allowable copper concentrations to 1 mg /L (1 ppm) in tap water (USEPA 2024). The typical 66" Morozko Ice Bath holds a little more than 500 liters (about 80 gallons), so the drinking water standard suggests that no more than 0.005 grams of copper be present in your Morozko to remain below the threshold.
You can purchase a copper test kit from Amazon using this link https://amzn.to/3SLXJtw to monitor and maintain your copper concentration. When used as an algicide in swimming pools, target concentrations are typically about the same range as the UEPA drinking water standard — i.e., between 1 mg/L and 1.5 mg/L of copper. Higher copper concentrations have been demonstrated to be more effective for inhibiting algae growth (Bishop 2017), however ingesting copper at drinking water concentrations exceeding 4 mg/l copper can create nausea (NPIC 2012). Drinking larger concentrations elicits stronger adverse reactions, such as vomiting. Because skin exposures at these concentrations are non-toxic, and may promote transdermal absorption, you might decide to experiment with copper concentrations that exceed 2 mg/l (2 ppm). At those concentrations, please avoid drinking the water in your Morozko.
Copper as a swimming pool treatment (not human consumption) is typically sold as copper sulfate pentahydrate, which is CuSO4-(H2O)5 -- that is, a copper sulfate molecule with five water molecules attached for a total molecular weight of 250 g/mol. About 25% of the total molecular weight of the larger molecule is from copper, so to get 0.0065 grams of copper in your Morozko, you'd have to add only 0.026 grams (26 mg) of copper sulfate pentahydrate. That’s a very small amount — only 0.000057 lbs, or a small fraction of an ounce. To put that amount in perspective, swimming pool guides recommend 1 tablespoon of copper sulfate pentahydrate per 1000 gallons of pool water. For a Morozko that holds less than 100 gallon, the swimming pool recommendation is for less than one tenth of a tablespoon. That’s just a few grains of the blue crystals of copper sulfate pentahydrate.
WARNING regarding copper supplements
If you are taking a copper supplement, such as copper glycinate, adding copper to your Morozko may result in total dose of copper that exceeds beneficial levels. For example, your total dose will be whatever you get from your supplement plus any transdermal (thru the skin) absorption obtained from your cold plunge practice. When adding copper sulfate pentahydrate to your Morozko, consider discontinuing or reducing your supplement dose. Should you experience adverse effects consistent with copper toxicity, such as nausea, vomiting, stomach pain, headache, dizziness, weakness, diarrhea, or a metallic taste in the mouth, discontinue your supplement and replace your Morozko water to remove the copper for several days.
Zinc
The USEPA drinking water standard for zinc is about four times greater than that for copper, at 5 mg/L (5 ppm). Although zinc is considered relatively non-toxic (Fosmire 1990) at high concentrations symptoms of zinc toxicity that are similar to copper toxicity may emerge -- i.e., nausea, vomiting, stomach cramps, and headache.
Although I haven't tested their accuracy, Zinc test strips for monitoring concentrations in water are available here https://amzn.to/3yF6LBM.
The molecular weight of zinc sulfate monohydrate (ZnSO4-H2O) is about 180 g/mol, of which 65 g/mol or 35% is zinc. To achieve a concentration of 5 mg/L zinc in a Morozko that holds 500 L of water, you need to add about 2500 mg of zinc, which is equivalent to 7000 mg (7 g) of zinc sulfate monohydrate. That's only a quarter of an ounce, or about one-eighth of a tablespoon.
You might want to add more -- up to a full tablespoon (which is what I do). Although this suggests that you will exceed the drinking water standard by eight times, zinc toxicity is far less concerning than copper (c.f. NPIC 2017). Symptoms of zinc toxicity typically do not emerge until total intake exceed 1000-2000mg (1-2 g) per day (Barceloux 1999) -- equivalent to drinking 200 liters (50 gallons) of water with zinc concentration at the USPEA drinking water standard. So even though I'm dosing my Morozko with eight times more zinc than that, I'm not expecting to ingest four gallons a day of my Morozko water.
The same caveat with regard to supplements applies to zinc, just as it does to copper. Discontinue use of zinc supplements and replace the water in your Morozko to remove zinc should you experience adverse side effects, including nausea, vomiting, diarrhea, or metallic taste in the mouth.
Recipe for Ice Bath Salts
A good place to start when adding ice bath sulfate salts to your Morozko is with small quantities that allow you to make observations of any changes. For example, I've had success with:
4 lb Epsom salt
1 lb potassium sulfate
1 tablespoon zinc sulfate monohydrate
several grains of of copper sulfate pentahydrate (in stainless steel tubs only)
I've also experimented with much larger quantities, including up to 10lb magnesium sulfate and up to a 1/4 lb each of zinc sulfate monohydrate and copper sulfate pentahydrate. My water quality was pristine and I never experienced symptoms of toxicity related to any of these metals.
However, I'm not recommending these levels for you. In fact, nothing in this article represents a recommendation. The information provided here is for your general education purposes only. Your results may be different from mine, and nothing is more important to your own health and well-being than your own N=1 experiment.
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 adding these salts is because they make the filtration more efficient.
So why don't I recommend addition of aluminum to your ice bath, 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 (e.g., Zhang et al. 2021), and when it comes to brain health, I'm not taking any chances.
References
Bishop WM, Lynch CL, Willis BE, Cope WG. Copper-based aquatic algaecide adsorption and accumulation kinetics: influence of exposure concentration and duration for controlling the cyanobacterium Lyngbya wollei. Bulletin of Environmental Contamination and Toxicology. 2017 Sep;99:365-71.
Boone C, Gervais J, Luukinen B, Buhl K, Stone D. Copper Sulfate Technical Fact Sheet; National Pesticide Information Center (NPIC). 2012.
Boone C, Bond C, Buhl K, Stone D. Zinc Sulfate General Fact Sheet; National Pesticide Information Center (NPIC). 2012.
Chasapis CT, Ntoupa PS, Spiliopoulou CA, Stefanidou ME. Recent aspects of the effects of zinc on human health. Archives of toxicology. 2020 May;94:1443-60.
Choi S, Liu X, Pan Z. Zinc deficiency and cellular oxidative stress: prognostic implications in cardiovascular diseases. Acta Pharmacologica Sinica. 2018 Jul;39(7):1120-32.
Collins JF. Copper nutrition and biochemistry and human (patho) physiology. InAdvances in food and nutrition research 2021 Jan 1 (Vol. 96, pp. 311-364). Academic Press.
DiNicolantonio JJ, Mangan D, O’Keefe JH. Copper deficiency may be a leading cause of ischaemic heart disease. Open heart. 2018 Oct 1;5(2):e000784.
Fosmire GJ. Zinc toxicity. The American journal of clinical nutrition. 1990 Feb 1;51(2):225-7.
Franchitto N, Gandia-Mailly P, Georges B, Galinier A, Telmon N, Ducassé JL, Rougé D. Acute copper sulphate poisoning: a case report and literature review. Resuscitation. 2008 Jul 1;78(1):92-6.
Garlid KD. Cation transport in mitochondria—the potassium cycle. Biochimica et Biophysica Acta (BBA)-Bioenergetics. 1996 Jul 18;1275(1-2):123-6.
Keatinge WR, Cannon P. Freezing-point of human skin. The Lancet. 1960 Jan 2;275(7114):11-4.
Juravlyova OA, Markin AA, Kuzichkin DS, Saltuikova MM, Loginov VI, Zabolotskaya IV, Vostrikova LV. Features of human metabolic reactions under extreme cold exposure. Human physiology. 2018 May;44:330-5.
Kardalas E, Paschou SA, Anagnostis P, Muscogiuri G, Siasos G, Vryonidou A. Hypokalemia: a clinical update. Endocrine connections. 2018 Apr 1;7(4):R135-46.
Prasad AS. Discovery of human zinc deficiency: 50 years later. Journal of Trace Elements in Medicine and Biology. 2012 Jun 1;26(2-3):66-9.
Zhang T, He F, Lin S, Wang X, Li F, Zhai Y, Gu X, Wu M, Lin J. Does aluminum exposure affect cognitive function? a comparative cross-sectional study. PloS one. 2021 Feb 16;16(2):e0246560.
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.
Great article, as always. As I'm now using subzero temperature for months using lot of salt, I can give you more feedbacks on my try&errors. It seems it is more the salty ice that is very corrosive and create frostnipes. More than the temperature itself. I can now stay 10min in my ice bath at -6.5°C without any injury. Huge disclaimer, I'm training for 2 years now and I'm checking a lot of stuff on myself after the bath (such as my foot temperature) to avoid frostbites. Risk is VERY high and danger REAL. So for 99.999% of the persons, I highly recommands to not go down 32°F/0°C. Why am I doing it? Because my body seems not stimulated enough at…
i am also curious how much to put in?
What kind of copper and zinc sulfates are you using and approximately how much to your bath? The epson salt is working pretty well in my bath so far 3 pounds and has been about 2 weeks. Water still looks pretty good with minimal filter running