The 1,000+ pounds of Epsom salt in every float tank is not just for buoyancy. Magnesium sulfate is a biologically active compound, and many float advocates argue your skin absorbs some of it during a session. The evidence for that is thinner than the marketing suggests, but the question matters because roughly 48% of Americans fall short of the estimated average requirement for magnesium (NIH Office of Dietary Supplements, 2022).
The Science of Transdermal Magnesium Absorption
What the Research Actually Shows
When you float in a saturated Epsom salt solution, the concentration gradient between the high-magnesium water and your lower-magnesium body theoretically drives passive diffusion through the skin. Theoretically. The skin is built to keep things out, and metal-ion absorption across intact stratum corneum is expected to be low (Gröber et al., Nutrients, 2017).
The most-cited piece of evidence for transdermal magnesium uptake is a 2004 pilot study by Dr. Rosemary Waring at the University of Birmingham. Nineteen healthy volunteers bathed daily in magnesium sulfate solutions for up to seven days, and 17 of 19 showed elevated serum magnesium afterward (Waring, University of Birmingham pilot, 2004).
That study has serious limits. It was never published in a peer-reviewed journal, had no control group, used a sample of 19 people, and has not been replicated under controlled conditions. A 2017 review in Nutrients — the most thorough look at this question — concluded that "the propagation of transdermal magnesium is scientifically unsupported" and that the authors cannot yet recommend transdermal magnesium application (Gröber et al., 2017).
So the honest summary is: float tanks contain a massive amount of magnesium sulfate, a small pilot study suggested some absorption is possible, and the broader scientific literature is not yet convinced. If you float for the relaxation, the magnesium hit is a possible bonus, not a guarantee.
Why People Argue Transdermal Might Help
Oral magnesium has well-known absorption issues. Gastrointestinal uptake ranges from roughly 20-50% depending on the form, and high oral doses cause diarrhea because magnesium is an osmotic laxative (NIH Office of Dietary Supplements, 2022).
Magnesium oxide, the cheapest form on supplement shelves, has bioavailability as low as 4%. If transdermal delivery worked, it would bypass the gut entirely. The Gröber review notes that hair follicles may offer a small absorption pathway, but the data on how much actually crosses into circulation remains weak (Gröber et al., 2017).
What Magnesium Does in Your Body
Magnesium is a cofactor in more than 300 enzyme systems regulating protein synthesis, muscle and nerve function, blood glucose control, and blood pressure (NIH Office of Dietary Supplements, 2022).
Muscle Function
Magnesium acts as a natural calcium antagonist, regulating the calcium influx that triggers muscle contraction. Deficiency is associated with cramps, spasms, and tetany because muscles cannot relax properly without adequate magnesium to gate calcium channels (Volpe, Adv. Nutr., 2013).
Sleep Quality
A 2012 double-blind RCT in 46 elderly subjects with insomnia found that 500 mg/day of oral magnesium for 8 weeks significantly increased sleep time and sleep efficiency, raised serum melatonin, and lowered serum cortisol and insomnia severity scores (Abbasi et al., J. Res. Med. Sci., 2012).
That study used oral supplementation, not float-tank exposure. Whether floating delivers enough magnesium to produce similar effects is unproven, though the parasympathetic shift and sensory reduction of a float session likely contribute to sleep benefits through separate mechanisms.
Stress Response
Magnesium modulates the hypothalamic-pituitary-adrenal axis, and deficiency is associated with greater cortisol reactivity (Volpe, 2013). Float therapy itself produces acute anxiolytic effects after a single 60-minute session in anxious and depressed populations, independent of magnesium absorption (Feinstein et al., PLOS ONE, 2018).
Bone and Cardiovascular Health
About 60% of body magnesium is stored in bone, and chronic low intake is associated with higher cardiovascular and metabolic risk (NIH ODS, 2022).
Epsom Salt Concentration in Float Tanks
| Parameter | Typical Float Tank |
|---|---|
| Water volume | 200-300 gallons |
| Epsom salt | 800-1,200 lbs |
| Specific gravity | 1.25-1.28 |
| Skin-contact temperature | ~93.5°F |
| Session length | 60-90 minutes |
The North American Float Tank Standard recommends operating specific gravity between 1.25 and 1.30 (NAFTS, 2017). A home Epsom salt bath uses 1-2 lbs of salt, so the concentration gradient in a float tank is several orders of magnitude higher than what the Waring study tested.
Float Therapy Outcomes — What Is and Isn't Established
- Anxiety reduction: Significant decreases in state anxiety and muscle tension after a single 60-minute session (Feinstein et al., 2018). Well established.
- Sleep improvement: Reported subjectively in float surveys; the mechanism likely combines parasympathetic activation, sensory reduction, and possibly some magnesium contribution.
- Cortisol reduction: Mixed evidence — some studies find reductions, others (Schulz & Kaspar, 1994) find no significant change at 60 minutes post-float.
- Transdermal magnesium loading: Plausible but unproven beyond one unpublished pilot. The 2017 Gröber review remains the strongest published assessment, and it is skeptical (Gröber et al., 2017).
Optimizing Magnesium Absorption During Floats
These tips assume the Waring pilot is directionally correct — they cost nothing and may help if any transdermal absorption occurs.
- Shower before floating to remove oils, lotions, and sunscreen that block skin contact
- Float for the full 60-90 minutes to maximize contact time
- Float regularly — weekly or bi-weekly — for any cumulative effect
- Hydrate before and after to support renal magnesium handling
- Skip pre-float lotions that create a barrier on the skin
If your goal is specifically to correct a known magnesium deficiency, oral supplementation has stronger evidence behind it than float therapy. Float for the things floating reliably does — anxiety, muscle tension, sensory rest — and treat magnesium as a possible side benefit.
Frequently Asked Questions
Does floating actually increase magnesium levels?
Maybe. The only direct evidence is a 2004 University of Birmingham pilot study (n=19, unpublished) reporting a 35% serum magnesium increase after 7 daily Epsom salt baths (Waring, 2004). A 2017 peer-reviewed review in Nutrients concluded that current evidence does not support transdermal magnesium absorption claims (Gröber et al., 2017). The honest answer is that the science is unsettled.
Can I get the same magnesium benefit from an Epsom salt bath?
The Waring pilot used roughly 600 g of Epsom salt in a standard bath, so a home bath is closer to that protocol than a float tank is. Float tanks have far higher concentrations and longer contact time, which could increase absorption — but no controlled study has tested float-tank exposure specifically.
How much magnesium does your body absorb during a float?
Unknown. No published study has quantified transdermal magnesium uptake during a float session. The Waring pilot reported a 35% serum increase after seven daily baths, but extrapolating that to float tanks is speculative.
Is too much magnesium from floating dangerous?
No. Healthy kidneys efficiently excrete excess magnesium. Hypermagnesemia is rare and typically occurs only with IV administration or in people with significant renal impairment (NIH ODS, 2022).
Should I still take magnesium supplements if I float regularly?
Probably yes, if you are deficient. Oral magnesium supplementation has decades of evidence behind it for outcomes like sleep (Abbasi et al., 2012) and muscle function. Don't rely on floating as your primary magnesium source. Talk to your healthcare provider for personalized guidance.
Related Reading
- Float Tank Benefits: The Science of Sensory Deprivation
- Float Tank Tips: How to Maximize Your Session
- How Often Should You Float?
-- The Float Finder Team