Pressure chambers are a weird concept which look like something from Doctor Who. What’s also odd is that when I started reading about them one of the most googled questions was: Did Michael Phelps sleep in a hyperbaric chamber?
There are different types of pressure chambers. Hyperbaric chambers create a high-pressure environment and hyperbaric oxygen therapy is the addition of 100% oxygen to that environment. They have typically been used to treat decompression sickness, or “the bends.” In this setting, the increased pressure helps shrink harmful nitrogen bubbles that form when divers surface too quickly from depth. Hyperbaric oxygen therapy also has recognised medical uses beyond diving medicine, including promoting wound healing, and it is approved in the US for conditions such as severe burns and crush injuries.[1]
Hypobaric chambers do the opposite by decreasing the surrounding pressure thereby simulating high altitude. This reduced oxygen environment stimulates the body to produce more red blood cells, increasing its oxygen-carrying capacity. Despite frequent claims online, Michael Phelps seems to have slept in a hypobaric rather than hyperbaric chamber. His chamber simulated an altitude of around 2,500 metres, helping to improve his aerobic performance and reduce recovery time.[2]
To many, this may sound like blood doping. In fact, the World Anti-Doping Agency ruled in 2006 that altitude simulation methods could enhance performance and were contrary to the “spirit of sport.” However, they were not added to the list of banned substances, meaning their use remains legal.[2]
So, while hyperbaric and hypobaric chambers serve very different purposes, the confusion around Phelps highlights how easily medical technologies can be misunderstood when they cross into the world of professional sport, where science, performance, and myth often collide.
👋 For the new joiners: I’m Suraj, also known as Dr Sooj - a primary care doctor & health content creator. I love navigating the complex world of health and wellness and breaking down complicated concepts.
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References:
[1] Kahle AC, Cooper JS. Hyperbaric Physiological And Pharmacological Effects of Gases. [Updated 2023 Jul 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470481/
[2] Braconnier D. Michael Phelps: Hyperbarics vs Hypobarics [Internet]. BaromedicalHBO.com; [cited 2025 Dec 16]. Available from: https://www.baromedicalhbo.com/industry-news/michael-phelps-hyperbarics-vs-hypobarics/