It is easy to become overwhelmed by the myriad of breath work practices of our time, and times past. Especially, as many seem to contradict each other. Some are gentle and seemingly benign, while others seem outright life threatening (and some are). What makes it even more interesting is that many, if not all practices, have some validity to them. At some point, they have helped some if not many people. The opposite is also true. Most all, if not all, at some point have left people frustrated, worse than before they started, or simply “unmoved.” We want you to be moved by the practices you engage. We want you to feel safe, focused, confident, and certainly better off than before you started. That is the goal of this work. This piece aims to make sense of the current landscape of breath work.
What follows are some of the more popular breath work practices we see today. The practices presented paint a general picture of the landscape and are not completely inclusive of all breathwork. They are presented roughly in chronological order of their perceived origin. Please appreciate that these descriptions merely scratch the surface of each practice. A true picture can only be obtained by fully experiencing each practice. And a good, or even profound experience likely has to do with many things less visible than the practice itself. In this, we humbly present what we see in the current landscape.
Freediving has ancient origins to gather food from the sea, like sponges and pearls. It has also been used to reclaim sunken valuables and to help aide military campaigns. The earliest documented evidence is from ancient Greece, by Plato and Homer in 400 BCE. There is record of the Japanese divers collecting pearls in the sea depths at the beginning of the first-century . We do not know how closely these ancient practices mirror that of modern-day freediving, but there likely core similarities.
In our time, and arguably sometime previous (given human nature), freediving has become a sport. In addition to the utility and sport of freediving, most free divers also describe a spiritual connection to nature.
It’s important to note that freediving breath training is not hyperventilation. It is the practice of improving tolerance to CO2. While hyperventilation can make a dive more comfortable, it tricks the body into thinking that is it is doing okay, while it may not be. This may be the primary cause of shallow water blackouts. These blackouts can occur in the final stages of the assent are often fatal if an experienced dive partner is not present. Drowning occurs on or close to the surface.
Freediving breathwork training generally involves the following steps .
- Use the starting breath-hold time to determine 1-month training expectations
- Breath holding technique
- Preparation relaxation
- The final breaths
- Breath hold
- Practice schedules – 1 hour a day. Dry and wet.
- Anaerobic and Aerobic training
- Eating well
Visit www.freediveuk.com for more training and information.
Also dating into ancient times is the practice of Pranayama. Pranayama is a Sanskrit term for breath control in yoga. This can include the synchronization of breath during movements or asanas, or it can be a stand-alone practice of regulating the breath. The practice of breath holds, or suspension of the breath has been more formally described in the Hindu texts Bhagavad Gita and Yoga Sutra of Patanjali  .
According to Monier-Williams, the technical definition of pranayama refers to three processes .
- Taking the breath in (purak)
- Retaining the breath (kumbhak)
- Discharging the breath (rechak)
Examples of pranayama include
- Ujjayi breath(“Victorious Breath”)
- Bhastrika(“Bellows Breath”)
- Kapalabhati(“Skull-shining Breath”, a Shatkarma purification)
- Surya Bhedana (“Sun-piercing Breath”)
- Bhramari (buzzing like a bee)
K. S. Iyengar, a well-known and respected modern-day yogi, cautions that pranayama should only be undertaken when one has a firmly established yoga practice, and then only under the guidance of an experienced Guru .
Pranayama is a general term, referring to breath control in yoga, and involving varying degrees of breath awareness, manipulation, and retention. There is contextual recognition that practicing breath work comes with benefits and risks.
Tummo is a goddess of heat and passion in the Tibetan Buddhist tradition. As a breathing exercise, it is used to generate heat in the body. Tummo is also a Tibetan word for “inner-fire” and is pronounced “Dumo.” . Tummo breathing is part of tantric meditation cycles in which the purpose is to gain control of the body processes. Arguably, one of the more notable recorded practices includes a group of female Tibetan monks living in the upper reaches of the Himalayan mountains. These women used Tummo breathing to withstand (live in) freezing temperatures for extended periods of time. This practice employs significant over breathing, followed by breath retention and visualization – generally centered in the sub-navel region. A bodily mechanical lock, or pressure building maneuver is performed between the diaphragm and pelvis and is generally used to help focus heat in the heart and brain. There are several peer-reviewed studies indicating that skilled practitioners can increase directed body temperature up to 14 degrees [8, 9]. This practice seems to have been the inspiration behind the Wim Hoff Method breath practice .
The Buteyko Method was developed by Konstantin Pavlovich Buteyko, a Soviet doctor in the 1950’s. The method was designed to treat asthma and other respiratory conditions. The method assumes that many medical conditions are exacerbated by chronically increased respiratory rate and hyperventilation. The method claims that restoring normal acid-base balance of blood through better breathing practices will improve health. The practice entails nasal breathing, reduced breathing, and relaxation. The scientific community is very mixed on the efficacy of this treatment for airway and respiratory conditions, yet the method is used world-wide. A British study concluded that the Buteyko Method could improve asthma symptoms and quality of life, but that it had little impact on lung function . More recently the method is used to improve general health.
Holotropic Breathwork is a trademark practice used to reach “non-ordinary” or “holotropic” states of consciousness. It was developed by the Czechoslovakian psychiatrist Stansilav Grof, MD, PhD after his LSD-based psychedelic therapy practices were deemed illegal in the late 1960s. The practice employs rapid breathing, music, and breath holds. This aims to change brain activity in the default mode network, the part of the brain responsible for conscious processing. The practice acknowledges the complex nature of the psyche, including the perinatal or consecutive stages of birth, the transpersonal or past life experiences, the phylogenetic or ancestral memories, memories of animal ancestors, and the archetypal or experiences of mythological figure and domains. These culminate in COEX, or systems of condensed experiences. Holotropic states function like a radar that finds unconscious content with emotional charge. These states identify problems and mobilize inner healing intelligence that reveals unconscious content in the best order. Holotropic states activate the emotional and physical systems, and then link them to spirituality. This is consistent with ancient cultural practices that believed the breath connects the external world with the body and activates the psyche to take us to the spiritual domain. There is some evidence that the practice may recover adaptability of the sites in the brain that influence personality [12, 13]. There is no consensus in medical and psychiatric communities that Dr. Grof’s full claims are valid.
Rebirthing breathwork was created by Leonard Orr in the 1960s and ties to Tantric and Kundalini yoga . This technique claims to access and resolve traumatic childhood memories and experiences by erasing the thoughts that create unhappiness and misery with thoughts that are in harmony with the Universal Truth. The 4 elements (earth, water, air, fire) are integrated through mantra meditation practices. The practice includes cyclical breathing, without gaps between the inhale and exhale, with an emphasis on relaxing. There is no consensus in the scientific community that this approach achieves its claims.
Mindfulness breathwork in its general terms is simply the practice of noticing the breath. The practice does not aim to change the breath. Non-judgmental awareness is practiced, noticing where in the body the breath occurs, what it feels like, and what other areas of the body might feel like. The practice generally centers on feeling the breath go in the nose (cooler air) and out the nose (warmer air). It’s recognized that the attention will likely drift away from the breath many times. Gentle, nonjudgmental reorienting attention to the breath is key to the practice .
This practice is core to the Mindfulness-Based Stress Reduction meditation program, or MBSR. This practice was developed in 1979 at the University of Massachusetts Medical Center by Jon Kabat-Zinn. Jon Kabat-Zinn is rooted in the scientific community, earning a PhD in molecular biology in 1971 from MIT. After protesting military research and the Vietnam war at MIT, he went in pursuit of his “karmic assignment.” He studied meditation with Buddhist teachers such as Thich Nhat Hanh and Seungsahn . In 1979, he founded the Stress Reduction Clinic at the University of Massachusetts Medical School. The MBSR program is an 8-week, patient-centered, evidence-based intervention that focuses on teaching mindfulness meditation, breathwork, basic yoga, and other relaxation methods  .
The practice has reached a high level of scientific validity for stress and anxiety reduction, as well as positive improvement in other markers of health. It likely has positive influences in athletic performance, though that is less clearly examined in the literature [18-27].
Of all the breath work practices, breath mindfulness is probably the simplest in concept. It’s worth noting that it also requires a significant level of time and patience. While this practice may seem entry level, we also see it as very advanced.
Sudarshan Kriya is a specific form of pranayama founded in India on September 17, 1981 by Sri Sri Ravishandar. Sri Sri Ravishandar reported obtaining the vision for the practice after 10 days of silence and fasting along the banks of the Bhadra river in India. The cyclical breathing practice has roots in traditional yoga that aims to provide relief for depression and anxiety for clarity of the mind [28, 29].
Sudarshan Kriya Yoga (SKY)
It is taught by the nonprofit Art of Living Foundation . The following are key components.
- Ujjayi or “Victorious Breath”
- Bhastrika or “Bellows Breath,” during which air is rapidly inhaled and forcefully exhaled at a rate of 30 breaths per minute
- “Om” is chanted three times to create a prolonged expiration
Training for this practice is available worldwide and can be found on the Art of Living webpage.
The Wim Hof Method
Wim Hof (aka Iceman) is an extreme athlete, performer, investigator, and yogi born in 1959 in The Netherlands. He is most known for his ability to withstand extreme freezing temperatures. He holds double-digit Guinness World Records for feats such as swimming under glaciers and running a half-marathon barefoot in the artic circle – all while successfully regulating his body temperature within safe and normal limits (for the most part). There are documented cases of near catastrophe events, such as freezing his corneas . He has demonstrated an ability to teach others to do the same [32, 33].
The Wim Hof method is centered around three pillars: The Breath, Cold Exposure, and Commitment (meditation). Like Tummo breathing, the WHM uses controlled hyperventilation, followed by breath holding. Specifically, the WMH uses 30-40 cycles of big, deep breaths, followed by 1-3 minutes of a breath hold at the end of a normal exhale. Then a smaller breath-hold of 15 – 20 seconds with the first breath in. The sequence is repeated 3 or more times .
Scientific investigation seems to indicate that the practice can have temporary effects on the immune and autonomic nervous system response . Some claims of potentially curing cancer and autoimmune diseases has not been substantiated by rigorous peer reviewed research . It is worth noting that two Dutch sources have sited cases in which drowning has occurred – seemingly while practicing the WHM [32, 36]. This practice seems to attract a certain level of bravado, which may also be a factor in the accidents. Suffice it to say a good dose of common sense is warranted when practicing the WHM, and this practice has been shown to have at least short-term positive health effects.
Intermittent Hypoxia Training (IHT)
Intermittent hypoxia (IH), and Intermittent Hypoxia Training (IHT) refers to breath work that includes intentionally alternating periods of normoxia and hypoxia. Normoxia is defined as approximately 21% O2. Hypoxia is anything less and is often associated with negative health consequences . Hypoxia may be achieved by breathing oxygen-deficient air (less than 21%), by breathing through devices that offer resistance and by breath holding. IHT practice has been employed by athletes to simulate altitude training to enhance physical performance.
Intermittent hypoxia may have clinical use to alleviate certain pathological conditions. IH works by creating long-term facilitation (LTF), which selectively increases synaptic activity and motor output . Reductions in oxygen partial pressure are sensed by the carotid body, a chemo-afferent receptor . Activation of the carotid center releases serotonin, which improves motor activity. Specifically for the respiratory system, IH, by way of LTF, significantly improves phrenic nerve motor response and respiratory recovery [40,41].
IH-induced LTF has also been witnessed in carotid denervated rats, suggesting that synoptic plasticity due to intermittent hypoxia may also exist outside of the carotid body .
Therapeutic applications include improved phrenic nerve motor nerve output (improved diaphragm function), decreased sleep apnea, and improved conditions related to COPD . IH has been shown to facilitate hippocampal neurogenesis in ways that improve cognition and memory (be it rats) , and has antidepressant-like effects (still rats) . Nitric oxide production has been shown to be elevated with IH. This has a further positive effect on cardiac and respiratory function, as well as bone mineral density [46-48].
IHT has also been used to enhance physical performance . This was a signature training strategy of Doc Councilman, arguably one of the greatest swimming coaches of all time. We recognize that IHT should be done carefully and with skilled supervision by qualified coaches.
IHT does not come without risks. The damage is caused by reactive oxygen species (ROS), which decreases the effect of the antioxidant defense system. There is documented evidence of negative changes to most hemodynamic variables, as well as other system processes  .
Given the risks associated with hypoxia, proper dosing and good physical condition are crucial. Current evidence seems to point to hypoxic events of up to 10 repetitions a day, 1 minute duration, at 12-10% O2, alternating days of the week .
Oxygen Advantage Breathwork
Patrick McKeown, the author of The Oxygen Advantage is an Irish born Buteyko Breathing practitioner He has taken the concepts of The Buteyko Method and Intermittent Hypoxia Training (IHT) into the athletic performance world. He is the founder of the Oxygen Advantage training program and websites and trains athletes and practitioners the world over.
In addition to the benefits toted by IHT, The Oxygen Advantage program highlights that hypoxic training can induce splenic contractions, thereby delivering more red blood cells into the blood system. Hypoxic training is also said to facilitate the natural release of erythropoietin (EPO) from the kidneys . A fundamental activity in this program includes hypoxic breath holding during short bouts of sprint-like activities. Studies of rugby players demonstrated improved repeated-sprint ability (RSA) after training in the technique for 2 weeks  and one month . This effect has been demonstrated in other team sport activities when hypoxic events were performed during max efforts in the same context as the sport-specific efforts . The RSA effects on endurance do not seem to be as significant , although reduced breathing frequency has been demonstrated to improve endurance performance .
Making Sense of It All
First, Holy Buckets!
Reviewing the evidence and seeing the contradictory nature of the full spectrum of practices is enough to make anyone’s head spin. On one hand we have practices that advocate a subtle and gentle approach to the breath, using it as a tool to build inner, then hopefully outer awareness. On the other hand, we have approaches that are anything but subtle – and feel almost violent. And that group is divided up into practices biased toward hypercapnia by breathing less – or elevating CO2 levels, versus hypocapnia by over breathing (dramatically lowering CO2 levels). We have scientific research supporting some aspects of these practices, yet not completely embracing them as their authors had hoped. And yet…there is enough scientific evidence to say “something is there.” Lastly, scratch the surface of any of these practices and you’ll find people whose lives have been changed for the better, and on a few occasions, fatally for the worse.
- Mindful Observation Breathwork: Mindfulness breathwork, MBSR
- Hypercapnia (elevated CO2, reduced breathing, , hypoxia): The Buteyko Method, Intermittent Hypoxia (IH) and Intermittent Hypoxia Training (IHT), The Oxygen Advantage, Free Diving Breath Training
- Hypocapnia (reduced CO2, over breathing): Tummo Breathing, Wim Hof Method, Sudarshan Kriya Yoga (SKY), Holotropic Breathing, Rebirthing Breathing
“Yes, and” over “either, or”
It is highly likely that there is a place for all mentioned practices – and all have merit given the right individual, dosing, and timing. Individual preferences and abilities need to be considered on the part of the coach and explorer. If a practice doesn’t feel right, it probably is not. Each practice could be everything in the hands of the right coach. An explorer and a coach should not feel pressured into a practice if it does not resonate or if they have not had proper training. Both are important.
Still, it’s hard to not see an apparent, blatant contradiction in the different strategies. One group advocates subtle-reduced breathing to improve CO2 levels and uses intermittent hypoxia improve tolerance to CO2 (hypercapnic). While the other group employs the opposite practice – dramatically blowing off CO2 through intentional hyperventilation, to “improve” blood pH toward alkalinity. This practice is followed by breath-holding that dramatically reduces oxygen blood oxygen levels, often to otherwise considered dangerous levels. In essence, the hypercapnic and hypocapnic approaches are both reaching toward hypoxia. Maybe the key to improve health through breathwork revolves around intentional and controlled hypoxia – and the potential rebalancing hormetic stress associated with hypoxia. Maybe. And we think there is more. In addition to the afore mentioned timing, skill and individual preference, the key to success may also have a lot to do with intention, choice or free-will and the level of consciousness engaged with a breathwork practice. Until more research is done, we will rest on this principle to help identify the best practice for an individual.
Breathe Your Truth takes the approach that first we help an explorer figure out how to improve their CO2 levels at rest (Phase 1), then take that ability into daily activities (Phase 2), lastly and optionally, leverage the benefits of hyper and hypoventilation (Phase 3) for those who want to play that way.
Go easy, keep eyes wide open, stay clear on goals
Until the evidence provides a clear and comprehensive path through breath work, we recommend moving gently into practices with eyes wide open. Watch closely for signs that the practice is helping and for signs that might indicate the practice is not helpful. For example, use a CapnoTrainer® to make sure that a hyperventilation (hypocapnia) practice is not making baseline sitting ETCO2 worsen over time. If so, consider removing the practice or balancing it with intermittent hypoxic training. Lastly, establish goals early and allow them to shape the path. Two important questions in goal setting are: 1-What do you notice about your breathing? And 2-What would you like to see changed by breathing better? Let the answer to these questions direct the breath practice.
No place for competition
There is no place for competition in breath work – with the self or others. We understand that it is hard to not compare breath hold durations, the number of breaths used to hyperventilate, the tolerance time to cold exposure, etc. Work to transcend judgements in comparison. This will be the fast(est) track to success.
Science says…(a brief summary)
The following is a summary of what we may be able to extrapolate from the evidence, to create breath work practices that are evidence informed:
- Intermittent Hypoxic Training has potential health benefits.
- Intermittent Hypoxic Training has helped some team sport athletes, though not all.
- Dosing IHT is important. It is possible to under-do and over-do.
- There is a large body of individual testimonials for hypocapnic work. Science may indicate that some aspect of our personal sense of self can be expanded with these practices. Peer-reviewed research has not supported many of the broader claims.
- Hundreds of Wim Hof practitioners have been successful at reproducing the “super-human” feats of autonomic regulation under extreme cold conditions. The long-term implications of these practices do not have scientific consensus.
- Mindfulness training, orienting toward observing the breath has solid scientific evidence to support stress reduction. It takes patience.
- Slowing the breath has solid evidence to improve parasympathetic tone.
- We know that a properly functioning diaphragm also helps engage the muscles we associate with good core stability.
Lastly, let us know if we can help
Breathe Your Truth has developed a systematic way to help explorers find their path. We use technology, the latest evidence and ancient practices to make this work meaningful.
Contact us at email@example.com
David Deppeler, PT, DSc, is the founder of Breathe Your Truth. He fuses wisdom teachings, modern science, adult learning theory, and clinical experience to create rich, collaborative, team-learning experiences. Passionate about his paddle board, Labrador, and stretching his own thinking. Past lives: Executive Partner and Director of Clinical Education at Therapeutic Associates, TAI BreathWorks designer, Fellow of the American Academy of Orthopaedic Manual Physical Therapists, breath work certifications through Better Physiology, Professional School of Behavioral Health Sciences and The Oxygen Advantage. Holden Qi Gong and Les Mils RPM instructor certifications. College All-American swimmer. Physical Therapy degree from the University of Wisconsin – Madison, and a Doctor of Science in Physical Therapy from Andrews University.
 Ivanova, D., H. Nihrizov, and O. Zhekov, The Very Beginning, in Human Contact With the Underwater World. 1999, Think Quest.
 Donald, I. How to Hold Your Breath for 5 Minutes in 1 Month – Freediving Training. 2015; Available from: https://www.freediveuk.com/how-to-hold-your-breath-for-5-minutes-in-1month-freediving-training/#:~:text=Breathe%20calmly%20and%20slowly%20for,some%20deep%20inhales%20to%20recover.
 Springmann, T., Bhagavad-gītā. 8. ed. 1952, Büdingen-Gettenbach,: Lebensweiser-Verlag. 115 p.
 Patañjali, M.N. Dvivedi, and Theosophical publication fund. [from old catalog], The Yoga-sutra of Patanjali. 1890, Bombay: Pub. by Tookárám Tátyá for the Bombay Theosophical publication fund. 1 p.l., ii p., 1 l., viii, 99, vii p.
 Mishra. Available from: https://en.wikipedia.org/wiki/Pranayama#CITEREFMishra.
 Iyengar, B.K.S., Light on prāṇāyāma : the yogic art of breathing. 1981, New York: Crossroad. xxiv, 294 p.
 Yeshe, L.T., The Bliss of Inner Fire: Heart Practice of the Six Yogas of Naropa. 1998, Boston: Wisdom Publications. 22.
 Kozhevnikov, M., et al., Neurocognitive and somatic components of temperature increases during g-tummo meditation: legend and reality. PLoS One, 2013. 8(3): p. e58244.
 Benson, H., et al., Body temperature changes during the practice of g Tum-mo yoga. Nature, 1982. 295(5846): p. 234-6.
 Hof, W. and K. De Jong, The Way of the Iceman. 2016, Little Canada, MN: Dragon Door Publications, Inc.
 Society, B.T. and S.I.G. Network, British guideline on the management of asthma. Thorax, 2014. 69 Suppl 1: p. 1-192.
 Ivanov, I.V., O.P. Zheltyi, and P.V. Ishchenko, Using the Method of Special Breathwork and Ideomotor Exercises for Psychocorrection of Adaptation-Related Disorders in Flying Personnel. Hum Physiol, 2019. 45: p. 789-792.
 Grof, S., Human nature and the nature of reality: conceptual challenges from consciousness research. J Psychoactive Drugs, 1998. 30(4): p. 343-57.
 Bhajan, Rebirthing : breath-vitality-strength : Kundalini Yoga as taught by Yogi Bhajan. 2011, Santa Cruz, New Mexico: Kudalini Research Institute. xii, 450 pages.
 Kabat-Zinn, J., Full catastrophe living : using the wisdom of your body and mind to face stress, pain, and illness. Revised and updated edition. ed. 2013, New York: Bantam Books trade paperback. xlv, 650 pages.
 Wilson, J., Mindful America : the mutual transformation of Buddhist meditation and American culture. 2014, New York: Oxford University Press. viii, 265 pages.
 Paulson, S., et al., Becoming conscious: the science of mindfulness. Ann N Y Acad Sci, 2013. 1303: p. 87-104.
 Andersen, E., et al., Effects of Mindfulness-Based Stress Reduction on Experimental Pain Sensitivity and Cortisol Responses in Women with Early Life Abuse: A Randomized Controlled Trial. Psychosom Med, 2020.
 Errazuriz, A., et al., Effects of mindfulness-based stress reduction on psychological distress in health workers: A three-arm parallel randomized controlled trial. J Psychiatr Res, 2020.
 Felsted, K.F., Mindfulness, Stress, and Aging. Clin Geriatr Med, 2020. 36(4): p. 685-696.
 Kang, H., J.B. Seo, and I.Y. Hwang, Mindfulness-Based Stress Reduction for Stress, Anxiety, and Psychological Well-Being: Effects in a Republic of Korea Navy Fleet Crew. J Psychosoc Nurs Ment Health Serv, 2020. 58(11): p. 48-55.
 Kriakous, S.A., et al., The Effectiveness of Mindfulness-Based Stress Reduction on the Psychological Functioning of Healthcare Professionals: a Systematic Review. Mindfulness (N Y), 2020: p. 1-28.
 Lamothe, M., et al., Changes in hair cortisol and self-reported stress measures following mindfulness-based stress reduction (MBSR): A proof-of-concept study in pediatric hematology-oncology professionals. Complement Ther Clin Pract, 2020. 41: p. 101249.
 Lee, E.K.P., et al., Effect and Acceptability of Mindfulness-Based Stress Reduction Program on Patients With Elevated Blood Pressure or Hypertension: A Meta-Analysis of Randomized Controlled Trials. Hypertension, 2020. 76(6): p. 1992-2001.
 Mirmahmoodi, M., et al., The Effect of Mindfulness-Based Stress Reduction Group Counseling on Psychological and Inflammatory Responses of the Women With Breast Cancer. Integr Cancer Ther, 2020. 19: p. 1534735420946819.
 Robert-McComb, J.J., et al., The Effects of Mindfulness-Based Movement on Parameters of Stress. Int J Yoga Therap, 2015. 25(1): p. 79-88.
 Wasson, R.S., et al., Feasibility and Acceptability of a Community-Based Modified Mindfulness-Based Stress Reduction Program for the Under- and Unemployed. Glob Adv Health Med, 2020. 9: p. 2164956120973636.
 Parimala, S. and D. Kanchibhotla, Association Between Yogic Breathing Practice With Perceived Impact of COVID-19: A Cross-Sectional Study From India. Asia Pac J Public Health, 2020: p. 1010539520975282.
 Hamilton-West, K., T. Pellatt-Higgins, and F. Sharief, Evaluation of a Sudarshan Kriya Yoga (SKY) based breath intervention for patients with mild-to-moderate depression and anxiety disorders. Prim Health Care Res Dev, 2019. 20: p. e73.
 Zope, S.A. and R.A. Zope, Sudarshan kriya yoga: Breathing for health. Int J Yoga, 2013. 6(1): p. 4-10.
 Carney, S., The Iceman Cometh. 2011.
 Tijmstra, F. and L. Bomers. Iceman onder vuur” [Iceman’ under fire] (in Dutch). 2016; Available from: https://eenvandaag.avrotros.nl/item/iceman-onder-vuur/.
 Kox, M., et al., Voluntary activation of the sympathetic nervous system and attenuation of the innate immune response in humans. 2014, PNAS: Nijmegen, Het Nederland.
 Hof, W., The Wim Hof method : activate your full human potential. 2020, Sounds True,: Boulder, CO. p. 1 online resource.
 Houtman, A., et al., Endocrine and Immune Systems, in Biology Now. 2015, W. W. Norton & Company. p. 388–405.
 Duin, R., Iceman oefening eist opnieuw leven [Iceman exercise claims a new life]. 2016, Het Parool (in Dutch).
 Perry, G., Wheel-well and cargo compartment temperatures of large aircraft in flight: implications for stowaways. Aviat Space Environ Med, 2002. 73(7): p. 673-6.
 Hayashi, F., et al., Time-dependent phrenic nerve responses to carotid afferent activation: intact vs. decerebellate rats. Am J Physiol, 1993. 265(4 Pt 2): p. R811-9.
 Millhorn, D.E., F.L. Eldridge, and T.G. Waldrop, Prolonged stimulation of respiration by a new central neural mechanism. Respir Physiol, 1980. 41(1): p. 87-103.
 Blick, C., et al., Hypoxia regulates FGFR3 expression via HIF-1alpha and miR-100 and contributes to cell survival in non-muscle invasive bladder cancer. Br J Cancer, 2013. 109(1): p. 50-9.
 Navarrete-Opazo, A. and G.S. Mitchell, Therapeutic potential of intermittent hypoxia: a matter of dose. Am J Physiol Regul Integr Comp Physiol, 2014. 307(10): p. R1181-97.
 Sibigtroth, C.M. and G.S. Mitchell, Carotid chemoafferent activity is not necessary for all phrenic long-term facilitation following acute intermittent hypoxia. Respir Physiol Neurobiol, 2011. 176(3): p. 73-9.
 Haider, T., et al., Interval hypoxic training improves autonomic cardiovascular and respiratory control in patients with mild chronic obstructive pulmonary disease. J Hypertens, 2009. 27(8): p. 1648-54.
 Lu, X.J., et al., Hippocampal spine-associated Rap-specific GTPase-activating protein induces enhancement of learning and memory in postnatally hypoxia-exposed mice. Neuroscience, 2009. 162(2): p. 404-14.
 Zhu, X.H., et al., Intermittent hypoxia promotes hippocampal neurogenesis and produces antidepressant-like effects in adult rats. J Neurosci, 2010. 30(38): p. 12653-63.
 Martel, J., et al., Could nasal nitric oxide help to mitigate the severity of COVID-19? Microbes Infect, 2020. 22(4-5): p. 168-171.
 Bian, K. and F. Murad, Nitric oxide (NO)–biogeneration, regulation, and relevance to human diseases. Front Biosci, 2003. 8: p. d264-78.
 Lyamina, N.P., et al., Normobaric hypoxia conditioning reduces blood pressure and normalizes nitric oxide synthesis in patients with arterial hypertension. J Hypertens, 2011. 29(11): p. 2265-72.
 Serebrovska, T.V., Z.O. Serebrovska, and E. Egorov, Fitness and therapeutic potential of intermittent hypoxia training: a matter of dose. Fiziol Zh, 2016. 62(3): p. 78-91.
 Hinghofer, H., Intermittent hypoxic training: Risks versus benefits. European journal of applied physiology, 2009. 108: p. 417.
 McKeown, P., The oxygen advantage : the simple, scientifically proven breathing techniques for a healthier, slimmer, faster, and fitter you. First edition. ed. 2015, New York, NY: William Morrow, an imprint of HarperCollinsPublishers. xiv, 352 pages.
 Beard, A., et al., Repeated-Sprint Training in Hypoxia in International Rugby Union Players. Int J Sports Physiol Perform, 2019. 14(6): p. 850–854.
 Galvin, H.M., et al., Repeated sprint training in normobaric hypoxia. Br J Sports Med, 2013. 47 Suppl 1: p. i74-9.
 Faiss, R., O. Girard, and G.P. Millet, Advancing hypoxic training in team sports: from intermittent hypoxic training to repeated sprint training in hypoxia. Br J Sports Med, 2013. 47 Suppl 1: p. i45-50.
 Hamlin, M.J., et al., Hypoxic Repeat Sprint Training Improves Rugby Player’s Repeated Sprint but Not Endurance Performance. Front Physiol, 2017. 8: p. 24.
 Kapus, J., A. Ušaj, and M. Lomax, Adaptation of endurance training with a reduced breathing frequency. J Sports Sci Med, 2013. 12(4): p. 744-52.