Hot from the Inside: How Hot Yoga Heats Up 100% of Your Body Down to the Cellular Level and Puts You at Risk of Heat Stroke

The American Council on Exercise (ACE), self-identified as the largest non-profit health and fitness certification, education, and training organization in the world, just published a scorching report on the effects of Bikram yoga on core body temperatures.  Bikram-franchised studios teach trademarked Bikram Yoga, a 26 posture sequence with two breathing exercises in a room heated to 105° with 40-50% humidity for 90 min. Copycat studios teach this same sequence under the taglines of hot yoga, hot hatha, or hot 26 and under similar conditions of heat and humidity for a duration of 60-90 minutes. I have experience with one studio that regularly allowed its temperature to creep up towards 110° and 60% humidity.

Is hot yoga safe??

The obvious question has always been “is it safe to practice yoga in such extreme heat  and humidity?” Now we have an answer.  The study found that 60 minutes into a 90 min class, nearly half of the study’s participants had core temperatures over 103°. Core temperature is that of your insides including your blood, guts, and other organs. Outside of the context of a hot yoga experience, adults at 103° or 104° are heading to urgent care centers and emergency rooms.

According to Bikram Yoga’s founder, Bikram Choudhury: [Parenthetical use is Choudhury’s not mine. MM]:

“The room is kept at this temperature or more for the following:

  • Keeping the body from overheating (contrary to popular misconception) 
  • Protecting the muscles to allow for deeper stretching
  • Detoxing the body (open pores to let toxins out)
  • Thinning the blood to clear the circulatory system
  • Increasing heart rate for better cardiovascular workout
  • Improving strength by putting muscle tissue in optimal state for reorganization
  • Reorganize the lipids (fat) in the muscular structure”

How do you actually keep from overheating?

A lesson on thermoregulation is warranted. When you exercise, you generate energy, which is released as heat. Excess core heat, a complex result of muscle activation, is transported from your core to your skin, where it is lost to the environment via several methods – radiation, conduction, convection, and evaporation.

Radiation. When your body is hotter than your surroundings, which it almost always is, then a greater quantity of heat radiates from your body than to it. In a thermal comfortable room, about 60% of heat loss occurs as radiant heat.

Conduction. The transfer of heat from your body to an object, say your mat in yoga. In a normally heated room, 3% of heat loss occurs due to conduction.

Convection. Heat transfer via moving gas/liquid, which is almost always occurring. In a reasonably temperatured room, 15% of heat loss occurs via convection.

Evaporation. Heat loss that occurs through ventilation/diffusion and sweating. This accounts for 25% of heat loss in a comfortable environment.

Metabolic heat is transported to the skin while activation of sweat glands causes sweat to be secreted onto skin surface, promoting heat loss by evaporation of the water portion of sweat, which does not contain toxins, by the way.

Body cooling by radiation and convection, which is about 75% of heat loss, depends on a significant difference between your core temperature and the air around you. When that air exceeds 98.6°, heat exchange is reversed and your body now gains heat by radiation and convection instead of losing it. Read that sentence again.

Now you are left with only one mechanism to cool your core temperature – evaporation of your sweat. Sweat can only cool you if it is allowed to evaporate. When the surrounding air is dry, sweat readily evaporates, cooling your skin. However, if the humidity is high as it is in hot yoga studios, evaporation is impeded, resulting in sweat accumulating on your skin and insufficient cooling of your body. You now have no way to lower your core temperature other than leaving the room. Ironically, ignorantly, and inexplicably, many hot yoga studios discourage you from leaving the room and instead suggest you lie down on your mat, where you are now at the mercy of gaining more body heat from conduction of heat from your mat to your body. Double jeopardy for studios with floor heating systems. You are now in a position of experiencing a critical heat load and developing the mother of all heat illness – heat stroke.

If you have not experienced a hot yoga class, I cannot impress upon you enough how much sweating happens. Many participants are soaked and dripping with sweat before the end of the first breathing exercise at the start of class. When sweating becomes your last resort to cool your core temperature, it must be matched by water intake or dehydration occurs.  Ironically, ignorantly, and inexplicably, many hot yoga studios discourage drinking outside one or two sanctioned water breaks.

When your core temperature reaches 104°, which happened to one of the study’s male participants (interestingly, male participants experienced significantly higher heat-induced heart rates and core temperatures), heat stroke becomes a real possibility. The Mayo Clinic defines heat stroke as “a condition caused by your body overheating, usually as a result of prolonged exposure to or physical exertion in high temperatures. This most serious form of heat injury, heatstroke can occur if your body temperature rises to 104 F (40 C) or higher.” Heatstroke can result in a number of complications including permanent damage to your brain and other vital organs and/or death. Heatstroke requires emergency treatment. Damage worsens the longer treatment is delayed, increasing your risk of serious complications or death.

What should hot yoga studios and teachers do?

I know enough hot yogis to also know it is futile to preach to them about the risks of the practice they love, so I offer, instead, best practices for minimizing that risk.

The ACE study made several recommendations for improving safety of hot yoga classes.

  • Shorten classes to 60 minutes. Study participants began experiencing dangerous core temperatures at the 60 minute mark
  • Lower the room temperature
  • Encourage students to stay hydrated by reminding them to drink water throughout the class
  • Stress to students that toweling off sweat undermines evaporative cooling in an already gravely compromised evaporation system

I would go bolder and say:

Hot yoga studios should reject the Bikram heat/humidity standard and implement a temperature cap of 98.6°, which would hopefully keep core temperatures below the 104° heat stroke threshold.  Humidity levels should also be lowered to allow for more efficient evaporative heat loss. I would hope to see further research that explores a range of lower temperatures and humidities as well include a greater number of participants, including novices and those unacclimatized, and an active control group.

New students should be offered a shortened, modified class to allow them to acclimatize. This study used regular practitioners of Bikram Yoga, who were acclimatized to 90 min at  105°/40%. However it takes up to 14 days to fully acclimate to these excessive conditions.

Hot yoga studios should provide ongoing training to its staff and students on how to recognize heat illness and what to do about it.

Namaste, Michele