Linda Pickett, of Lower Hutt, asks :-

Here’s a question for us hot yogis. Bikram Yoga is a series of 26 hatha yoga postures performed in a heated room (40°C) over 90 minutes.

Usually in these classes the standing postures are where you feel hottest and lying on the floor in Savasana is the coolest time – as I would expect given heat raises.

However occasionally in a class the floor is definitely the hottest place to be and you feel a sense of compression and cannot wait to at least be up on your knees for some nice cool air.

I’m guessing this is to do with humidity levels but don’t really understand how it works – please help explain.

Jim Cotter, a physiologist who specialises in environmental and exercise physiology, at the University of Otago's School of Physical Education, Sport and Exercise Sciences, responded.

We need to think about a few things here; some physical, regarding evaporation, and some physiological, regarding body temperature.

Let's deal with the physical things first. The rate that liquid water (e.g., sweat on your skin) can evaporate into air depends mostly on the difference in vapour pressure between your skin and the air. So, sweat evaporates faster and more easily - making you feel cooler - when vapour pressure of the air is much lower than on your skin, and also when air moves across your skin (because sweat-humidified air is removed more quickly).

Warmer air can hold exponentially more water in its evaporated state (vapour); the amount roughly doubles for every 10°C increase in air temperature. Warmer air rises, so air higher in the room can also hold much more water vapour than the air near the floor can, even though the absolute humidity (mass of water in the air) will probably be similar throughout the room. The absolute humidity in the room will rise over time as water vapour from sweat accumulates, possibly even reaching a point where the relatively(!) cooler air near the floor is fully saturated (called its dew point, because water condenses out of air at that temperature).

In a Bikram studio, even this air may not be cooler than your skin, so you can't heat it up to make it hold more water and help cool you through evaporation.

We also need to think about a couple of physiological things. Your body core temperature will also rise steadily as time goes on, so anything that makes your skin warmer and/or wetter will feel more humid and more uncomfortable. Your skin in contact with your yoga mat will worsen this sensation.

It's a couple of years since I did Bikram yoga, but I recall the floor-based postures being more later in the session, i.e., a time at which you would be hotter, with a wetter skin, more uncomfortable, and when the room would presumably be more humid. Air closer to the floor may also be more stationary, hindering evaporation, and so making you feel even warmer.

A couple of things that you may also be interested in: (i) Higher rates of sweating produce wetter skin, but any sweat that drips off is wasted, and (ii) women tend to sweat less rapidly than men do, from a similar number of sweat glands (1-2 million) on a smaller skin surface area, and therefore sweat more efficiently.