The main changes made by the body during heat acclimatisation are:

Heart rate and blood volume

Radiation and convection at the skin's surface are less efficient at higher external temperatures. So to compensate more blood has to flow to the skin. This is like turning the valves on a household central heating radiator up to get more heat out.

The chart shows the time for average healthy people to make a 95% adjustment.

The body increases the flow by making the skin's blood vessels dilate becoming wider and letting more blood through. More blood volume is needed to provide the extra flow. The body starts to make the extra blood.

At the start of acclimatisation, there is not enough blood for both heat transportation to the skin and carrying oxygen to the muscles.

In the first few days the body tries to cope by increasing its heart rate to make the same volume of blood do more. As the body gradually increases it's blood volume the heart rate comes down. It can take seven days to complete these blood volume and heart rate changes.

The first week of acclimatisation is particularly dangerous because the heart cannot beat fast enough to do both jobs ie transport extra heat and carry oxygen. This means that heat from the muscles and the core is retained more than when the body was in temperate conditions. The core can heat up much more easily than at any other time in a new hot environment. It must also be remembered that the body has not bought its sweating up to the level necessary to support the same level of exercise as you could undertake in temperate conditions.

For the same level of exercise/work the first day's heart rate can be 30 beats per minute higher than seven days later.

Reaction to work/exercise

An important reaction to heat for most of us, when we first experience it, is to get tired very quickly. We probably evolved this response as a protection. It is however very variable between individuals and cannot be relied on to protect you from undertaking too much exercise in the early days of acclimatisation, as surprisingly low levels of work can create overheating in the body at this time. As can be seen from the chart above, it takes on average between 3 and 5 days for a 95% level of adaptation to be reached where a healthy person would no longer feel worn out by the heat.

One of the consequences of this change is that the second half of the first week of adaptation is also dangerous as people will be starting to feel better in the heat and "up for it" while sweat production can be a long way short of the levels required to prevent significant activity being dangerous. The chart above shows that the time for healthy people to reach 95% of the sweating increase needed is highly variable and can take between 7 and 14 days.

Salt Retention

Salt is vital to the way the body works chemically, and increased sweating causes levels to be lowered. The body adjusts by reducing the concentration of salt lost in both sweat and urine. As can be seen above this takes between 3 and 9 days.

Increased sweat production

The amount of sweat to be produced when exercising in hot conditions can be three times or more that produced in temperate conditions.

Temperate sweating Hot conditions

Volume per hour - litres

0.33 - .45 1.8 - 2.0+

Volume per hour - pints

¾ - 1 3 - 3 ½

Remember that the sweat has to evaporate on your skin to give it's cooling effect.