There are lots of articles that suggest that hot water are not advisable for hair. Let’s take a deeper look as to why.
First of all, a bit of biochemistry: it is a relief to know that hot water in the shower will not damage the internal protein structure of hair. Protein starts to denature (changes its structural conformation) at a temperature of 105.8F (41°C). That change causes the proteins to loose their functionality. In hair, the denaturing of the fibrous protein implies a weakening of the internal structure. If someone sits in a tub of water maintained at 105F (and above) for a while, he or she would surely start to feel overheated. In real life, our body does not reach that internal temperature in a shower unless you stay there for hours.
The impact of hot water on hair
Hot water even at 100F has some negative impacts on hair. The heat causes the intercuticular tissues/membranes to swell more. When that happens, lipids, small protein fragments and other water soluble components in those layers get extracted from the hair more easily. Additionally, any free lipids (from sebum) on the surface of the cuticles get cleaned off more efficiently in hot water.
The loss of too much lipids can make the hair feel squeaky clean. That translates to drier, less pliable, less soft and less shiny hair.
Lipids act as a moisture barrier in hair because they are hydrophobic (repel water). With the lipid loss, moisture is able to leave the hair as well. The loss of moisture also contributes to the dry feel of the hair.
Note that the loss of the proteins fragments can also make hair more porous and less able to trap moisture.
What to do
Conditioning the hair after shampooing can minimize the effects of the hot water, but for those already suffering from dry hair, the drying effects might outweigh the conditioning benefits.
For that reason, it is often advised to use lukewarm/cool water instead. Lukewarm water helps to clean the hair well without stripping too much oil from the hair. Cool water on the other hand, can cause the intercuticle membranes to contract and force the cuticles flat. Many people have seen more shine after using cool water because the cuticles are more aligned.
With that being said, you should use water temperatures that you feel comfortable with. Most people will choose lukewarm because they are in the shower when they wash their hair. But, if you able to wash your hair outside the shower, then you can always opt for cooler temperatures.
Note that hot water is also bad for the skin. In a similar manner it causes the skin to dehydrate.