As you already knows a high temperature generates inside a processor. Normally it is about 125 Fahrenheit. Normally this type of disease occurs while temperature increases over 111 Fahrenheit. Using a laptop for some time on thighs there will some sense of heat. This heat is the reason to burn the skin. This will not happen to everyone. This happens to the people who use laptops for more than 6-8 hours per day.
In this syndrome we can see the skin comes to red. If you see carefully you are able to identify some pattern there.
How to avoid this?
Check whether the ventilation systems are working well. Do not cover the ventilation holes when holding the laptop. Do not keep it directly on thighs. Keep a heat barrier on thighs and keep the laptop on it (I.e. a pillow). Do not keep the laptop on thighs for long time. Keep it on another surface if you use it for a long time.
Men face another danger
Using laptops on thighs of males can increase the temperature around testis. Therefore it will clog sperm generation. Therefore this can be caused for lack of fertility.
Take previously described precautions when you using your laptop for a long time.