"Insect bites itch because insects leave proteins in your skin when they bite you, your body reacts to the proteins and you get really itchy. Heat can break down these proteins and stop the bites from itching.
"If you want to read more about the science involved check out this research paper:
"I normally use a hair dryer, but when I haven’t had one to hand I’ve used a lighter, candles and even a coffee mug. Just be careful not to actually burn yourself.
"Also please be aware that heat does not help protect you from Malaria or Dengue Fever – it only stops the itching."
For using heat to treat poison ivy and such, Google
"use heat to treat poison ivy".