A MacBook Air gets hot, just like any other laptop, so not to worry there. 

Although 100°C is relatively hot, you should not worry about it. There are multiple reports where users complain about their temperature ([here][1] and [here][2]).

Things you can do to prevent getting the Mac too hot is making sure you haven't covered his ventilating holes. Place it on a flat, hard surface (no lap, pillow or blanket as developers tend to do. At least, that's what I do once in a while :-)). 

What you should keep in mind is the MacBook Air will shut itself down when there is any chance of damage.  
So you've got nothing to worry about. Your Mac won't get harmed. 

  [1]: http://gdgt.com/question/how-hot-does-the-macbook-air-get-during-normal-operation-hth/
  [2]: http://9to5forums.com/discussion/2390/how-hot-does-a-macbook-air-get