Hello,
Thank you for participating in the Apple Support Communities.
It looks like you're wondering if there's need for concern over the temperature of your MacBook Air. Are you being prompted by OS X or is the "Platform controller hub die" temperature being reported by a third party app?
I recommend visiting the following resource for more information on operating temperature as well as tips on working with your MacBook Air.
Mac notebooks: Operating temperature
According to this link, The bottom surface and some areas between the keyboard and LCD hinge of your Mac notebook can become warm after extended periods of use. This is normal operating behavior.
All the best.