If you mean when you play a video, your laptop temperatures increase (thus increasing fan speed), thats normal. Its just to try keep the MBA cool.
Heat=bad for computers.
Cold = also bad for computers.
So let's try to be more specific before spreading FUD to the uninformed who will then proceed to create hundreds of threads with topics like "OMG, MY COMPUTER IS OVERHEATING IS IT BAD!?"
When playing 1080p mkv files using VLC you can expect around 20-25% CPU usage and around 70-75 degrees Celsius CPU temperatures. If you're playing back m4v and mp4 files then the CPU will use quicksync CPU usage will be around 5% since the work is offloaded to the integrated graphics chip.
This is all documented in various threads if you do a quick search. I've created one where I documented various use cases and corresponding temperatures.
Your MBA will happily run at 95 degrees Celsius all day long. The fans will come on. If it's not shutting down, then everything is fine. Heat does not simply means bad for computers. Computers are designed to be used and to handle the heat they produce. If they don't, then you return them. It amazes me how many people on these forums feel the need to be baby sitting their computers and spend their every waking moment worrying about something. Just the use the damn thing since that is what it's designed for.