The dangers or microwave radiation are more in the long run, its not like you will get sick immediately. But it is not good to be exposed for any length of time.
Your microwave should have some sort of shielding that prevents the waves from getting out, and I doubt if that has anything to do with the fan at the back. However it is not something that you should play around with. There may be more than one problem with your microwave, and I would suggest that you not use it until you have it repaired. Exposure to microwave radiation is not healthy.