How does selenium protect against mercury poisoning? I have read that it is not essential to avoid fish with mercury in them if they also have a correspondingly high level of selenium because mercury poisoning is primarily primarily the result of mercury disabling selenium based antioxidants.  

How does selenium intake protect against mercury poisoning?  Can enough selenium counteract the effects of high mercury intake and does it matter what kind of exposure took place (breathing vapors etc)?
Selenium has a very high affinity for mercury. It strongly binds mercury to itself, thus neutralizing the reactivity of mercury. The complex formed Hg - Se is not absorbed by body and excreted out from the body. 

In this way mercury is unable to lodge in fatty tissues, resulting in no damage to the body.