Oxygen Leak - Is X calculated once, or constantly?

So we had sort of a weird game last night.  We were fighting The Chairman on Wagner Mars Base.  We had sort of a bad draw, since we got an Underboss for the first and second cards played, which means that with The Operative helping out, we had 4 underbosses in play by the end of turn 2.  Luckily, the base played Meteor Storm, so we decided to take some turns to build things up and see what we could do.  Well, without any damage being dealt, we ended up with all Underbosses and Thugs in play very soon, which brought up the question of how to clear the table.

In comes Oxygen Leak.  We figured we could build up some cards, skip a turn just before the environment's turn, and (while taking a hit ourselves), let it clear the board for us.  This led to an interesting situation though.  All 3 Villanous Weaponries were out, and there were 6 cards in the environment trash.  Oxygen Leak deals X damage where X = the number of cards in the environment trash.  So it deals 6 damage to each of us, then 6 damage to Villainous Weaponry....so does Oxygen Leak now start dealing 7 damage?  8 after the next Weaponry dies?

We just weren't sure if it only checked X once when it started, or it was constantly evaluated as damage was dealt.  The "nothing happens simultaneously" rule leads me to believe it checks X before it deals each damage, but it wasn't 100% clear.

Thoughts?

Yep - we play that each attack is independent, and the damage dealt is based on game state at the time that attack happens.

So we had sort of a weird game last night.  We were fighting The Chairman on Wagner Mars Base.

That is weird.  How'd The Chairman and all his street thugs get on Mars...? ;)

I also agree with Rabit's expert analysis.