Dead Space - need some help with the "Environmental Hazard" level

[darkely]

Information...

Game Level: Medium
Guns: Plasma Cutter, Line Gun, Pulse Rifle, and Flame Thrower (some upgraded)

I'm at the point where you have to go through some sort of green house (Environmental Hazard) and kill infected people who are poisoning the air by simply being around. The problem is there are 8 of them spread around (they don't attack me - but they just trigger the oxygen counter) and when you kill one of them 4 monsters come out of the vents (2 being the normal ones (the ones you start out with) and the other the infecting monster (idk the name but its the one who turns corpses into other monster)). After I kill the smokers (who poison the air) the 4 others come out but I can run out of the room and they wont follow (apparently they cant open doors). When I simply walk into the level area 4-6 monsters begin attacking me (2 normals and 3 that shoot rockets (they have 3 or 4 tentacles that shoot stuff at you)). I have only 3 health packs and they quickly dwindle because I can't kill them all fast enough... help?

Note: sorry for the weird names I gave the monsters...