You will get this for completing the Labs (3-1 to 3-4). These are a lot harder than previous levels as they include green goo that burns a more deadly green fire, and if you stand in it you will get hurt. Put it our with the extinguisher and use the hose to wash it away.
The levels also include green gas that comes from vents. You must extinguish all of those in a room and will be hurt whilst doing so. Unless you buy and use the Gas Mask upgrade. Then you will take no damage from it.
I found I often ran out of time in these levels as the green fire can be tricky to put out. Starting from Office and saving everyone on the way through builds up time which can really help.