The Gas Mask effect allows Sabitsuki to wear a gas mask. Pressing Shift will make her lift up the mask, leaving only the left side of her face visible. The Gas Mask effect is found in the Plant Labyrinth World by touching the plant emitting smoke in one of the rooms.
Go left in the nexus, find the small house by going southeast. Go through that, down the stairs, then go past the elevator. You should proceed to the next area and you'll pass some doors. The first door you pass is the one you enter. Enter another door to enter the Industrial Snow World. You go northeast till you see what looks like a place blocked up a smoking pipe. This is simple! If you go directly east of it, there should be a cylinder with a door you can enter.
Once there, interact with the flower! Then go out, and slightly southwest (very slightly), and the way should be cleared! Good to mention that once you go down the stairs, you cannot go back. Enter, and you should be in the Plant Labyrinth (Past) area! Go all the way right, whether you need to go down or up, just go right! Then you should find a door. Go through the next door. Once through there, you should teleport near two heartbeat monitors.
Go northwest of the monitors, and you'll see a place you can enter along with what looks like a nurse walking around. Enter, but there is no return. Continue left, then left again to each door till there is no door in the area to the far right. Then go up and there should be a red monster smoking. Click it!
- The effect is ガスマスク in Japanese.