Grey or White Tasks Edit
A duplicant cannot reach more than 3 blocks above them. I learned this the hard way by screaming at my duplicants to do their job, and I saw that they couldn't reach. A few Tiles later, and the problem was solved. You can see if they reach or not if the task is grey instead of white, which means they cant reach.
Gas Permeable Tiles Edit
Gas Permeable Tiles can be great for spreading Oxygen throughout the base. Here's how: Build a ton of Oxygen-creating buildings on the lowest floor, and put at least 2 gas permeable tiles in every floor. This way, Oxygen will spread throughout the base, and make all floors breathable. If Contaminated Oxygen takes over, try using Air Deoderizers on the floor that needs it, and that should (hypothetically) cause the Contaminated Oxygen be regular Oxygen.