The air is pulled from outside the house to the inside through a hepa filter, the whole point of the posi system is to eliminate the need for an air-tight structure. My first exposure to this actually came from the alpha rubicon article on the subject. I don't worry too much about chem/bio problems, but I just wanted to start a discussion. I think the rubie motto facta non verba is worthwhile, but I also believe that the ability to discuss things prior to trying them or simply being aware of a method, even if you haven't tried it, is also worthwhile. This is not, afterall, the Rubicon which has a completely different and in my opinion somewhat elitest view of survivalism.