VENTILATING MACHINES IN HOSPITALS USE NEGATIVE PRESSURE
WRONG!!
Well doctors in hospitals talk about "positive pressure" (when you breathe out) and "negative pressure"
(when you breathe in)
In fact, pressure of the air is transmitted by the air molecules constantly bombarding us randomly. If there are less molecules (or if they are made to go slower by cooling the air) there are not so many collisions with us per second so the pressure is less.
As there are fewer and fewer molecules the pressure goes down and down until when there are no molecules there is no pressure.
you cannot have less pressure because you cannot have less molecules than zero - ie you cannot have a negative pressure. or..if you are a bit more advanced: pressure is not a vector it is a scaler, it has no direction.
What the doctors mean (even if they don't realise it) is that they are talking about the pressure in the machine RELATIVE TO ATMOSPHERIC PRESSURE. ie "negative pressure" means less than atmospheric pressure.