Why are modern computers cooled the way they are? When I first started working with computers (almost 50 years ago) the whole computer room was cooled using a positive pressure system of filtered air, which had the obvious benefit of preventing dust etc. getting anywhere near the components. Yet from the very first PCs the boxes were fitted with extraction fans, ensuring that dust is sucked into the components through every nook and cranny of the casing. Is there any logical reason why this method was employed rather than a positive pressure system with a simple disposable filter? I suspect that cost, reliability and convenience were the deciding factors. A positive pressure cooling system would require larger or more powerful (and noisy) fans to overcome the resistance of the filters, and some sort of easily accessible filter holder. The designers probably took the view - quite rightly -- that most users could not be relied upon to change their filters on a regular basis, even assuming that they were cheap and readily obtainable. The inevitable result would be a high incidence of damage, due to overheating, caused by blocked filters. It may also be significant that PCs typically have a 3 to 5 year working life, before they are replaced, which is usually before clogged fans, cooling fins and ventilation slots becomes a serious problem.