Case air pressure gets discussed as though it were a major tuning decision. It has one real practical consequence, and it is about dust rather than temperature.
What the terms mean
Positive pressure means intake fans move more air in than exhaust fans move out, so the case is very slightly pressurised and air leaks out of every gap.
Negative pressure means the reverse: exhaust exceeds intake, and air is drawn in through every gap and seam.
The differences involved are tiny. This is not pressure in any sense you would feel.
Why it matters for dust
With positive pressure, air enters mainly through the intake fans, which have filters. Air leaves through the gaps. Dust is largely caught at the filters, where you can clean it.
With negative pressure, air enters through every unfiltered gap and seam as well as the fans. Dust accumulates inside the case, on components, where cleaning is much harder.
This is the whole practical argument, and it is a good one. Slightly positive pressure with filtered intakes keeps a machine noticeably cleaner over years.
What it does not do
It does not meaningfully change temperatures. What cools components is the volume of air moving across them, not the pressure differential. A balanced configuration and a slightly positive one perform about the same thermally.
Do not chase a specific pressure at the expense of airflow. A case set up for positive pressure with restricted intakes is worse than a balanced one that breathes freely.
In practice
Two or three filtered intakes and one or two exhausts gives slightly positive pressure without any calculation, which is why it is the common recommendation.
Clean the filters occasionally. That is the point of having them, and a clogged filter removes the benefit entirely while restricting airflow.
Fan count is not the same as airflow
A common mistake is counting fans rather than air moved. Three intake fans and three exhaust fans is not necessarily balanced, because fans do not all move the same volume and they are not all working against the same resistance.
An intake fan pulling through a dense dust filter and a restrictive front panel moves considerably less air than the same fan exhausting freely through an open rear grille. So a case with equal fan counts front and back is usually running slightly negative, not neutral.
This is another reason the practical recommendation errs towards more intake than exhaust: it compensates for the resistance the intakes are working against.
How to check what you have
You do not need instruments. Hold a light tissue near an unfiltered gap in the case, such as an unused expansion slot or a seam, while the machine is running.
If it blows away from the gap, the case is positive. If it is pulled towards the gap, it is negative. That is the whole test.
If you find it strongly negative and dust accumulation bothers you, add an intake fan or reduce exhaust speed in the fan curve rather than buying anything.
Where the dust actually comes from
Worth knowing before optimising anything: most case dust comes from the room, and the largest single factor is where the machine sits.
A machine on carpet draws in far more than one on a desk, because it is sitting in the layer of disturbed dust that foot traffic kicks up. Raising it a few inches off the floor helps more than any fan configuration.
Pets, smoking and open windows near roads all matter more than pressure differential. Filters and positive pressure make cleaning easier; they do not change how much dust is in the room.