If water vapor was the only thing airborne, then this would be mostly plausible. But the reality in any typical environment is for small particles of dust, soot, microplastics, COCs, etc to be in the air, in addition to the usual suspects of oxygen, nitrogen, carbon dioxide, etc. Some of those will increase the conductance of water, when condensed upon a cool surface. Think of water vapor as a lint filter that floats around the room until it lands on something.

But even in a hermetically sealed environment with only the typical atmospheric mix of oxygen and nitrogen and other trace elemental gases, and then water vapor, there’s still a problem. Air has a conductivity – measures in Siemens, the inverted unit of Ohms which is resistance – of 3-8 x 10^15, meaning it will not conduct much at all. But compared to condensation upon a PCB in this sealed environment, DI water has a conductance of 5.5 x 10^6. That is 100,000,000x times more conductive, although it’s still a tiny amount.

The reality is that all circuits and electronics leak small currents here and there, even through the air or through their PCB substrates. But the sum total of these leakage and creepage currents will be negligible in all but high-voltage cifcuits. Though that’s only under the rated environmental conditions.

When air is fully saturated at 100% humidity, some of those currents become noticeable. And for high-voltage switchgear, it can become an issue very quickly. But iutright water on most circuit would be disastrous due to arcing or shorting, or both.