Someone asked me today if there is a minimum voltage for required an electrostatic discharge (ESD) to happen. He was probably thinking of the Paschen minimum - this is a minimum voltage to break down an air gap between two electrical conductors. When two electrodes are brought closer together, the breakdown voltage between then reduces nearly linearly with distance. However a minimum occurs at about 0.1 mm - at this distance an air discharge requires about 300-350 V across the gap. If you bring the conductors closer, the breakdown voltage theoretically increases! In practice it may not, because a different type of discharge occurs at small gaps. So really there is no minimum - at the limit an ESD will occur when the conductors touch.