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Sound pressure level uses 20 log10 rather than 10 log10, because intensity goes with the square of pressure. Against the 20 micropascal reference, 0.2 Pa is 80 dB. That reference is chosen so the pressure and intensity scales read the same number for a plane wave in air.
Sound pressure level
L = 20 log10(p / p0)
Reference
p0 = 20 uPa
Because intensity is proportional to pressure squared, and the square inside the logarithm becomes a factor of two outside it.
Around 2 Pa at 100 dB - still only 20 millionths of atmospheric pressure, which shows how sensitive hearing is.