Assume that a speck of lsd is 10 um in diameter. This is a typical powder dust size https://blissair.com/how-small-one-micron-why-matters.htm/amp Assume thumb is 1cm2 and lsd has a density of 1 ton/m2. Thumbprint is 0.00001x0.01x0.01=1e-9 m2. Thats 1e-9m2x1e6g/m2 = 0.001g lsd. Thats 1 milligram or 10 hits. Considering losses, it might only be 3-5 hits.