If I recall the factoid I read correctly it’s correlated with the value of the bill with the maximum sensitivity “wet/solvent sampling” methods they were basica

If I recall the factoid I read correctly it’s correlated with the value of the bill with the maximum sensitivity “wet/solvent sampling” methods they were basically able to always produce a trace from at least one substance considered to be a “narcotic” by federal law on lower denominations like $1 and $2. Were talking amounts so low normal that field testing swabs will miss them, but it’s clear evidence of how random the distribution of drug related chemicals on US currency is. It’s not a tainted or not situation… for small denomination bills it’s really just a question of how tainted. If I recall their conclusion section correctly, the issue is with these chemicals becoming macroscopically embedded into the fibres of a bill along combined with repeated exposure to liquids (mostly sweat and other water exposure) dissolving them in to the fibres at the microscopic scale