Science of Currency Sniffs Examined by Seventh Circuit

There are a select few decisions that must be read frequently by anyone interested in dog law and forensics, including the Supreme Court's U.S. v. Place, 462 U.S. 696 (1983) (baggage sniffs) and Terrell v. State, 3 Md.App. 340, 239 A.2d 128 (1968) by the Maryland Court of Special Appeals (review of history of tracking decisions throughout the U.S.). A more recent case that arguably may belong in this exalted category is a 2005 decision of the Seventh Circuit, U.S. v. $30,670, 403 F.3d 448 (7th Cir. 2005), where the issue was whether a large amount of cash was properly forfeited as the proceeds of crime. The case is not so important for its review of prior case law but rather for the detailed review of research on the significance of a drug detection dog's alert to cash.

After detaining the cash but not the individual in possession of it at Chicago's Midway Airport on September 5, 2000, agents performed a sort of luggage lineup in the DEA offices of the airport, allowing a drug dog named Bax to sniff among empty suitcases and one containing the confiscated currency. The dog, whose handler had not been informed as to which suitcase contained the cash, alerted to the one that contained it. A forfeiture action was instituted.

Because there was scientific and legal debate over the issue of the probity of a dog’s alert to currency, the trial court asked the parties to supplement their arguments with “publicly available empirical information” on the question. Calhoun, the individual from whom the cash was confiscated, asserted a “contaminated currency” argument--i.e., that most currency in circulation in the U.S. has some level of cocaine residue which could have come from anyone.

The trial court delved into the research, noting first that dogs probably alert not to cocaine but rather to methyl benzoate, which is “volatile enough to evaporate from the currency within a short period.” The court cited research arguing that dog sniffs are reliable despite currency tainting by narcotics because dogs do not alert to trace amounts of drug odor after a period of dissipation. That period may be relatively short. One study found that 90% of methyl benzoate evaporates from the surface of currency in two hours. Indeed, one researcher was cited as stating that dogs would not alert to byproducts other than methyl benzoate and would not alert to pure cocaine unless methyl benzoate was added. Thus, dogs do not alert to innocently tainted currency in general circulation because the amount of cocaine and/or methyl benzoate is too small for detection.

One expert was quoted as writing that a "positive alert to U.S. currency by a properly trained ... canine indicates that the currency had recently ... been in close or actual proximity to a significant amount of narcotics, and is not the result of any alleged innocent environmental contamination of circulated U.S. currency by microscopic traces of cocaine.” The research further indicated that, in contrast to the levels found on general circulation currency, the “threshold level of cocaine and methyl benzoate required for a canine to signal an alert was substantial and reproducible." Further:

"Calculation from the amount of methyl benzoate required for a reliable detector dog alert (>85% [detection success] at 10 [micrograms]), the amount of methyl benzoate observed in street cocaine sample[s,] ... and the diffusion rates of methyl benzoate from individual bills (ca. 10% remaining after 2 hours) indicate a required amount of recently contaminated cocaine residue of ca. 500 [milligrams] cocaine (initially)… This required amount is “50,000 higher than the amount reported on circulated currency (ca. 10 micrograms/bill).”

The significance of this was that "generously assuming" all bills in general circulation are tainted by 10 micrograms of cocaine, it would take at least fifty thousand bills to generate enough methyl benzoate to trigger a dog alert. Experts stated that "the odor produced by contaminated bills stacked together does not increase proportionally to the number of bills, but rather is a function of the available surface area” of the bills…. This indicates that stacked or bundled bills, which obviously have less contaminated surface area exposed to the air, would exude a correspondingly smaller odor signature and the 50,000 figure therefore may be too small by orders of magnitude when tainted bills are bundled together (although stacked bills do retain the methyl benzoate for longer periods)."

One researcher, and the Seventh Circuit, criticized the Ninth Circuit decision in U.S. v. $30,060, 39 F.3d 1039 (9th Cir. 1994), where the general contamination argument was accepted and defeated the government's forfeiture claim. The Seventh Circuit summed up its own conclusion by stating that “it is likely that trained cocaine detection dogs will alert to currency only if it has been exposed to large amounts of illicit cocaine within the very recent past.” Thus, an individual who had been on an airplane for many hours would have trouble asserting that he had only come into possession of a particular lot of currency in the last few hours. After reviewing the alerting dog’s record, the court affirmed the forfeiture.

There has been additional canine olfactory and currency residue research since the Seventh Circuit spoke in 2005, which I am reviewing for publication here and elsewhere. Nevertheless, what I have seen so far seems rather to support the 2005 decision rather than undermine it.