Are doorknobs "public" or "private"? Either supposition has implications that limit their evidentiary value in secret search cases, and that's the thorny issue in the cases of three Salt Lake City men, who are challenging the legality of search warrants obtained after Utah police secretly used test swabs to collect drug and firearms residue samples from their homes' doorknobs. From the Salt Lake Tribune:

[F]ederal prosecutors insist no warrant is needed to swab a doorknob and run the test - called an Ionscan - to detect whether occupants and visitors have been in contact with drugs. The exterior of a home cannot be expected to remain untouched, they say, as friends, solicitors, proselytizers, campaign workers and delivery people come to the door.

Assistant U.S. Attorney Leshia Lee-Dixon pointed out that visitors to [plaintiff Anthony Diviase] Mora's home were directed by a sign to go around to the back door. The invitation meant officers could touch the door, she wrote in a court filing. Mora did not have a reasonable expectation of privacy in the door handles to his screen or inside back doors, Lee-Dixon wrote.

In an Ionscan, a sample is taken by rubbing a sterile cloth over a surface to collect residue. The swab is placed in a machine that analyzes particles and gives an alert when certain substances are found, such as cocaine and methamphetamine. The technology is used at airports, including Salt Lake City's International Airport, to check suitcases for explosives.

TalkLeft quotes a ruling by Utah Judge Ted Stewart,

...cit[ing] a 2001 U.S. Supreme Court opinion that required Oregon police to get a warrant before using thermal imaging technology, which senses the use of heat lamps.....Taking the sample from Mora's door was similar, Stewart wrote. "The swab of the outside of the doorknob reveals something about the details of the interior of the home that is unknowable without physical intrusion - that persons who have handled drugs have entered," the judge said.

While I agree with the ruling because it limits a questionable stretching of the Fourth Amendment, I also see a fundamental problem with its logic: if traces of drugs [or firearm residue] are found on a doorknob, this fact alone does not prove that anyone possessing or handling these substances actually entered the premises, much less that the person[s] in question lives or spends time at that location. All it proves is that someone who came in contact with illegal substances touched the doorknob - which could be any of us considering that public surfaces are known to be contaminated with everything from nose candy to E. coli. Some studies have shown that up to 90% of U.S. paper currency in circulation contains traces of cocaine. ["Drug Contamination of U.S. Currency," Amanda J. Jenkins, Forensic Science International Oct. 1, 2001, pp. 189-93 (abstract)]

Drug WarRant puts it this way:

The legal question is quite interesting, academically. I lean to the notion that either the doorknob is private, in which case a warrant is needed to test it, or it's public, in which case there's no way to know who touched it and a positive test is not justification to search the house.

That's the crux of the problem. I live in a busy Chicago apartment complex, where dozens of people come and go from the premises every day. Delivery people, mailmen, movers, and visitors come and go, and they probably touch a lot of doorknobs. If someone's visitor who'd recently used drugs knocked on my door and tried the knob before realizing they had they wrong apartment, I'd have heaven-knows-what on my doorknob (I probably have heaven-knows-what on my doorknobs anyway, but I'd rather not think about that right now). Even if police tested the door on a single-family home in a remote location, the knob may have been touched by many people not connected to the residence in any meaningful way. That's why fingerprint samples taken from doorknobs are virtually useless as legal evidence.

What about the "garbage can" search loophole, that allows police to search using the argument that trash becomes public property once it is discarded by the owner? Orin at Volokh Conspiracy states,

On the other hand, the residue on a doorknob may be seen as analogous to the garbage bags left out on the street in California v. Greenwood.

I would hope that the court wouldn't give the two situations similar weight, for a few reasons. Garbage, even if not bagged or otherwise separated into containers easily linked to a given residence or business, is usually used by investigators to obtain documents, DNA samples, hair and fibers, and body fluids - mainly individual-characteristic evidence, primarily useful in linking a specific piece of evidence with a specific suspect. Class-characteristic evidence, under which drug and firearm residue would fall, is not generally useful in making these specific one-to-one matches, and its value depends chiefly on the circumstances of how and where the evidence was found. Garbage taken from a single-family home trash receptacle would be more useful than that found in a communal Dumpster - for example, if police found cocaine traces in my apartment building's common trash bin, there is no reasonable way they could link it with a particular occupant.

Last spring I took an introductory forensic science class, where we learned how easily evidence is transferred and spread around on publicly-handled objects. That's one of the reason why illnesses like pinkeye, colds and flu spread so quickly; when you grasp a door handle, you're not just leaving evidence (and viruses) - you're picking it up as well and subsequently transferring those particles onto the next object you come in contact with. If fingerprints and germs disseminate so widely with normal handling of doorknobs, on what basis can police claim that drugs and firearm residue are reasonably connected to the occupants of a home or apartment? Sounds to me that these Utah "soldiers in the war on drugs" are grasping at straws...er, doorknobs.

MORE: "The police want to rub your knobs." on Drug WarRant
"Swiping Doorknobs and the Fourth Amendment," on Volokh Conspiracy
"No Privacy Right in Doorknobs," TalkLeft
"Doorknob Swabs Challenged" on the Salt Lake Tribune
IonSCAN® Product details
"False Alarm," [AlterNet] an article on the civil rights implications of a competing chemical analysis product called Itemiser®
Free Republic thread on doorknob swabbing

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