Don’t put your money where your mouth is

When banknotes roll off the presses they are perfectly clean, but they soon become contaminated with all sorts of stuff.

You may remember that, in 2016, Massey University toxicologist Dr Nick Kim showed that every bank note collected in Auckland supermarkets, corner stores, bakeries, and takeaway bars carried traces of methamphetamine, many of them well above health department guidelines.

The previous year, more than 400 tenants were evicted from State Houses throughout New Zealand because traces of methamphetamine had been detected in their homes.

Kim argued that the health department’s tests were far too sensitive. He said the trivial amounts of meth detected posed no threat to anybody’s health, so the tenants had been wrongfully evicted.

The remarkable thing is that New Zealand bank notes carry more traces of meth than the bank notes of any other country.

Kiwi druggies might favour meth, but evidence from bank notes show that traces of cocaine are found on more than 90 per cent of American banknotes, 89 per cent of Canadian money and 80 per cent of British money. Spanish, Irish and German notes carry somewhat less. Almost everywhere, the contamination rate is highest in big cities and lowest in smaller communities.

Cocaine is spread by handling the drug, by using the notes to snort the stuff, or it is unwittingly spread by banks stacking notes together, or in their note-counting machines. But not only drugs. In 2013, an American team tested circulating $1 bills from a Manhattan bank. The project involved 544 million ‘‘reads’’ of DNA on the notes. This month, the team reported that by far the most DNA was from human skin, human mouths and guts, but also DNA from food, horses, pigs, dogs and a fungal mould or two. Then came lesser amounts of DNA from 397 kinds of bacteria.

The horse and pig DNA in New York money was a surprise. It is supposed that farmers must have handled the notes down in the country before spending up in the Big Apple. Most of the bacteria on American notes were harmless commensals that had flaked off hands, faces and mouths, but there were others that cause acne, strep throat, the H-bug, two species infecting vaginas, and other microbes associated with contaminated food, with dairy production, and with fermentation.

Collectively, our scientists call this DNA cocktail a ‘‘monetary microbiome’’. A lot of these bugs are thought to be spread by our habit of licking our fingers before we count bank notes.

The American team also cultured live microbes from the bills, but that was hard work as the living bugs grew in such infinitesimal quantities.

Although billions of the world’s bank notes change hands in commerce, in food service, travel, and the sex trade, very few of us will catch infections from money.

We are more likely to contract diseases from the far more heavily contaminated public transport, door handles, telephones, ATM machines and PC keyboards.