Read more: 鈥�Secrets of the body鈥�
YOU might think that the physical bounds of your body are a neat cut-off point where 鈥測ou鈥� end and the rest of the world begins. Yet everywhere you go you leave behind a calling card. Whether it is hair, skin cells, sweat or waste material such as urine and faeces, you are constantly shedding material that started off as part of you.
How much of ourselves do we unknowingly dump each day on our clothes, on the way to work, in the office, on door handles, cellphones and in the bathroom? And once it has left our body, does it still belong to us?
Advertisement
A commonly cited figure for hair loss is 50 hairs a day. 鈥淓veryone says it, but no one really knows where it originated,鈥� says founder of the Henry C. Lee Institute of Forensic Science in West Haven, Connecticut. How many skin cells and how much sweat we deposit in the wider world is even more of a mystery. As for things such as earwax, spit, snot, faeces and the like, it seems we prefer not to think about them, let alone quantify them.
What we do know is how much of our DNA is cast out in bodily detritus. 鈥淏lood and semen contain the most,鈥� says Lee. Every millilitre of semen has up to 300,000 nanograms of DNA. For blood, the figure is 30,000 nanograms, while saliva, another rich source, contains a tenth of that. There is no DNA in hair unless the root is shed, which is very rare with head hair but less so with pubic hair. Urine contains a smidgen of DNA, but faeces is likely to have much higher levels as many cells are sloughed from the gut lining as it passes through, though this hasn鈥檛 been tested.
Surprisingly, skin cells are not the main source of DNA left behind on objects you touch. 鈥淚t鈥檚 DNA from sweat that gets left,鈥� says of King鈥檚 College London. Although sweat contains just 11.5 nanograms per millilitre, .
鈥淪weat contains enough DNA to identify who touched an object鈥�
Knowing we leave all this stuff behind is one thing but a much bigger legal, ethical and philosophical question is whether it is still part of us.
鈥�,鈥� says Graeme Laurie of the University of Edinburgh in the UK. Generally speaking, bodily waste or cells that we leave in the environment are considered 鈥渁bandoned鈥�, allowing lawyers to argue that we have relinquished our rights to them. This has led to a bizarre situation in the US. 鈥淚f you鈥檙e a man and you have sex with a woman, the semen is no longer yours, but hers,鈥� says Lee. Likewise, biological deposits on a computer keyboard or telephone receiver at work become the property of the company that owns those objects. Elsewhere, including the UK, the law is fuzzier. 鈥淎n accidental trail might not be tantamount to abandonment,鈥� says Laurie. 鈥淚 think you would need to show intention to abandon.鈥� Besides, he adds, taking and using genetic material without consent is a criminal offence in the UK and US.
More philosophically, the gradual loss of our tissue to the environment raises questions about our identity, given that cells die and are replaced every day. 鈥淓very cell has a finite life, and that means you鈥檙e not the same person you were five years ago,鈥� says Daniel. So if you are not quite feeling yourself today, now you know why.
This article appeared in print under the headline 鈥淏order control鈥�