At the outset, she complains that the global human hair trade is “a backstage business about which little is known to those outside the trade”. And after several years of combing the world to research her tome, the numbers she garners still seem very shaky.

The trade in human hair – yes, there really is a customs classification for wigs, beards and brows – may amount to around US$2.5 billion, and beyond this, the hair extension industry in hairdressing salons from London to Harlem to Lagos probably adds several billion more.

But getting more accurate numbers seems a forlorn task, unaided by any official trade data.

As someone who nowadays has almost no hair to boast about, the world of hair is about as mysterious as tree-sloth toenail clippings, but in many parts of the world it is a huge business, and has been for centuries.

It is strange but true that so many cultures attribute great mystical significance to hair and go to great lengths to prevent strangers from getting their hands on a strand and using it to inflict magical harm, and yet those same cultures have become huge traders in the stuff.

It seems the world’s largest sources of human hair are India, Bangladesh, Myanmar and China. And by far the dominant buyers are in the US (with a surprising demand from Nigeria and Ghana).

And yet, anyone spending US$2,000 to US$3,000 on a fine human-hair wig today would probably claim the hair comes from Brazil. If a trader wants a good price, he is unlikely to admit its true origins.

Even for scientists, determining the origin of hair seems a challenging business. Top-quality hair, called “remy” (which should be 30 inches or longer and should never have been chemically treated) can cost US$400 a kilo, and will almost always be black. Getting it to look silky blond takes weeks of chemical treatment.

For many years, hair from China was blacklisted as “communist hair”, and so entered the world’s markets as anything but Chinese.

There are constant stories of hair coming from corpses, prisoners, workhouses or hospitals.

Victoria Beckham back in 2003 joked that her extensions probably came from Russian prisoners: “so I've got Russian cell block H on my head”. The Moscow Centre for Prison Reform admitted at the time that it was possible.

A huge amount of “ethical hair” comes from Hindu temples like those in Andhra Pradesh, where women shave their heads for religious reasons and earn millions for the temple authorities.

Tarlo’s research finds Myanmar to be a huge source of remy hair, with the township of Pyawbwe, close to Yangon, boasting around 10,000 companies in the human hair trade, some of them employing more than 4,000 people. She estimates at least half a million Burmese women working in the business.

In poor districts of many developing countries, women gather and save these hairs daily from hairbrushes, waiting for the regular visit of the itinerant trader who buys their hair. It earns them welcome pocket money, and underpins a large part of the world’s human hair business.

India’s biggest human hair exporter sells about 10 tonnes of such “hair balls” every month. Go online, and you can find “waste human hair balls” for sale.

Tens of thousands of poor rural Burmese women spend millions of hours untangling these hair balls for traders in and around Pyawbwe. It takes about 50 hours to untangle a kilo of hair balls.

No wonder Tarlo writes: “Sitting on the ground untangling balls of comb waste must rank as one of the most monotonous, unhealthy, poorly paid jobs in the world, requiring levels of patience and endurance that only people who lack alternatives would tolerate.”

But for those living in poverty, it offers a fragile lifeline of sorts.

As communities are lifted out of poverty, it is perhaps unsurprising that their willingness to part with their tresses – or to unpick hair balls – has declined over the years, leading to a chronic surplus of demand over supply.

This inevitably led to the surge in demand for synthetic hair (which today accounts for perhaps 90 per cent of the global market in wigs and other hair products), and the boom in wigs as mass fashion.

For this reason, for a short while in the 1970s, Hong Kong was thrust into the heart of the business. From eight wig factories in 1963, business boomed to a point in 1970 when Hong Kong boasted 400 factories employing 40,000 people.

Like most Hong Kong manufacturing, it moved to the mainland in the 1980s, and today, Xuchang in Henan is the world’s “wig city”.

An estimated 88 per cent of the world’s real hair wigs and extensions are today exported from China, earning about US$1.6 billion in 2016.

I suspect that was not on US President Donald Trump’s tariff radar screen in the trade war, but given the president’s interest in hair art, perhaps it should have been.

David Dodwell researches and writes about global, regional and Hong Kong challenges from a Hong Kong point of view