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Mystery still surrounds the infamous Franklin expedition, a British voyage that set out in 1845 to map the Northwest Passage. The crew went missing in the Canadian Artic and some of their remains . But a big question lingers: what would make those sailors leave the relative safety of their two icebound ships and trek into the unforgiving wilderness?

One theory suggested that lead from the ship鈥檚 pipes and the tinned food stores leeched out and poisoned the sailors, clouding their judgement. But a growing body of evidence shows that lead poisoning is unlikely to have been a factor in their deaths. The latest addition comes from an analysis of the hair of one of the 129 crew members, tentatively identified as Harry Goodsir, who was buried on King William Island.

鈥淏ecause hair is inert, whatever trace elements or chemical signature that is there will stay in the hair,鈥� says Lori D鈥橭rtenzio at McMaster University in Ontario, Canada. 鈥淲e were able to go back at least three months prior to Goodsir鈥檚 death and see the lead levels he was exposed to.鈥�

D鈥橭rtenzio used a bundle of 55 hair fibres that were found stuck to the clothes Goodsir was buried in. She and her team washed the hair fibres with distilled water to eliminate any contamination from the surrounding environment, and then used a microscope to identify the root of each hair so they could work out which way it had been growing. The researchers then cut off three 1-centimetre-long segments from each hair beginning at the root. On average human hair grows one centimetre per month, so the researchers had a record of hair growth for the final three months of Goodsir鈥檚 life.

They analysed these segments for lead-205, which was found in the solder used to seal the food cans on the ships and also in three crew member bodies buried on another nearby island. D鈥橭rtenzio and her team found lead concentrations up to 84.2 parts per million (ppm) in the oldest segments from the tip of the hair, with lead levels lowering to 73.3 ppm near the root. They estimate that these lead levels in the hair correspond to blood lead contamination of between 53.6 and 61.3 ppm.

Lead levels

鈥淚n 19th Century Britain, levels of lead exposure were very high compared to today, from water and adulterated food,鈥� says Keith Millar at the University of Glasgow, UK. 鈥淭hese measurements indicate that while he was on board the ship, there was a moderate level of lead ingestion that you鈥檇 expect in those days. When he was away from the ship, the level of lead dipped.鈥�

Although high for today鈥檚 standards, these lead levels were probably not associated with the kind of poisoning that would take the expedition to its fatal end, he says.

Jennie Christensen, founder of biological tissue analysis company TrichAnalytics, has previously examined the fingernail of Franklin crew member John Hartnell. She says she found even lower levels of lead. 鈥淚 agree that lead poisoning wasn鈥檛 the main cause of death, but I think even these numbers are overestimated,鈥� she says. Christensen points out that hair has a porous structure which could allow lead from the general environment to enter it and become embedded.

So, if it wasn鈥檛 lead poisoning that brought the Franklin expedition鈥檚 search for the Northwest Passage to a mysterious end, what was it? 鈥淚 think part of it was just sheer bad luck,鈥� Millar says. The sailors set out during a period of relatively benign weather, but that was followed by a 鈥渕arked and severe change in the Arctic climate for a few years where there was really no summer or fall whatsoever,鈥� he says.

They were stuck on the ice, search parties couldn鈥檛 find them, and they eventually abandoned their ships and set out on foot. 鈥淏eing a naval expedition, they were not trained or equipped to escape overland, and they were fatally exposed,鈥� Millar says.

The search for clues about their last days on the two ships continues 鈥� the wrecks of the HMS Terror and HMS Erebus were recently discovered. 鈥淚 really want to see if a sample from the lead pipes could match the isotopic signature we found,鈥� D鈥橭rtenzio says.