Science & technology | Manganese poisoning

Subtle effects

Exploring the link between manganese and Parkinson’s disease

MANGANISM has been known about since the 19th century, when miners exposed to ores containing manganese, a silvery metal, began to totter, slur their speech and behave like someone inebriated. The poisoning was irreversible, and soon ended in psychosis and death. Nowadays workers are exposed to far lower doses and manganism is rare. But new research suggests it could be some way from being eradicated entirely. The metal’s detrimental effects on human health may be subtle but widespread, contributing to diseases known by other names.

For the past ten years Brad Racette, a neurologist at Washington University in St Louis, Missouri, has been tracking those effects, paying special attention to welders, since they are exposed to more manganese than most people. Being harder than iron, manganese is often used to strengthen steel and is present in many industrial emissions, including welding fumes.

In one study, Dr Racette found that symptoms resembling Parkinson’s disease (PD) were 15% more prevalent in welders than in other kinds of workers. In another, he found that in a small sample of welders who had not yet reported any neurological symptoms, brain scans showed signs of damage to a part of their brains called the striatum that co-ordinates movement and is damaged in PD. In the case of PD, it has been clearly shown that the first symptoms appear only after the striatum has lost more than half of its neurons.

These findings were controversial, and not just with the industries that make use of manganese. Though most neurologists agree that manganese poisoning and PD have a lot in common, others have found that the pattern of damage in the brains of PD patients and those exposed to high levels of manganese differs in important ways. The differences are reflected, Dr Racette’s critics say, in the fact that PD responds well to the drug L-dopa—a chemical precursor of dopamine, the neurotransmitter that patients lack because the brain cells that make it gradually die—whereas L-dopa does nothing for those with manganese poisoning.

This debate matters because on it rests the question of whether manganese in the environment could be one cause of PD—a disease that affects a significant fraction of the general population, not just manganese-exposed workers. Around 60,000 Americans are diagnosed with PD each year. In a 2010 survey, Dr Racette’s group found that PD was between two and ten times more common in the American Midwest and north-east—where industrial manganese emissions are highest—than in western and southern regions.

That was a dramatic finding, but it was only a correlation and not proof of a causal link. Now, however, Dr Racette thinks he has found the ideal population in which to search for that link. Some 80% of the world’s known high-grade manganese ore lies beneath the desert of South Africa’s Northern Cape Province, where it is mined. But South Africans also mine for other things, including gold and diamonds. For the past 100 years a national scheme has provided free heart-and-lung autopsies on dead miners, and compensation to their families if these reveal mining-related diseases such as mesothelioma.

About five years ago Dr Racette’s group persuaded the scheme’s organisers to include miners’ brains in the autopsies, and with Gill Nelson, an epidemiologist from Witwatersrand University in Johannesburg, and others he has now studied over 60 of them. The first results, published in Neurotoxicology late last year, suggest that those who were exposed to manganese had lost significantly more grey matter from the striatum than those who were not. Interestingly, says Dr Racette, the miners are on average exposed to less manganese than welders are in the United States.

The number of brains autopsied is still too small to provide convincing evidence that manganese causes PD. But Dr Racette says the miners’ brains could anyway reveal a lot about what does cause PD. They show signs of inflammation, for example; one theory holds that PD involves an inflammatory reaction in the brain.

The level of harm

Either way, it is now clear that exposure to high levels of manganese is not good for the brain. But what constitutes a high level? As a trace element in people’s diet, manganese is essential to keeping organs, including the brain, healthy. The American standard for the airborne concentration of manganese dust is now 5 milligrammes per cubic metre of air—a vast improvement on the doses of close to 1,000mg/m3 that some workers were exposed to only 60 years ago. Last year, Robert Park, a statistician with America’s National Institute for Occupational Safety and Health, published a review which concluded there was good evidence of neurological effects at concentrations lower than 0.2mg/m3.

Manganese is not the only toxin in industrial emissions whose effects on human health are not properly understood. Such controversies will continue.

This article appeared in the Science & technology section of the print edition under the headline "Subtle effects"

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