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Four people have died and six British backpackers are in hospital in southeast Asia after reports of suspected methanol poisoning.
Simone White, 28, from Orpington in Kent, is among those who fell ill after being offered "free shots" in the Vang Vieng resort of Laos.
Bianca Jones, 19, from Melbourne, Australia, two Danish women in their 20s, and a 56-year-old US national are reported to have died.
New Zealand has confirmed one of its citizens has also fallen ill.
Authorities are now warning travellers about the risks of methanol poisoning, how it happens, and how to avoid it.
What is methanol?
Methanol, or CH3OH, is very similar to ethanol - the pure form of alcohol in alcoholic drinks.
While like ethanol, it is an odourless, tasteless, and highly flammable liquid, it has a different chemical structure that makes it toxic for humans.
Otherwise known as wood alcohol, methanol is most often used to make solvents, pesticides, paint thinners, and alternative fuels.
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What makes it so dangerous is the way our bodies metabolise it.
Once consumed, our enzymes metabolise methanol into formaldehyde, the substance used to make industrial glue and embalming substances, before breaking it down into formic acid.
"The formic acid upsets the acid balance in blood and the major consequence is initially the effect on someone's breathing. There are effects on many other organs the kidney being one," says Professor Alastair Hay, emeritus professor of environmental toxicology at the University of Leeds.
"Formaldehyde attacks nerves, particularly the optic nerve and blindness is a potential risk," he adds.
How does it end up in alcoholic drinks?
In southeast Asia and other popular tourist destinations, methanol can be found in alcoholic drinks for two main reasons.
Firstly, it is cheaper than ethanol, so it is sometimes added instead to save costs, before the counterfeit alcohol is bottled and sold in shops and bars.
Alternatively, it can occur by accident when alcohol is homemade - something common across southeast Asia.
When alcohol is distilled and fermented without the appropriate monitoring, it can sometimes produce methanol in toxic quantities.
Because it is impossible to tell the difference between methanol and ethanol content without specialist equipment - homemade drinks are often offered to tourists without anyone knowing how dangerous they are.
What are the symptoms of methanol poisoning?
Methanol is highly toxic, so as little as 25ml can prove fatal.
Methanol poisoning can be treated by using ethanol to counter the effects on the body - but only within the first 10 to 30 hours after consumption.
This makes early diagnosis and warnings to others critical.
The most common symptoms are:
- Vomiting and nausea;
- Changes in vision, including blurring, loss of sight, and difficulty looking at bright lights;
- Abdominal and muscle pain;
- Dizziness and confusion;
- Drowsiness and fatigue.
Methanol poisoning symptoms are similar to those from alcohol poisoning - but are often more severe. If drinks were left unattended or your symptoms appear disproportionate to the amount you drank, it could be methanol poisoning, authorities warn.
How it is treated?
Professor Hay says treatment involves removing methanol from the blood via dialysis - while "keeping someone mildly drunk" by giving them ethanol at the same time.
"The principle behind administering ethanol is quite simple; it delays methanol metabolism," he says.
"Both alcohols are broken down by the same liver enzyme alcohol dehydrogenase. But the enzyme prefers ethanol.
"So ethanol acts as a competitive inhibitor largely preventing methanol breakdown, but markedly slowing it down, allowing the body to vent methanol from the lungs and some through the kidneys, and a little through sweat."
This avoids the process of methanol ultimately metabolising as formic acid, he adds.
How can you avoid it while travelling?
UK travel advice for Laos and other parts of the region advises the most commonly affected drinks are:
- Local spirits, such as rice and palm liquor, often labelled 'special' or 'happy' drinks;
- Spirit-based mixed drinks such as cocktails;
- Counterfeit brand-name bottled alcohol sold in bars and shops.
In order to minimse risks, travellers should:
- Buy alcohol only from licensed bars, hotels, or shops;
- Check labels for signs bottles may be counterfeit, including poor print quality or spelling errors;
- Avoid homemade alcohol;
- Check bottles are properly sealed before drinking from them;
- Avoid free drinks you have not seen poured yourself;
- Do not leave drinks or food unattended.
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