Half an onion by your bed during winter? Doctors say this about it

It turns up every winter on Facebook and in family chats: cut an onion in half, put it on the bedside table, and it will absorb the flu virus overnight. By morning, it's supposedly black "from all the germs."

The tip is centuries old. According to the US National Onion Association, the belief goes back to the 1500s, when cut onions were placed in rooms to protect against the plague, long before anyone knew about germs.

A Dutch ear, nose and throat specialist explains how the old remedy might offer some temporary relief, though not for the reason your elders claimed.

How a plague-era belief became a winter remedy

Centuries ago, people believed diseases spread through miasma, or unhealthy air. Placing onions around a sickroom was thought to absorb whatever was making people ill.

The practice survived long after scientists discovered viruses and bacteria.

During the 1918–1919 influenza pandemic, a story circulated about a farming family supposedly spared from the illness because they kept onions around their home. Versions of that story still appear on social media today.

But there's no reliable scientific evidence that onions protected anyone from influenza.

Why does an onion turn black overnight?

One popular version of the remedy claims that a darkened onion proves it has collected germs from the room.

Food scientist Ruth MacDonald of Iowa State University explains that onions don't work like air filters.

When a cut onion sits out, its cells gradually break down. Microorganisms can also grow on the exposed surface, contributing to discoloration and decay.

That doesn't mean the onion has pulled harmful germs out of the surrounding air.

There's another related myth: that leftover cut onions become poisonous. That's also misleading. The National Onion Association says properly handled cut onions can be refrigerated in a sealed container for up to seven days.

Why an onion might actually help a blocked nose

Despite all the evidence against it, there is one interesting detail behind Grandma's advice.

Digna Kamalski, an ear, nose and throat specialist at UMC Utrecht, explained to Dutch science magazine Quest that cutting an onion releases an irritating gas called syn-propanethial-S-oxide. It's the substance responsible for making your eyes water when chopping onions.

It can also irritate the lining of your nose, causing it to run. For someone struggling with congestion, that temporary watery discharge might feel like relief.

But Kamalski stresses that the effect doesn't last all night and won't shorten a cold.

In other words, an onion might briefly make your nose run, but it doesn't cure an infection or kill flu viruses in your bedroom.

What actually helps protect against flu?

Flu spreads mainly through virus-containing particles released when infected people breathe, talk, cough or sneeze. Contaminated hands and surfaces can also contribute to transmission.

Health information website Apotheek.nl recommends keeping your distance from sick people, coughing or sneezing into your elbow, using tissues and washing your hands regularly.

Fresh air and good ventilation also help reduce exposure to respiratory viruses indoors.

Unlike a bowl of onions, these measures address how infections actually spread.

This winter's flu jab: who is eligible?

In England, the NHS offers free vaccination to eligible groups, including adults aged 65 and over. In the United States, the CDC recommends annual flu vaccination for nearly everyone aged six months and older.

In the Netherlands, the RIVM offers a free flu jab to people aged 60 and older, including those turning 60 before May 1, 2027. People with certain medical conditions, including diabetes and heart, lung or kidney disease, are also eligible. Invitations are arriving later than usual this autumn because vaccine deliveries were delayed by several weeks. The vaccination is voluntary and helps reduce the risk of serious flu complications.

So there's no need to throw away Grandma's old recipes. But when it comes to protecting yourself against flu, an onion is better used in your dinner than on your bedside table.