If there is one kitchen frustration that unites home cooks of every age, skill level and nationality, it is this: the moment the knife hits the onion, the eyes start streaming. It happens every time, to almost everyone, and it has been happening since humans first started cooking.

But now, physicists actually studied it – properly, with high-speed cameras and microscopes – and what they found changes the advice somewhat. More on that shortly.

First, the science behind the tears.

Why do onions make you cry?

When an onion is cut, its cells are damaged and an enzyme called lachrymatory-factor synthase is released. This enzyme triggers a chemical reaction that produces a volatile gas called syn-Propanethial-S-oxide, which drifts upward, reaches your eyes and stimulates the tear glands. The tears are a genuine physical response to chemical irritation — not the onion personally attacking you, though it can feel that way.

The key insight from a new study published in the journal PNAS in 2025 is that the sprays were directly linked to the sharpness of the knife and the speed at which each knife stroke cut the onion. The sharper the knife and the slower the cut, the less the spray. Duller knives pushed down on the onion, forcing its layers to bend inward, and as the cut continued, these layers sprang back and forced juice into the air.

In other words, the single most evidence-based thing you can do about onion tears is also something every cook should be doing anyway: use a sharp knife and cut slowly.

Here are nine hacks ranked by how well they actually work:

1. Use a sharp knife — and cut slowly

The sharper the knife and the slower the cut, the less spray produced. Blunter knives require more force to break the onion’s skins, pressure builds in the onion’s juices, and quicker, forceful cuts with a dull blade propel droplets even further. This is now backed by peer-reviewed physics research. Sharpen your knife. It is the single most effective thing you can do.

2. Chill the onion first

Refrigerating your onion for at least 30 minutes before cutting – or popping it in the freezer for 15 minutes – significantly slows the chemical reaction that produces the irritating gas. Cold temperatures suppress enzyme activity. This is one of the most consistently effective hacks and costs nothing. Do not freeze completely, as this affects texture.

3. Turn on the range hood or open a window

Turning on a stove hood, opening a window or chopping near good ventilation helps move the gas away from the face before it reaches the eyes. Even small improvements in airflow can noticeably reduce irritation. This is simple, free and significantly underrated.

4. The wet paper towel trick

Place a damp paper towel flat on the cutting board next to the onion while you chop. The theory – which Dr Joe Whittington popularised and which has attracted millions of views – is that the moisture attracts the volatile gas before it reaches your eyes. Results are mixed in testing, but many people swear by it. It costs nothing and is worth trying.

5. Leave the root end intact until last

The root end has the highest concentration of enzymes. Cutting it last means the most irritant-heavy part of the onion is exposed for the shortest time. Always cut the root end off last rather than first. The Occasional Traveller

6. Cut near running water

Holding the onion near a running tap, or keeping a bowl of water nearby, can help absorb some of the volatile gas before it travels upward. Not practical for large quantities but useful for a quick single onion.

7. Wear contact lenses or goggles

Contact lenses create a physical barrier between the gas and the cornea, which is why many contact lens wearers report far less trouble with onions than glasses wearers. Dedicated onion goggles are also available – unglamorous but genuinely effective for anyone who chops onions frequently.

8. Soak the onion in water

Soaking the onion for 10 to 15 minutes helps dilute the tear-causing chemicals. This does slightly soften the onion, so it is best for recipes where texture is not crucial. Good for caramelised onions or soup, less ideal for fresh salads.

9. Chew gum while you chop

Chewing gum while cutting onions engages mouth breathing and may also reduce sensitivity to the gas. Some cooks swear by it. The evidence is anecdotal but the theory is plausible — and it costs almost nothing to try.

What doesn’t work

Microwaving the onion before cutting has been tested and generally produces an unpleasant smell without meaningful tear reduction. Holding a piece of bread in your mouth is a popular old trick — the jury remains firmly out. Breathing through your mouth helps slightly but not significantly.

The combination that the evidence most consistently supports is: keep your onions in the fridge, use a properly sharpened knife, cut slowly, and turn on the range hood. Add the wet paper towel if you like — it may help and certainly does not hurt. That combination will not eliminate tears entirely for everyone, but it will reduce them significantly for most people.

And if all else fails, the onion goggles are genuinely effective. Your dignity is a small price to pay for a good French onion soup.

@drjoe_md Cutting onions releases a chemical compound known as propanethial S-oxide. When this compound comes into contact with the moisture in your eyes, it forms sulfuric acid, causing irritation and stimulating the eyes to produce tears as a defense mechanism to flush it out. To minimize this effect: 1. Lay a wet paper towel next to the onion – The wet paper towel can help absorb some of the irritant gases released by the onion. 2. Chill the onions in the fridge before cutting – Cooling the onions slows down the chemical reaction that releases the irritating gases, reducing their impact on your eyes. 3. Use a sharp knife – A sharper knife causes less damage to the onion’s cells, resulting in fewer irritants being released into the air. #kitchenhacks #tipsandtricks #healthtips #medical ♬ original sound – Dr. Joe, M.D. 🩺