Onions are a common ingredient used in cooking and can add great flavor to many dishes. However, onions also contain compounds that can cause irritation when cut or eaten raw in large amounts. The most well-known effect is tearing up or a burning sensation in the eyes when you cut onions. But can eating a lot of raw or cooked onions also cause eye irritation? Let’s take a closer look at why onions cause burning eyes and if consuming too many onions can have the same effect.
Why Do Onions Cause Eye Irritation When Cut?
Onions contain sulfur compounds called thiosulfinates. When an onion is cut or crushed, these compounds mix with the air and react to form sulfuric acid. This sulfuric acid vaporizes and floats up towards your eyes, where it can interact with the water in your eyes and tissues to cause irritation and burning.
The main irritating compound is known as syn-propanethial-S-oxide. It stimulates the trigeminal nerve in your eyes, which senses pain and irritation. This trigeminal nerve stimulation causes your eyes to water, sting, and burn.
Onion Chemistry
Here’s a more in-depth look at the chemistry behind onion eye irritation:
- Onions contain amino acids called cysteine sulfoxides. The main one in onions is isoalliin.
- When an onion is cut, the enzyme alliinase is released and acts on the isoalliin to generate sulfenic acids.
- The sulfenic acids rearrange spontaneously into syn-propanethial-S-oxide, which is volatile and floats up towards the eyes.
- Syn-propanethial-S-oxide interacts with the lacrimal glands in the eyes to stimulate tear production and a burning sensation.
So in summary, cutting onions releases sulfur enzymes and compounds that create irritating sulfuric acid vapors.
Can Eating Onions Cause Eye Irritation?
Now that we know why raw cut onions can cause eye irritation, does consuming onions have the same effect?
Eating onions generally does not cause the same degree of eye irritation as cutting them. Here’s why:
- Chewing onions releases less of the irritating compounds compared to cutting.
- Cooking onions at high heat helps to break down and denature some of the thiosulfinates.
- The volatile compounds have less exposure to the eyes when onions are swallowed compared to when cut.
- Digestion also breaks down some of the thiosulfinates.
However, eating large amounts of raw onions or onion juice may potentially cause some mild irritation in sensitive individuals. The compounds are not completely broken down during digestion, so some can enter the bloodstream and make their way to the eyes.
But the effect is much less pronounced compared to cutting or crushing onions, which maximizes release of volatile eye-irritating compounds.
Onion Sensitivity and Intolerance
Some people may be more sensitive to the compounds in onions and experience stronger eye irritation from eating them. This is more likely in those with:
- Sulfur sensitivity – some people are sensitive to sulfur compounds in foods like onions and garlic.
- Allium intolerance – an intolerance to compounds found in onions, garlic, shallots, leeks, etc.
- Irritable bowel diseases – IBD and IBS may cause sensitivity to compounds like onions.
- Gastroesophageal reflux disease (GERD) – reflux of onion contents can irritate the eyes.
- Tearing disorders – dysfunction in tear production or drainage can worsen eye irritation.
People with these conditions may experience exacerbated eye burning and irritation from eating onions. They may need to limit or avoid dietary onions.
Amount of Onions That Can Cause Eye Irritation
So how many onions is too much? At what point might eye irritation set in?
There aren’t defined thresholds established, since it depends on the individual. But some general guidelines for amounts of onion that may cause issues:
- 1-2 onions in a single meal
- 1⁄2 cup to 1 cup of raw onion
- Large amounts of cooked onions with multiple meals throughout the day
- Juicing multiple onions and drinking the juice
Consuming onions within usual culinary amounts is unlikely to cause significant eye irritation in most people. But sensitive individuals may experience irritation with smaller amounts of onions.
If you develop stinging, watery eyes, or irritation after eating onions, take note of the amount. Try reducing onion intake to see if symptoms resolve.
Tips to Prevent Eye Irritation from Onions
Here are some tips to help prevent eye irritation when handling or consuming onions:
- Avoid cutting onions. Use pre-cut, frozen, or dehydrated onions in cooking instead.
- Chill onions before cutting to reduce volatile compounds released.
- Use a very sharp knife to cut onions to limit crushing.
- Cut onions under running water or submerged in water.
- Cook onions well to help break down irritating compounds.
- Limit intake of raw onions if you experience sensitivity.
- Take antihistamine eye drops before handling large amounts of onions.
- Wear goggles or safety glasses when cutting many onions.
- Improve ventilation over the cutting board when chopping onions.
These tips can help reduce exposure to the volatile onion vapors that affect the eyes.
Onion Eye Irritation Treatment
If you experience onion irritation in the eyes, here are some ways to get relief:
- Blink and tear – Allow yourself to tear up and blink. This flushes the irritants out of your eyes.
- Cold compress – Apply a cold wet cloth or cool cucumber slices to the eyes.
- Artificial tears – Use over-the-counter lubricating eye drops to flush the eyes and reduce irritation.
- Wash with water – Splash eyes with cool clean water.
- Wash hands – Wash hands well after cutting onions to avoid spreading compounds to eyes.
- Avoid rubbing – Rubbing your eyes pushes the irritants around and makes irritation worse.
Symptoms should resolve fairly quickly once the irritating vapors are flushed from the eyes. If irritation persists or worsens, see an eye doctor for evaluation.
Conclusion
Eating onions don’t typically cause the same severe eye irritation as cutting onions. Onion compounds are mostly broken down during chewing and digestion. But eating large amounts may cause mild to moderate eye irritation in sensitive people prone to allium intolerance. Limiting dietary onions and cooking onions thoroughly can help prevent irritation. Try various remedies like cold compresses, eye drops and eye flushing to treat onion eye irritation. See an optometrist if symptoms don’t subside. Overall, enjoy onions in moderation as part of savory dishes, implement cutting tips to minimize vapors, and be aware onions can occasionally irritate eyes when consumed in excess.