The tears and stinging that come with cutting an onion range from irritating to outright painful. This can make using chopped onions in your recipes a chore. Instead of ignoring the problem or buying pre-cut onions, there are several ways to reduce or eliminate the issue, which is caused by certain enzymes in the onion being released into the air.
Preparing the Onion
Keeping the onions in the refrigerator, or otherwise keeping them cold, inhibits the release of the enzyme into the air, reducing or eliminating the tears and stinging. You can also make cuts into the onion and soak it in water, letting the enzymes and juices out.
Onion Cutting Techniques
Most of the tear-causing enzyme is stored near the root, so avoiding the root until the end and cutting the top and middle first is a good technique. Cutting the onion underwater or under running water has much of the same effect as chilling the onion, as discussed in the previous section, but takes less preparation time. Use a sharp knife for a cleaner cut, allowing fewer vapors to get out.
Outside Methods
Instead of changing the way you cut, you can change the outside environment. Light a candle and place it within a couple of feet of your chopping area. The flame draws the vapors from the onion and prevents tears. This tip is especially useful during grilling season, when you're outdoors near an open flame grill. The flames burn off the sulphurous compounds in the vapors that cause the tears and stinging, rendering them as harmless as water vapor.
If you don't want to change your technique or prepare your onion in advance, you can always buy specially made onion goggles, designed with the same sort of proofing as scuba goggles. That is to say, as with scuba goggles, no particles are getting in or out.