Lots of tips exist to help you prevent crying caused by cutting onions, including chilling the onion, soaking the onion in water first and holding a piece of bread in your mouth while you chop. If you missed these preventive measures and are a blubbering mess, you have options.
Cutting into an onion ruptures its tissue, which contains essential enzymes. When these enzymes come in contact with the air, they turn into a gas, called propanethial S-oxide, with properties similar to those used in tear gas. The fumes waft up toward the small glands in your eyes and cause you to tear in an attempt to flush away the chemical gas.
The first step in relief is prevention -- not exposing yourself to the gas or minimizing its release. If you failed to do that, then treatment is called for.
Wash your hands thoroughly after chopping the onions to prevent exacerbation of the irritation.
Flush your eyes with cool water or an eye wash rinse. Alternatively, blot your eyes and eye area with a damp, clean kitchen rag or paper towel to remove all residue of the gas.
Symptoms should disappear relatively quickly and at least within a few hours. The gas release peaks at about 30 seconds and subsides after about five minutes. If you don't experience relief within this time frame, visit an eye-care specialist immediately.
An exhaust fan in your kitchen or a ceiling fan can help fumes clear out more quickly and soothe you sooner. Expose yourself to fresh outdoor air, such as standing on a porch or in front of an open window, to dilute the gas.