I hate chopping onions. They make me cry within seconds, and those tears both hurt and obscure my view of onions, knife, and fingertips (which can lead to additional injuries).
The chemical mechanism by which onions cause this agony is well known. Less well known are effective methods to prevent or mitigate this agony in order to get through chopping the quantities of onions that need to be chopped for a Thanksgiving meal.
So, I canvassed sources (on Twitter) for possible interventions and tested them.
Materials & Methods
1 lb. yellow onions (all room temperature except 1/2 onion frozen, in a plastic sandwich bag, for 25 min)
stop-watch (I used the one on my phone)
video capture (I used iMovie)
slice of bread
metal table spoon
1. Put proposed intervention in place.
2. Start the stop-watch and start chopping onions.
3. Stop stop-watch when onion-induced tears are agonizing; note time elapsed from start of trial.
4. Allow eyes to clear (2-5 min) before testing next intervention.
Here are the interventions I tested, with the time to onion-induced eyeball agony observed:
Slice of bread in the mouth: 46 sec
Metal spoon in the mouth: 62 sec
Candle burning near cutting-board: 80 sec
Onion chilled in freezer: 86 sec
Fan blowing across cutting-board: 106 sec
Swim goggles: No agony!
Note that each intervention was tested exactly once, by a single experimental subject (me) to generate this data. If there's any effect on an intervention due to being tested right after another particular intervention, I haven't controlled for it here, and your onion-induced eyeball agony may vary.
Also, I did not test these interventions against a control (which here would be me chopping an onion with no intervention). So, on the basis of this experiment, I cannot tell you persuasively that the worst of these interventions is any better than just chopping onions with no interventions. (On the basis of my recent onion-chopping recollections, I can tell you that even the slice of bread in the mouth seemed to help a little -- but THIS IS SCIENCE, where we use our tearing eyes to look askance at anecdata.)
The most successful intervention in my trials was wearing goggles. This makes sense, as the goggles provide a barrier between the eyeballs and the volatile chemicals released when the onions are cut.
The fan and the burning candle deal with those volatile chemicals a different way, either by blowing them away from the eyes, or … well, with the candle, the likely mechanism is murkier. Maybe it's that those volatile compounds get drawn to the flame and involved in the combustion reaction there? Or that the compounds released by the candle burning compete with those released by the cut onion for access to the eyeball? However it's supposed to work, compared to the barrier-method of the goggles, the candle method was less successful. Even the fan couldn't keep some of those volatile compounds from getting to the eyeballs and doing their teary work.
Cooling the onion was somewhat successful, too, likely because at a lower temperature those compounds in the onion were less ready to make it into gas phase easily. There may be a side effect of this method for those chopping onions for culinary use, in that freezing long enough may change the texture of the onion permanently (i.e., even when returned to room temperature).
I am not sure by what mechanism a slice of bread or a metal spoon in the mouth is supposed to protect one's eyes from the volatile compounds released by onions. Maybe it's just supposed to distract you from your eyes? Maybe the extra saliva produced is supposed to get involved somehow? Who knows? However, note that it was possible for us to empirically test these methods even in the absence of a proposed mechanism.
If you have lots of onions to chop and don't have a proper fume hood in your kitchen, a pair of goggles that makes a secure seal around your eyes can provide some protection from onion-induced eyeball agony. Failing that, chilling the onions before chopping and/or setting up a fan to blow across your chopping surface may help.