A couple weeks ago I read an article posted on facebook about how eating leftover onions can be toxic. The article went into detail about how if you were to cut an onion and leave if in your fridge it would pull in bacteria form all around and essentially become a breeding ground for very toxic bacteria.
"Please remember it is dangerous to cut an onion and try to use it to cook the next day, it becomes highly poisonous for even a single night and creates toxic bacteria which may cause adverse stomach infections because of excess bile secretions and even food poisoning. "
Me being the crazy food safety germaphobe that I am, I immediately freaked out. When I cook I barely ever use a whole onion, I usually put what is left in a baggie and throw it in the fridge until next time I need one. Was I just lucky all of those times?
After seeing this article a couple more times and realizing how many other people thought the same, I decided to do a little research. And I'm going to go ahead and call bullshit on this article.
I did not find one article from the scientific sources I normally use that said this was true; neither did I find any information saying that onions are a common source of food poisoning (like the article said).
Here is my reasoning for saying this is false:
First of all bacteria is not going to just flood to a certain area, it needs to be put there (by handling food with dirty hands, by placing it on a dirty surface, sneezing on it, etc.) If you put your onion in a zip lock bag, granted no one sneezes on it before hand, you should be safe.
Second of all, the reason we put food in a refrigerator is so that bacteria does not grow on it. You fridge should be at a temperature that is below 41 degrees, a cold temperature like that makes it very difficult for bacteria to reproduce. The only bacterium that thrives in a condition like that is listeria, and if you have that in your fridge you have bigger problems than eating an old onion. Therefore it is very unlikely that in one day (like the article said) you can become seriously ill from the bacteria on your onion slice.
Third of all bacteria grows in very moist conditions, granted onions do hold a lot of water, it is mostly intracellular. Therefore the bacteria would have to work its way through the cell wall and so on. Which if that is possible, is not going to happen over night.
Eating a left over onion slice has the same risk as eating a left over tomato, or cucumber slice. I assure you that you will not die if you eat an onion the day after you cut it.
However you should treat a cut onion the same way you would treat any other sliced food, store it in a dry, cool place (your fridge) and put it in a sealed container (such as a baggie or tupper ware). You should throw it away after a reasonable amount of time, cut veggies should last 2-3 days in the fridge. This should go without saying, but if they smell or look funky, don't eat them; they will probably make you sick.
Don't believe everything you read folks! This may be the onion industry's way of getting you to buy more onions ;)