Q: “Is it really toxic to leave cut onions out? I thought it was an old wives’ tale…but then a friend of mine (who I would generally trust) recently told me it is. Something about them soaking up toxins in the air or something?”
A: I was not aware of this idea until yesterday. I knew potatoes can be toxic if green, and honey can have botulism spores in it which can be dangerous to infants, but I had never heard of the notion that onions serve as a sort of magnet for bacteria. The good news is that there does not appear to be any scientific basis for this idea. In fact, onions may have antibacterial chemicals which would likely prevent most bacteria from growing on them. At any rate, they do not have the preferred combination of factors to encourage bacterial growth. Bacteria prefer moist, sugary and/or protein-filled foods with a neutral pH. Onions, on the other hand, are somewhat acidic (pH ~5.3-5.4; neutral pH=7), which makes them relatively inhospitable for pathogens. As for soaking up other toxins in the air, there isn’t any particular reason an onion would be a good toxin sponge.
Conclusion: It’s safe to leave cut onions out. The only thing likely to happen is that they’ll dry out.