Onions are known to absorb toxins. In fact, during the days of the Plague in England, folks would keep chopped onions around to absorb toxins and clean the air. This helped protect them against getting the plague.
Do not save an onion. If you use an onion, try to use all of it. It will absorb all the toxins in the air (and in the fridge) and brew bacteria which may cause stomach infection. Chopped onion could be an excellent air purifier as it is is a natural anti- microbial and anti- bacterial.