Q: What is the black substance that appears on the outside of onions? It looks like mold but also seems a little like dirt or soot.
— R.H., Westerville, Ohio

A: That black, sooty-looking substance on the skins of onions is indeed a mold, specifically, Aspergillus niger. It is common on onions, both in the field and once dried, bagged and stored.

The fungus typically is found on the outside skins of an onion, and it is harmless, for the most part.

Simply peel away the skins and give the onion a good washing.

Discard the onion if the mold has grown beyond a little dusting on the outside skins.
While not too harmful if ingested, Aspergillus niger can cause sickness, particularly in those with weakened immune systems. It is not nearly as dangerous as its cousin, Aspergillus fumigatus, a common airborne pathogen that can cause severe respiratory illness.

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