A Question About Pennies.

My guess is swimming pool chlorine.

Sounds like she might have a bag of 1943 zinc coated steelies. That would explain the 'white' and the dustiness and the rust.

As others have stated, it does depend on what, exactly, your "pennies" are. We are assuming, since you're in the United States, that you are talking about American 1 cent coins and not "real" pennies from Britain.

1943 American cents are made of zinc-plated steel. Zinc can turn white and powdery, and the unplated steel edges of the coins can easily rust.

Pre-1982 American cents (except for 1943) are made of bronze. Bronze can, under the wrong atmospheric conditions (humid with a little salt seems to be the worst), form a type of corrosion called "bronze disease". Bronze disease is pale green, powdery and can spread throughout an entire bag of coins, coating them in pale green powder. That might explain the "powder" but not the "rust". Coins dug up by metal detectorists can also be covered in greenish verdigris.

Post-1982 American cents are made of copper-plated zinc. Zinc can, as stated above, turn powdery white. Copper-plated cents ("Zincolns") can develop nasty corrosion if the copper plating is worn or damaged.

It is entirely possible that it's some sort of chemical reaction, though if they are regular copper (bronze) cents, then it would be a very strange reaction. Virtually all copper corrosion by-products are blue or green in colour.

It's also entirely possible that they've simply been coated in chalk or some similar substance, which would wash right off.