what is the actual chemical in shellfish that results in the allergy?

I doubt fish emulsion would cause problems as it's fish, not shellfish used in the processing, but the compost I really can't say.

There are people so allergic to something they will go in to anaphlactic shock just by being in the same room and breathing the substance that triggers the reaction. Anaphlaxis is a life threatening emergency because it causes a persons respiratory system to swell shut and they cannot breath.
While I doubt that the foods grown with that "fertilizer" would absorb the allergin (my medical sources have not confirmed that however), you would need to be carefull, I'd think, spraying that around when your daughter in law is present, especially if just smelling the allergin is enough to trigger a reaction.

thank you for your advice. I ended up calling the company that makes it and they told me there is absolutely no shellfish in the fish emulsion.

I’m also wondering about sea compost that says it has shrimp in it and I have an anaphylactic reaction to shrimp and scallops...just wondering if it changes the allergin -such as iodine -in the composting process....

Which brand of fish emulsion did you ised that contains no shellfish? i cant fisn any

ALL fish emulsions contain no shellfish. They are a byproduct of the fish oil and fish meal industry.....essentially recycling a waste product. No shellfish are included in this process. They are too valuable for that sort of processing.

I am allergic to fish & someone used fish fertilizer in thier veggie garden. they shared the vegetables with me & I had a mild reaction.