Q.Killing Dandelions
I want to add dandelions to my salads. I live in a house built in the 70s. I don’t know what chemicals have been put on the ground previously. How long do chemicals stay in dandelions? Do the chemicals transfer to the seeds? Is there any time when the chemicals are no longer a threat to human kind? Thanks for your help.

Certified GKH Gardening Expert
You'd have to know what the area's been treated with and what kind of soil you have. Some herbicides break down quickly; some pesticides can take up to five years. You may want to contact your local university extension service for more information. There is no way I know of to test soil for chemicals at home, but perhaps they will know more.
I hope these links will help:
https://www.gardeningknowhow.com/extension-search
https://www.canr.msu.edu/uploads/234/68557/rec_chem_soil_test_proce55c.pdf
You could raise the dandelions in a raised bed that you added soil to if you don't particularly want to eradicate them from your yard.