If, Like other "plastics",
exposure to the sun will degrade the styrofoam more
and faster.
I would assume that during breakdown, chemical compounds will be
changing and that some noxious chemicals "may" be present.
If they are bad enough to harm a plant, or generations of plants,
is the question.
Like others, I have either used or had bought something that did have
styrofoam in the planting mixture. In my own experience, I did not
notice any bad effects, however I find the product too light to work with
as it wants to come out of mixture, floating horribly when watering a loose
mixture. And again, it is on the surface where the sunlight hits the mixture,
potentially encouraging the styrofoam to breakdown.
I do avoid using it, as I said I don't like the way it functions...
HOWEVER I do sometimes place pieces of styrofoam in the bottom of a
pot, when I don't have anything else on hand to use or waste in the bottom
of a pot. (I don't like to waste too much Chilean Sphagnum moss just to line
the bottom of a pot!)
So far, no bad side effects, and it is a way to use something that would otherwise
go to land-fill.
Good Luck and good growing!
Paul