A report by the American Academy for the Advancement of Science has concluded that much of the currently uncontrolled dioxins that North Americans are exposed to come from the incineration of wastes
The red part is a restrictive relative clause that modifies uncontrolled dioxins
that is a relative pronoun.
The second sentence means the same
A report by the American Academy for the Advancement of Science has concluded that many of the currently uncontrolled dioxins to which North Americans are exposed come from the incineration of wastes.
most part of the currently uncontrolled dioxins, we are exposed to which.
which and uncontrolled dioxin are coreferential.
Which = relative pronoun and object of the preposition of exposed