  We review the heuristic arguments suggesting that any thermal quantum field
theory, which can be interpreted as a quantum statistical mechanics of
(interacting) relativistic particles, obeys certain restrictions on its number
of local degrees of freedom. As in the vacuum representation, these
restrictions can be expressed by a `nuclearity condition'. If a model satisfies
this nuclearity condition, then the net of von Neumann algebras representing
the local observables in the thermal representation has the split property.
