An enclosing structure (S), in particular for enclosing and isolating outer environment of a packaging machine includes a plurality of enclosing panels (2), suitably and air-tightly assembled. At least one of the panels (2) defines a separating surface (1) for separating two different environments (A,B). Conveying means for conveying flows of purified air (F,F1,F2,F3,F4) are associated to the panel (2) to define, together therewith, a fluid-dynamic barrier, which avoids relative contamination between one environment (A) and the other environment (B).