The first order variation of critical loads of thin-walled columns with bisymmetric open cross-sectiondue to some variations of the stiffness and location of bracing elements is derived. The con-siderations are based on the classical linear theory of thin-walled beams with non-deformablecross-section introduced by Vlasov [1]. Both lateral braces and braces that restraint warping andtorsion of the cross-section have been taken into account. In the numerical examples dealing withI-column, the functions describing the influence of location of the braces with unit stiffness on thecritical load of torsional and flexural buckling are derived. The linear approximation of the exactrelation of the critical load due to the variation of the stiffness and location of braces is determined.

A mass balance model lo calculate critical loads of' airborne acidity and cutrophication to forest ecosystems has been computerized. The SONOX soliware developed at the Institute of Environmental Protection replicates the sequence of' events on the path from sulfur and nitrogen emission through their deposition and next overlaying it on critical loads values lo identify the extent and areas of critical loads cxcccdance. To support decision making a converse direction is offered to assess the necessary emission reductions lo meet assumed environmental goals by eliminating or suitably abating the critical loads cxcccdance. This software originally developed and applied to support the Polish contribution lo the negotiations of the Oslo and Gothenburg Protocols or the Convention on Long-Range Transboundary Air Pollution was thereafter used to assess the capacity of achieving the, interim environmental quality targets ofthe NEC Directive in Poland, to support the development of air protection programs lor administrative units exposed to transboundary lluxcs and other decision making purposes.