the negative air pressure within the work space 13 by increasing the air flow pressure needed to move the closure 24 given a constant negative air machine performance within the work space 13 as hereinbefore described by referring now to fig2 and 4 of the drawings a modification of the invention can be seen comprising a building 50 having a doorway 51 a negative air machine 52 incorporating a hepa filter 53 and a blower is arranged to exhaust to atmosphere and secondary filter and blower 54 provides additional circulation in a work space 55 a portable temporary enclosure 56 is formed of rigid doorway panels 18a and solid doors 20a and a transom 17a which are positioned upright by tubing sections 25a secured thereto and a plastic film sleeve 57 as hereinbefore described in connection with fig3 and 7 of the drawings arrows show air flow through the enclosure 56 into the work space 55 and through an airlock 56a defined between said doors 20a in fig4 of the drawings a modification of the rigid doorway panel 18a may be seen to comprise the solid weighted transom closure 57 having an adjustable weight 58 secured to the closure 57 which allows for relative change in balance of the closure 57 within the transom 17a by varying the position of the adjustable weight 58 as clearly seen tubing sections 25a as hereinbefore described support the modified rigid doorway panels 18a which can also be used with the systems of fig1 and 3 of the drawings it will occur to those skilled.