▁the ▁release ▁of ▁inf ect ious ▁organ isms ▁into ▁the ▁immediate ▁labor atory ▁and ▁the ▁environment . ▁Add itional ▁features ▁to ▁prevent ▁transmission ▁of ▁CL 3 ▁organ isms ▁are ▁appropriate ▁resp ir atory ▁protection , ▁HE PA ▁fil tr ation ▁of ▁exha usted ▁labor atory ▁air , ▁and ▁strictly ▁controlled ▁labor atory ▁access . ▁Ex amples ▁include ▁t uber cul osis ; ▁West ▁N ile ▁virus ; ▁and ▁pan demic ▁H 1 N 1 ▁influen za . ▁ ▁A ▁small ▁percentage ▁of ▁labor atory ▁space ▁( 3 . 6 %) ▁is ▁devoted ▁to ▁Cont ain ment ▁Level ▁ 4 ▁( CL 4 ) ▁at ▁N ML . ▁These ▁agents ▁have ▁the ▁potential ▁for ▁a eros ol ▁transmission , ▁often ▁have ▁a ▁low ▁inf ect ious ▁do se ▁and ▁produce ▁very ▁serious ▁and ▁often ▁fatal ▁disease ; ▁there ▁is ▁no ▁lic ensed ▁treatment ▁or ▁v acc ine ▁available . ▁This ▁level ▁of ▁contain ment ▁represents ▁an ▁isolated ▁unit ▁independent ▁of ▁other ▁areas . ▁CL 4 ▁emphas izes ▁maximum ▁contain ment ▁of ▁the ▁inf ect ious ▁agent ▁by ▁completely ▁se aling ▁the ▁facility ▁per imeter ▁with ▁confirm ation ▁by ▁negative ▁pressure ▁testing , ▁isol ation ▁of ▁the ▁research er ▁from ▁the ▁path ogen ▁by ▁an ▁en closed ▁positive ▁pressure ▁suit , ▁and ▁de cont am ination ▁of ▁air ▁and ▁all ▁other ▁materials . ▁Ex amples ▁include ▁E bol a , ▁N ip ah , ▁Mar burg , ▁and ▁ 1 9 1 8 ▁pan demic ▁influen za . ▁ ▁Str ucture ▁▁ ▁N ML ▁programs