3 
assessed risk or in the applied methodology, (iv) The guidelines will 
be subjected to periodic review (at least annually) and modified to reflect 
improvements in our knowledge of the potential biohazards and of the 
available safeguards. 
In constructing these guidelines it has been necessary to define 
boundary conditions for the different levels of physical and biological 
containment and for the classes of experiments to which they apply. We 
recognize that these definitions do not take into account existing and 
anticipated special procedures and information that will allow particular 
experiments to be carried out under different conditions than indicated 
here without sacrifice of safety . Indeed, we urge that individual investi- 
gators devise simple and more effective containment procedures and that 
study sections give consideration to such procedures which may allow change 
in the containment levels recommended here. Our recommendations should, 
therefore, be considered as guidelines in the most literal sense. 
II . Containment 
Effective biological safety programs have been operative in a variety 
ot laboratories for many years. Considerable information therefore already 
exists for the design of physical containment facilities and laboratory 
procedures applicable to organisms carrying recombinant DNAs (4-13). The 
existing programs rely upon mechanisms that, ■For convenience, can be 
divided into two categories: (i) a set of standard practices that are 
ge n eral ly used in microbiological laboratories, and (ii) special procedures, 
ecu 1 - ent, and laboratory installations that provide physical barriers 
whic r a-'e applied in varying degrees according to the estimated biohazard. 
[ 74 ] 
