National Institutes of Health 
Dr- Fredrickson 
9/21/78 
Page 2 
Adding the above statements makes it clear that periodic 
certification is required. 
We are recommending semi-annual certification for cabinets 
in P3 laboratories (and moderate risk and clinical laboratories 
working with class 3 agents) because we have found that many 
Biological safety Cabinets can increase in pressure drop by 
50% or more in a 6 month period of heavy use. NSF Std 49 
permits a decrease in airflow of up to 10% as a result of a 
50% increase in pressure drop. Thus, over the course of a year, 
a loss of airflow of 20% or more could occur. We understand that 
Dr. Barkley has data showing that a 20% loss in airflow results 
in one order of magnitude decrease in containment. At P3 level, 
we think this is sufficient reason to warrant twice annual inspect- 
ions. At P4 level, the exhaust systems of class III cabinets 
are typically adequate for once annual certification, and class 1 
and class 2 cabinets in suit rooms are adequate if certified 
once annually, but class 1 or 2 cabinets used with HV2 Biological 
Containment in the absence of a suit room should be certified twice 
annually for the reasons noted above. 
B. We have a number of comments to paragraph II-B-3-c-(7) 
and are splitting them up for ease of description and clarity. 
The first suggestion is to add after the second sentence "The 
system shall contain an alarm which will operate if loss of 
directional airflow occurs and shall preferably be interlocked 
to cut off any incoming air if loss of directional airflow 
occurs." The directional airflow property of the P3 laboratory 
is an important containment feature. We have seen a number of 
system failures which would have caused reversal of the air- 
flow direction in systems without interlocks . 
C. The end of paragraph II-B-3-c-(7) of the proposed 
guidelines states, "The exhaust air from the (P3) laboratory 
shall be discharged to the outdoors so that it is dispersed 
clear of occupied buildings and air intakes. The exhaust air from 
the laboratory can be discharged to the outdoors without filtration 
or other treatment." This is a change from the 1976 guidelines. 
We propose that this part of paragraph be rewritten as follows: 
"The exhaust air from the (P3) laboratory shall be either 
HEPA filtered and discharged outdoors or shall be discharged 
to the outdoors so that it is dispersed clear of occupied 
buildings and air intakes in which case it need not be filtered 
or otherwise treated." 
We believe that this revision, in conjunction with comments 
below, will clarify the guidelines and avoid some possible legal 
[A-219] 
