36 
earlier by Dr. Berg. The first principle, as stated in the guidelines, is 
that there are certain experiments which in the light of currently available 
information may be judged to present potential hazards of so serious a na- 
ture that they should not be attempted at this time. 
The second principle is that there is a large group of feasible 
experiments which appear to pose lesser or no potential hazard, and there- 
fore can be performed, provided that the information to be obtained or the 
practical benefits anticipated cannot be obtained by conventional methods, 
and provided that appropriate safeguards for containment of potentially 
hazardous organisms are incorporated into the design and execution of the 
experiment. 
Third, that the more serious the nature of any possible hazardous 
event, the more stringent should be the safeguards against escape of the 
potentially hazardous agents. Since the estimation of potential hazards 
is currently conjectural and speculative, the levels of required containment 
for potentially hazardous organisms should be set high initially, and modi- 
fied only when there is substantial relevant information to advise such 
modifications . 
The fourth and final principle is that the guidelines are to be review- 
ed at least annually, in order to account for new information. 
There are three approaches to the problem of containing potentially 
hazardous organisms which form the basis of the safeguards recommended by 
the guidelines. Each of the three may be viewed as setting up barriers to 
the dissemination of potentially hazardous organisms from the laboratory 
situation, and as setting up barriers between the laboratory worker and the 
organisms. Two of these approaches involve the limitation of the actual 
physical escape of the organisms and are referred to as physical containment. 
The first such approach is the set of standard microbiological prac- 
tices that have been developed over a period of many years and are widely 
used for handling pathogenic organisms both in research and clinical labora- 
tories. In the hands of well-trained personnel, these procedures are proven 
to be effective in safeguarding both the worker and the environment from the 
spread of pathogenic agents. 
The second approach to physical containment involves the use of special 
kinds of equipment and facilities to limit the spread of aerosols and for 
decontamination and for containment of laboratory air and wastes, as well as 
limitation of access to laboratories. 
As with the standard microbiological techniques, these types of equip- 
ment and facilities are not new, but have been developed and used previously 
for containment of known pathogenic organisms. The guidelines go into 
some detail concerning the practices and facilities required for physical 
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