This is a rule to be followed when doing experiments with organisms in the laboratory to ensure safety. Pathogenicity of microorganisms is the most important decisive factor affecting biological hazard assessment. The risk classification of infectious microorganisms is the basis of laboratory biological hazard assessment and the establishment of biological safety protection level of pathogens to be identified. Generally speaking, microorganisms with high hazard level need higher biosafety protection. However, even for the same microorganism, different experimental operations may have different potential hazards. Therefore, in determining the appropriate biosafety level, other factors should be considered besides the harm degree of pathogens. For example, microorganisms classified as risk level II usually need to be treated under BSL-2 conditions. However, if certain experiments may produce high concentrations of aerosols, it is more suitable to operate under BSL-3 conditions. For another example, highly pathogenic avian influenza virus and AIDS virus with the second degree of harm are required to be operated in the BSL-3 protection laboratory, while uncultured infectious substances are only required to be operated in the BSL-2 protection laboratory.
Biological hazard assessment may also be carried out under the condition of insufficient relevant information. At this time, according to the requirements of the third edition of WHO Laboratory Biosafety Manual, some conservative specimen handling methods should be carefully adopted.