Noun | 1. | ![]() tier, grade, level - a relative position or degree of value in a graded group; "lumber of the highest grade" biosafety level 1 - exposure only to infectious agents that do not ordinarily cause human disease biosafety level 2 - exposure to infectious agents that can cause disease in humans but whose potential for transmission is limited biosafety level 3 - exposure to infectious agents that can be transmitted by the respiratory route and which can cause serious infection biosafety level 4 - exposure to exotic infectious agents that pose a high risk of life-threatening disease and can be transmitted as an aerosol and for which there is no vaccine or therapy |