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Why leave the area for 30 minutes following a BL2 spill?

Have you ever wondered where the recommendation to leave the area for 30 minutes before cleaning up a BL2 level spill comes from?  The 30 minute timeline for re-entering following a BL2 spill is a standard recommendation to prevent aerosol exposure. This recommendation appears in spill procedures for nearly all companies and academic labs that conduct work with biological material. In addition, it is referenced in CDC, NIH, WHO, and ABSA publications. But what is this recommendation based off of?

In large part this recommendation is based off of a 1994 CDC study published in the Guidelines for Preventing the Transmission of Mycobacterium Tuberculosis in Health-Care Facilities (MMWR 1994; Vol 43, RR-13). The study showed that the time required for removing a given percentage of airborne particles from a room depends on several factors. These factors include the number of air changes per hour (ACH), which is determined by the number of cubic feet of air in the room and the rate at which air is entering the room at the intake source; the location of the ventilation inlet and outlet; and the physical configuration of the room.

Lab air exchange rates have been historically set at 8-12 ACH.  The study showed that it would take between 23 minutes (at 12 ACH) and 35 minutes (at 8 ACH) for a 99% removal efficiency for airborne aerosol contaminants. This is consistent with the standard 30 minute recommendation for leaving an area following a BL2 spill.  However, for a 99.9 % removal efficiency, the time required was actually significantly higher than 30 minutes (35-52 minutes depending on the air exchange rate).

Of significance is that the pendulum has started to swing on what is considered an acceptable lab ventilation air exchange rate due to ever increasing energy costs.  Depending on the work being conducted, 6 ACH is now often considered acceptable.  However, at 6 ACH it takes considerably longer for aerosol contaminants to be removed (46 minutes for 99% removal efficiency and 69 minutes for 99.9% removal efficiency.)  It will be interesting to see if trends to decrease lab air exchanges rates will result in a change in standard biosafety recommendations related to the time needed to allow aerosols to settle following a biological spill.

For additional information on the CDC study, or for tips on biological spill response procedures, please email [email protected].

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