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 Subject : How long to wait after a case or to allow providers without N95 to ent.. 05/05/2020 06:43:11 AM 
Leopoldo Rodriguez, MD, MBA, FAAAP, FASA, SAMBA-F
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Subject :How long to wait after a case or to allow providers without N95 to enter the room after aerosolizing

Each ASC has an "ENVIRONMENTAL SAMPLING VENTILATION TESTING" Air Exchange report. My facility has this done twice a year.

When reading the report, they label the room number, post a target number "Minimal Ventilation Requirements" based on building code at the time the building was built and a measured "Minimal Ventilation Requirements"

The Measured should always be at or above Target.
The measured number can then be checked on the CDC website:
CDC Airborne Contaminant Removal: https://www.cdc.gov/infectioncontrol/guidelines/environmental/appendix/air.html#b1

The table tells us based on the Measured air exchanges (ACH) 1st column on the left, how many minutes will it take for the ACH to remove 99% of particles (2nd column), or 99.9% on the third column.

Older buildings have a Target ACH in GI of 6 ACH; and Target ACH in OR of 15 ACH.
Newer buildings have a Target ACH in GI of 15 ACH; and Target ACH in OR of 20 ACH.

In the CDC table if your Measured ACH is 6: then it will take 46 minutes for 99% removal or 69 minutes (3rd column) for 99.9% removal.
In the CDC table if your Measured ACH is 15 to 20: then it will take 18 minutes for 99% removal or 28 minutes (3rd column) for 99.9% removal.

However, these numbers change if the door is opened or if there are people in the room.
 Subject : Re:How long to wait after a case or to allow providers without N95 to .. 05/05/2020 06:48:44 AM 
Leopoldo Rodriguez, MD, MBA, FAAAP, FASA, SAMBA-F
Posts: 53
Location: 
Subject :Re:How long to wait after a case or to allow providers without N95 to enter the room after aerosolizing

1. Airborne Contaminant Removal
Table B.1. Air changes/hour (ACH) and time required for airborne-contaminant removal by efficiency *
The number of air changes per hour and time and efficiency:
ACH § ¶ Time (mins.) required for removal. 99% efficiency 99.9% efficiency
2 138 minutes 207 minutes
6+ 46 69
15+ 18 28
20 14 21

* This table is revised from Table S3-1 in reference 4 and has been adapted from the formula for the rate of purging airborne contaminants presented in reference 1435.

+ Denotes frequently cited ACH for patient-care areas.

§ Values were derived from the formula:

t2 – t1 = – [ln (C2 / C1) / (Q / V)] X 60, with t1 = 0

where

t1 = initial timepoint in minutes
t2 = final timepoint in minutes
C1 = initial concentration of contaminant
C2 = final concentration of contaminant
C2 / C1 = 1 – (removal efficiency / 100)
Q = air flow rate in cubic feet/hour
V = room volume in cubic feet
Q / V = ACH

¶ Values apply to an empty room with no aerosol-generating source. With a person present and generating aerosol, this table would not apply. Other equations are available that include a constant generating source. However, certain diseases (e.g., infectious tuberculosis) are not likely to be aerosolized at a constant rate. The times given assume perfect mixing of the air within the space (i.e., mixing factor = 1). However, perfect mixing usually does not occur. Removal times will be longer in rooms or areas with imperfect mixing or air stagnation.213 Caution should be exercised in using this table in such situations. For booths or other local ventilation enclosures, manufacturers’ instructions should be consulted.
 
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