Smoke visualization for the invasion of pollutants during door-opening process in pharmaceutical cleanrooms: Effects of initial pressure differential and airlock barriers

•Smoke invasion was revealed for normal door operation in pharmaceutical cleanrooms.•Effects of pressure differential and delay of opening airlock doors were studied.•At 20 and 25 Pa, the migrated smoke was less, and it was almost invisible at 18 s.•Once the airlock was set, no visible smoke invaded the cleanroom from the corridor.•Increasing time delay was beneficial for eliminating polluted air in the airlock.