I found this pic here. Aren't these Koreans overkilling by wearing half face disposable respirators, inside their 3M powered air respirators? Isn't a 3M powered air respirator enough?
Raymond J. Roberge (2008) discusses the rationale for concurrent use of N95 FFR masks and PAPR hoods. The primary benefit is to reduce the number of particles the wearer inhales, and secondarily to provide backup protection in the event of power loss in the PAPR.
It's not clear that they are actually wearing a 3M powered air purifying respirator system as those have a valve and filter at the front of the mask.
Anyway, even if it is a 3M PAPR the data suggests you get a lot more protection
Objective: To determine if using an N95 filtering face-piece respirator concurrently with a loose-fitting powered air-purifying respirator (PAPR) offers additional protection to the wearer.
Methods: We used a breathing mannequin programmed to deliver minute volumes of 25 L/min and 40 L/min. We measured the baseline protection factor of the PAPR with its motor operational and then deactivated (to simulate mechanical or battery failure). We tested 3 replicates of 3 different N95 models. We glued each N95 to the breathing mannequin and obtained a minimum protection factor of 100 at 25 L/min. We then placed the PAPR on the mannequin and took protection factor measurements with the N95-plus-PAPR combination, at 25 L/min and 40 L/min, with the PAPR operational and then deactivated.
Results: The N95 significantly increased the PAPR's protection factor, even with the PAPR deactivated. The effect was multiplicative, not merely additive.
Conclusions: An N95 decreases the concentration of airborne particles inspired by the wearer of a PAPR.