I don't believe that is entirely accurate. Positive pressure systems are designed to blow 99.999% sterile air over the work area to prevent contamination. Commercial PCR workstations will typically take dirty outside air, run it through a prefilter, through a HEPA and then over the workstation either horizontally or vertically - blowing away any contamination that might be brought towards the sample. I'm not sure I've run into anyone who do this kind of sterile work who has necessitated the need for the kind of negative pressure system you're suggesting.

Also, I'm learning how to do DNA barcoding and typically, the PCR product isn't opened but rather sent off for sequencing or analysis. Anyway, thank you! Maybe I'm wrong so I'm happy to hear your thoughts. And thanks for the support.