Positive air pressure is a must if you work in the CDC or Center for Disease Control. In their level four containment facilities they always maintain air pressure very carefully.
These level four containment facilities contain the most deadly diseases on the earth and they need to be contained.
In these labs if there happens to be a spill the fumes would be whisked into a ventilation system designed with the best filters in the world. These filters are designed with deadly bacteria and virus control in mind.
In the book 'Virus Hunter: Thirty Years of Battling Hot Viruses Around the World' by C. J. Peters and Mark Olshaker they discuss this when explaining the work they participated in in one of these facilities.
There aren't many of these facilities throughout the world, but they are constantly working with some of the worst things on the face of the planet.