Typically negative air effectively air scrubbers (like what you see in the picture below) which have the ability to attach ducting to it to exhaust the air outside. Exhausting the air outside creates negative pressure inside the environment and helps inhibit the spread of polluted air to other areas inside the building.

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An air scrubber can be used also to recirculate the air inside the property without exhausting outside. The configuration does not affect room pressurisation.