Are small flying insects coming from seemingly nowhere? Check the roof. Flat roofs are famous for collecting water in low spots, and breeding all manner of flies and beetles.
When these insects originate under air intakes for heating, ventilation and air-conditioning (HVAC) systems, they can sometimes get past the filters and show up inside. One of the first clues is that the insects are showing up in more than one room as they are being distributed by the ductwork. Often, there is no obvious reason for them to be there.
One quick check, besides looking on the roof, is to cover the vents with cheesecloth to see whether insects are trapped there. It may be easier to check the roof and the intakes, but it can be deceivingly difficult to find these issues with a casual inspection. It may also be one of many intakes. You may need to remove the filters, or have the facility provide trained staff to do it for you. You will occasionally find the breeding site behind the filters on the inside of the intake.
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