Here's a fascinating article "Workers Say Courthouse is Unhealthy" from North Carolina, USA, about a problem building.
It seems it hasn't been a great building for 36 years.
Regarding air hygiene, various tests and investigations have been done, but none have produced results convincing enough for the employer/landlord to do anything concrete about duct cleanliness
A couple of quotes from the article:
"Employees also spoke of black, dustlike particles falling from ceiling vents. County officials said that was a concoction of dust and dead skin and was not a health hazard"
“People start wondering what the test results mean,” he [State Health Dept investigator] said. “Interpretation of the results is difficult. (People) want to link mold in the environment to their health issues, and that’s a problem because we don’t have a benchmark for what is safe and unsafe.”
Now take a look at the state of an HVAC diffuser grille and the nearby ceiling as an office employee takes a tape sample for mould / mold
And yes I know some of that dirt is induced from the served area onto the grille vanes and ceiling tiles, but really - how filthy does a building component have to be before somebody says: 'Hey, let's clean this'?
Think of the time and money that's been spent over the years doing investigations, discussing results etc. Is it really that complicated or difficult to see that it's just not right for a system to be that dirty?
It's the old story: just because it's out of sight, does not mean it should be out of mind.