Acoustical tile ceilings tend to get dirty, and are very difficult to clean. Some people just paint them, but that plugs up the acoustical holes. In this TV segment we walked through the cleaning of a very dirty ceiling in a community centre with a company called Enviro Clean from Cornwall, Ontario. The same service is available for homes.
They dust off cobwebs, spray on a special solution, wipe it off and the job is done. It actually works.