[Tennessee_Bob]

Wonder if there's new carpeting in the building? Or if the HVAC system has been cleaned recently?

[bluefish]

Article said they changed the filters while the school was shut down. This of course allows more smaller particles behind the filter to get through initially I believe. So if the source is on the pre-filter side and still present, changing the filters is likely to allow more through. If the source was a one time event, changing them would be a good idea. Perhaps the filters should have been sent to a lab of some sort for a very thorough examination. They should have been sent to the CDC, who would probably appreciate an opportunity to inspect it and probably has all kinds of experts who would know what to look for.