Short answer: encapsulation is a valid and common practice. But it doesn’t sound like it was done well in this case.

It is legal to have asbestos in buildings, but in some cases (where is it is “friable” – making airborne fibers), it should be removed (carefully by trained people) or encapsulated.

One way to encapsulate it would be to wrap it in 6-mil or 10-mil polyethylene sheeting (heavier than what painters use) and secured with a good tape. 10-mil pipe wrap (2″ wide, black with white writing on it) or the 6-mil stuff for vapor barriers (2″ wide and red) are both good. Alumimum tape for use on actual ducts is fine too but not needed (it is more expensive).

“Duct tape” is not to be used on ducts (says so right on it) and it is not a good long-term solution. The adhesive only lasts a few years (a few months outside) and it doesn’t stand up to UV exposure.

In the short-term, the duct tape would stop more fibers from being released. However, the fibers already released and the ones disturbed during their work (unless they sealed off the area and operated negative-pressure blowers), will remain in the area for months, based on controlled studies.

better wear a mask take it out and replace it. and dont use a vacuum at all. its not to code for one thing why do you think they have been replacing it in all public buildings for the past 25 years and blaming lung cancer on smoking ?

I do not think it is legal to even have the stuff anymore. It is effective for cutting down on the dust as long as nothing happens.