Off-gassing
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The mechanism
Deteriorating plastics emit acidic vapours that contaminate and degrade other materials. In an open room they disperse. In a sealed jar they accumulate against the contents.
Which is the sealed jar's one risk
Everything else about sealing is an advantage. The cost is that a wrong material inside becomes concentrated rather than diluted, so the benefit depends entirely on what goes in.
The coupon test
Metal coupons can be used to detect off-gassing plastics, because metals corrode in the presence of acids. A clean piece of a susceptible metal, sealed with the material in question, and inspected later.
Which is a test a collector can actually run
It needs a jar, a doubtful holder and a bright coin you do not care about. Nothing else. For anybody with a drawer of inherited holders of unknown material, it is the practical answer.
Silver has its own enemy
Sulphur, from paper, board, rubber bands and some adhesives, tarnishes silver without any plastic being involved. The same logic applies: in a sealed jar the source and the object are in a closed room together.
So sort by material before sealing
Silver with paper is a sulphur problem. Copper with a doubtful holder is an acid problem. A jar of mixed everything is both, held together.
Open a jar occasionally and look
A sealed system that nobody inspects is a system failing invisibly. Once a year, look at the contents and the indicator, and change the desiccant while it is open.
Adhesives are the quiet source
Tape, label glue, old album mounting and self-adhesive pages all age, and several of them acidify as they do. In a sealed jar an aged adhesive is a pollutant source sitting inside the microclimate.
New plastic smell is information
A container that smells strongly of anything is emitting something. It is not a quantitative test and it is a reasonable reason to air a new container before sealing a collection into it.