Humidity
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The metal number
Conserve O Gram guidance on metal objects is that they corrode in the presence of moisture and pollutants and should be housed below 35 percent relative humidity.
The plastic number
The Library of Congress warns that "photographic emulsions may stick to plastic enclosures if they are exposed to high RH, especially above 70% RH". That is about photographs and it is a general caution about plastic in damp conditions.
A room does not hold either
Domestic relative humidity swings with the weather and the heating. A loft and a basement are the two worst places in a house and the two places collections are most often kept.
A sealed jar does hold it
Which is the real argument for this shelf. A small sealed volume with a desiccant in it can be held below 35 percent indefinitely, and that is not achievable for a whole room without equipment.
Seal quality is therefore the whole game
A jar that leaks slowly simply exhausts its desiccant against the house. The liner and the thread are what decide whether you are maintaining a microclimate or decorating one.
Temperature matters through humidity
Cooler air holds less moisture, so a cold loft can be damp in absolute terms and a warm room dry. Stability matters more than the number: repeated cycling drives moisture in and out of materials.
Measure it
A humidity indicator card inside the jar, or a small hygrometer in the storage area. Both cost very little and both replace a belief with a reading.
Pollutants matter alongside moisture
The guidance names both. Urban air, a garage, a newly painted room and an attic full of cardboard all supply pollutants, and a sealed jar excludes them in the same move that it controls humidity.
Which is why sealing beats a dry room
A dehumidified room still exchanges air with the house. A sealed jar does not, so it controls both variables at once for a fraction of the cost and with no power.