"...prolonged periods without airing and drying usually means that the mattress accumulates moisture that never goes anywhere. Which gets rather nasty after a few months..."

That's a good point. Setting them out in bright warm sunlight would go a long ways to keeping them fresh. That might even be the reason behind the box springs that normally go underneath.

One thing I forgot to add: use plastic under the sheets (or whatever) with youngsters. And maybe even with elderly people.

Sue