There is an in-cabin air filter for the Prelude (they're in the 2001
models, so I would assume they are probably in the 97+ models too).
I've not changed mine yet or seen it, but the filter is located behind
the glove box. There is apparently some cutting involved after you
remove the glove box before you can access and see the actual filter
to change it. I would think that after the glove box is out, you
should be able to see where the filter is located. If you have access
to the Helm's manual, you might be able to get some details in there.