Worth auditing the cage even with a healthy hamster, because a lot of breathing irritation starts there. Dusty or scented bedding is the main one, so dust-extracted unscented paper. Cedar and pine shavings are widely advised against for small rodents because of the aromatic oils.
Then ammonia from the pee corner, which needs spot-cleaning more often than the rest of the cage, and anything scented in the room: air freshener, candles, cleaning spray.