Instruct: Classify a given Amazon review as either positive or negative.
Query: Good Air Purifier for the Price

It doesn't explicitly state it in the product description, but the Air Purifier comes with a permanent HEPA filter and one pre-filter. The pre-filter is supposed to be changed every three months and the HEPA filter is supposed to be vacuum cleaned every six months. The pre-filters cost between $10 and $15. I've only had it running for several says and the pre-filter is already covered with dust, so it's really doing what it's supposed to be doing. What's more is that the air is so clean and crisp. The instructions say that you should run the air purifier 24 hours a day, but I think that's a huge waste of energy and only run it when I'm in the room, which seems to work just fine. It does make a lot of noise, but it's white noise, like a window air conditioner or a box fan. I almost didn't buy it because of I was afraid the noise would keep me up at night, but was pleasantly surprised that the white noise helps me sleep more soundly through the night.