I can't recall the details, but when we were shipping our household goods to China from the US some years ago, our agent specifically advised us against importing air purifiers (even though at that time there was a significant price differential) due to levied import duties.

Since OP is proposing to bring with them as hand luggage, less need to 'declare' upfront, but I wouldn't be surprised if they were stopped by customs -- they may then be waved through (most likely) or may have to pay some form of duty.

Given that Xiamoi make pretty decent air purifiers at a fraction of the cost of the major international players, I would recommend against importing because it just doesn't make financial sense.

tb