The level of decibels (dB) indicates the minimum and maximum noise levels. At night, you will preferably be using the minimum power setting, in order to minimize the noise.

If you plan to use your air purifier in your bedroom, you must make sure the levels of noise are low. The following air purifiers have been selected for being specially silent. They are all silent night air purifiers that will guarantee a good night sleep. They all have a minimum noise level <35dB , which means that running them in sleep mode will be quite silent.
Dyson Air Purifier, Dyson AM09, Dyson Pure Hot Cool Fan Heater
PARTU Air Purifier Review, PARTU Air Purifier Filter, PARTU Air Purifier
Levoit Air Purifier Review, Best Deal On The Levoit Air Purifier
Levoit LV-H132, Levoit Air Purifier LV-H132 Replacement Filter UK
Philips AC3259, Philips AC3259/60, AC3259 Philips Air Cleaner
Running at the minimum power, the air purifier will clean the air more slowly. But at night this is not a problem, since activity at home (and traffic outdoors) tends to be minimal, which makes the air cleaner’s minimum power setting to be enough. During the day, however, higher levels of activity (people walking up and down, moving objects, etc) tend to make the dust disperse from the surfaces where it’s settled, back into the air. Also outdoors pollution, part of which gets into your home, is higher. Therefore during the day we recommend using a more powerful setting in the air purifier.