Air Purifier Buying Guide: CADR, Room Size, Noise and Filter Costs Explained Simply

Air purifiers are advertised with terms like CADR, HEPA H13 and room size that confuse more than they help when buying. Yet three numbers are enough to decide whether a device fits your room: the airflow, the noise level in night mode and the filter costs per year. I have tested the Levoit Core 300S and the Blueair Blue Pure Mini Max and explain here how to read the specs and which device suits which room. The Blueair review is currently only available in German.

CADR: the most important number

CADR stands for Clean Air Delivery Rate and indicates how much cleaned air a device delivers per hour. The Levoit Core 300S is rated at 240 cubic metres per hour, or 141 CFM. For the room size you multiply floor area by ceiling height: a 20 square metre bedroom with a 2.5 metre ceiling holds 50 cubic metres of air. 240 divided by 50 gives almost five air changes per hour, more than the usually recommended two to four. Levoit specifies rooms up to 50 square metres, where it comes to just under two air changes.

Room	Air volume at 2.5 m ceiling	CADR needed for 4 air changes	Suitable device from my tests
Child’s room, office 12 m² (130 sq ft)	30 m³	approx. 120 m³/h	Blueair Mini Max
Bedroom 20 m² (215 sq ft)	50 m³	approx. 200 m³/h	Levoit Core 300S
Living room 35 m² (375 sq ft)	88 m³	approx. 350 m³/h	Core 300S with two air changes, otherwise a larger model
Open living area 50 m² (540 sq ft)	125 m³	approx. 500 m³/h	larger device needed

Filters: HEPA H13, pre-filter and activated carbon

Both devices filter in several stages. The Levoit Core 300S combines a pre-filter, an H13 HEPA filter and activated carbon and removes 99.97 percent of particles from 0.3 microns according to the manufacturer. The Blueair Mini Max combines a particle and activated carbon filter with a fabric pre-filter and filters at least 99.97 percent of particles down to 0.1 microns according to the manufacturer. Activated carbon matters for odours and cooking fumes, HEPA for pollen, fine dust and pet hair.

Noise: night mode is what counts

An air purifier runs in the bedroom at night, so the quietest level counts. The Core 300S reaches around 31.5 decibels in night mode, which is whisper-quiet, while level 3 is 58 decibels and clearly audible. The Blueair Mini Max sits between 32.5 decibels on level 1 and 51.5 decibels on level 3. For a quick clean after cooking or airing you briefly use the high level, for the night the lowest one is enough.

Power consumption and filter costs

The Core 300S draws a maximum of 22 watts on the highest level, considerably less in night mode. Even in continuous operation the consumption stays low. The filters cost more than the electricity: on the Core 300S a filter should be replaced about every six months, and there are variants for pets or mould. On the Blueair Mini Max the filter is also replaced about every six months and is cheaper.

	Levoit Core 300S	Blueair Blue Pure Mini Max
CADR / room size	240 m³/h, up to 50 m²	up to approx. 20 m², 30 minutes for 20 m²
Filters	pre-filter, H13 HEPA, activated carbon	fabric pre-filter, particle and activated carbon filter
Noise	31.5 dB night mode, 58 dB level 3	32.5 dB level 1, 51.5 dB level 3
Power consumption	max. 22 W	low
Filter change	about every 6 months	about every 6 months
Controls	touch, app, auto mode, timer, laser sensor	two buttons, no app, no timer
Weight	2.72 kg (6 lb)	1.3 kg (2.9 lb)
My rating	4.8 / 5	4.5 / 5

App and sensor: do you need them?

The Core 300S measures air quality with a laser sensor and shows it as a colour code from blue to red on the device and in the app. In auto mode it adjusts the level itself, which is the real comfort in everyday life: after cooking it turns up, at night it stays quiet. Timer and schedules come on top. The Blueair Mini Max does without all that and is operated with two buttons, but at 1.3 kg it is light enough to carry between desk and child’s room.

Which device for whom?

• Bedroom up to 25 square metres, allergy sufferers, pets: Levoit Core 300S with auto mode and night mode.
• Desk, child’s room, travel: Blueair Mini Max, small, light, cheap to run.
• Living room over 30 square metres: A device with a considerably higher CADR or two devices.
• Summer with tilted windows: Both, because they pull pollen and fine dust out of the incoming air. They do not help against heat.

My verdict

Convert the CADR to your room, check the noise of the quietest level and the filter costs per year. That rules out most bad buys. The Levoit Core 300S is my recommendation for bedrooms and medium-sized rooms, the Blueair Mini Max for small rooms. You can find the reviews in the air purifiers category.

Frequently asked questions about buying an air purifier