Air purifier marketing has become very good at sounding technical while obscuring the things that would actually help you decide. CADR numbers appear prominently. HEPA certification is mentioned. Room coverage figures are given in square metres. What is usually missing is any information about what the device cannot do, how loud it is at the settings where it works, and what the ongoing filter cost is.
This is the information that determines whether an air purifier will actually be used, rather than turned off after two weeks because it is too loud to sleep next to.
CADR and what it actually means
CADR stands for Clean Air Delivery Rate. It measures how much air the purifier can move through the filter per unit of time, expressed in cubic metres per hour or cubic feet per minute depending on the market. Higher CADR means the device moves more air and cycles the room air through the filter more frequently.
CADR is measured at maximum fan speed. Most people do not run air purifiers at maximum fan speed because maximum fan speed is loud. The CADR at a speed that is tolerable to sleep in the same room with is considerably lower — typically around 40 to 60 percent of the maximum figure.
Room coverage figures quoted by manufacturers are almost always derived from maximum CADR. When a device is rated for 40 square metres, that figure assumes maximum fan speed and standard ceiling height. At a quiet setting, the effective room size is smaller.
HEPA certification
True HEPA filtration removes 99.97 percent of particles at 0.3 microns. This covers most allergens, dust, and fine particles from combustion. It does not capture gases, volatile organic compounds, or odours without an additional activated carbon stage.
Not all HEPA claims are equal. Some products use HEPA-type or HEPA-like filters that perform to a lower standard. True HEPA is a specific certification; look for this term rather than HEPA-style variants.
Some air purifiers include ionisers or UV-C lamps. Ionisers work by releasing ions that cause particles to stick to surfaces, including the device itself and surrounding walls and furniture — where the particles stay, rather than being removed. The evidence for UV-C lamp effectiveness in consumer devices at the dwell times achieved during normal operation is thin. Neither is a substitute for a properly rated filter.
Noise is the practical constraint
An air purifier that is placed in a bedroom will be used mainly at night. Night-time use means low-speed operation, which as noted above reduces effective CADR substantially. If the device at low speed produces around 25 to 30 decibels, most people can sleep with it. Much above that and the device migrates to a corner and stops being used.
Noise level at sleep mode or minimum speed is a more useful specification than CADR, and it is harder to find. Some manufacturers list it; many do not. Independent reviews that measure noise are worth seeking out for any device you are considering for bedroom use.
Ongoing filter costs
Filters need replacing. For most air purifiers with a true HEPA filter, the replacement frequency is somewhere between six and twelve months with regular use, occasionally longer. Replacement filter prices vary from very reasonable to considerably more than you might expect given the device purchase price.
Check the replacement filter price before buying the device. For some products, the filter costs as much as 30 to 40 percent of the device price per replacement. Over four years of ownership, the filter cost may exceed the device cost. This changes the cost comparison between differently priced devices significantly.
Also check availability. Some brands discontinue filter models without notice when a product line ends, which makes a previously functional device unusable.
Placement
An air purifier works on the air in its immediate vicinity. Placement in the centre of a room, or at least with clear space on all sides, outperforms placement in a corner where airflow is restricted.
Purifiers are also not substitutes for ventilation. A purifier that runs continuously in a sealed room will reduce particle count, but will not address CO2 buildup or high humidity. In practice, regular ventilation combined with a purifier running during closed hours is more effective than a purifier alone.
Before buying
Find the noise level at minimum or sleep speed, not at maximum. If the manufacturer does not list this, look for independent measurements.
Check replacement filter price and availability for the specific model. Multiply by the expected replacements over four to five years.
Calculate the room coverage at a realistic fan speed, not at maximum. Roughly halve the manufacturer's quoted room size as a conservative estimate for quiet operation.
Verify true HEPA certification if particle filtration is the reason for purchase. Check whether there is an additional carbon stage if odours or gases are also a concern.