Are the air purifiers and dehumidifiers useful? You may have used it wrong

Many families have bought air purifiers or dehumidifiers, but after using them for a period of time, they feel that they have no effect, or they keep running them but don’t know when to replace the consumables. These problems are often not problems with the product, but problems with the way they are used.

Air Purifier: Five Common Misuses

1. The room is too big and the purifier is too small

CADR (clean air volume) is the core parameter of the purifier. The unit is m³/h, which represents the amount of air that can be purified per hour.

Calculation formula: Room area (㎡) × floor height (m) × number of air changes ÷ 60 = purification requirements per minute

Simpler empirical value: CADR ≥ room area × 5

Room Size	Minimum Recommended CADR
15㎡	75 m³/h
20㎡	100 m³/h
30㎡	150 m³/h
40㎡	200 m³/h

Mistake: I bought a CADR 100 purifier and placed it in a 40㎡ living room, which is useless.

2. The filter is never changed.

When the HEPA filter accumulates dust to a certain extent, the filtration efficiency will drop significantly, while the wind resistance will increase and the motor load will increase.

Reference cycle for filter replacement:

Primary filter (prevents large particles from entering): Clean every 1–3 months
HEPA main filter: usually replaced every 6–12 months (depending on frequency of use and air quality)
Activated carbon layer: Replace every 3–6 months (to remove odor, it will become saturated and lose effectiveness)

Determine whether it needs to be replaced:

Refer to the "filter life" displayed on the purifier, but a better way is to look at the color of the filter (when it turns black/grey obviously)
When you turn on the maximum wind speed, you feel that the air volume is obviously reduced, which is usually due to blockage.

3. Don’t open the window after buying a purifier

The purifier purifies particulate matter in the existing indoor air, but it cannot produce fresh oxygen or discharge CO₂. If the room is not ventilated for a long time, the CO₂ concentration will continue to increase (exceeding 1000ppm will affect concentration and sleep quality).

Correct approach: Open windows regularly for ventilation (except on hazy days), and run the purifier at the same time when the windows are open.

4. Place the purifier in the corner

The purifier needs air circulation and should be placed in a corner to block the air intake.

BEST PLACEMENT: -More than 20cm away from the wall, and the air inlet should not be blocked

Place it in areas with the worst air quality (near human activity areas or pollution sources)
Do not place it next to bookcases or behind curtains

5. If you buy a purifier, you don’t need a fresh air system.

These are two products that solve different problems:

Purifier: Filters particles and odors in existing indoor air
Fresh air system: Introducing fresh outdoor air and filtering it at the same time to solve the problem of CO₂ accumulation

Indoor CO₂ exceeds the standard. Purifiers alone are useless. Ventilation must be exchanged.

Dehumidifier: Misunderstandings in use

Myth 1: The lower the humidity, the better

The human body feels comfortable within a humidity range of 40–60%.

Humidity below 30%: Dry skin, itchy eyes, and wooden floors prone to cracking
Humidity 30–40%: Dry, unfriendly to wooden furniture, uncomfortable for people with respiratory sensitivities
Humidity 40–60%: comfort zone, bacterial growth is inhibited
Humidity higher than 70%: Molds multiply, dust mites are active, and odors are produced

Recommended setting humidity target: 50–55%, the dehumidifier will automatically stop after reaching the target.

Misunderstanding 2: Turning on the dehumidifier but not closing the doors and windows properly

The dehumidifier extracts indoor moisture. If the doors and windows are wide open, the outdoor moisture will continue to come in and it will never run out, which is a waste of electricity.

Correct Operation: Close doors and windows when turning on the dehumidifier (or leave only light ventilation).

Misunderstanding 3: You won’t know until the water tank is full

The dehumidifier will automatically stop when the water tank is full. If it is not emptied in time, the machine will be out of service.

Solution:

For models with drain pipes, drain water directly (put it in the floor drain)
If not, set a reminder and clear it every day

Misunderstanding 4: Only used during the rainy season

The southern region has a high humidity period from March to September, and the air conditioning cooling season in the northern region will also bring about an increase in relative humidity after indoor cooling (because the temperature drops, the relative humidity feels higher).

Applicable scenarios:

Rainy season: persistent high humidity, dehumidifier is urgently needed
Summer air conditioning season: The air conditioner itself has a dehumidification function, but it is still needed when the air conditioner is not turned on.
Basements and garages: humid all year round, it is recommended to keep one

Misunderstanding 5: Not considering whether the daily dehumidification amount is enough

The core parameter of the dehumidifier is the daily dehumidification capacity (liters/day), which indicates how much moisture can be extracted from the air in a day.

Room size	Recommended daily dehumidification volume
≤ 20㎡	10–12 liters/day
20–40㎡	12–20 liters/day
40–60㎡	20–30 liters/day
Basements, high-humidity locations	30–50 liters/day

Purifier vs Fresh Air vs Dehumidifier: What problems do each solve?

Products	Problem Solving	What Not Solved
Air purifier	PM2.5, particulate matter, some VOCs, odor	CO₂, humidity
Fresh air system	CO₂, introduction of fresh air	Existing high concentration of pollutants (filter module required)
Dehumidifiers	Humidity Control	Particulate Pollution
Humidifier	Humidification in dry season	Other pollution

*This article refers to the national standard GB/T 18801 air purifier standard and GB/T 19411 dehumidifier standard. The specific data is subject to actual measurement by the manufacturer. *