Two Kinds of Feature
Every air purifier spec sheet mixes two kinds of feature. There are the ones that change
what the machine does — how much air it cleans, whether you can bear to
run it, whether it is telling you the truth. And there are the ones that change how it
feels to own — the app, the voice assistant, the remote, the finish. The second
kind is fine. The problem is that spec sheets present them as equals, and buyers routinely
trade a size up for a Wi-Fi chip.
This guide takes the features in order of how much they matter. The first two decide
whether the purifier works at all. The rest are worth having only once those are settled.
The core principle: CADR for the room, then noise at sleep speed, then a
sensor you can verify. If a feature is not one of those three, it should not cost you a size
class.
Noise — The Feature That Decides Whether You Use It
The most common failure mode of an air purifier in a bedroom is not filtration. It is that
it gets switched off at 11pm because it is too loud, and stays off. A purifier that is
off for the eight hours you are in the room is furniture.
What to read on the spec sheet:
- dB at the lowest speed, or sleep mode — not the maximum, which
is the number brands lead with when it flatters them and hide when it does not. You will
not run at maximum overnight. - CADR at that speed, if published. A purifier that is quiet because it
is barely moving air is not quiet; it is off with the light on. - Whether the display dims in sleep mode. A bright ring in a dark bedroom
is its own reason to unplug it.
The trade this creates is real: a larger unit running on low is quieter than a smaller unit
running on medium to move the same air. That is a second argument, after pollution, for
sizing up.
The Sensor — And Why It Shows Green
Almost every purifier now has a particulate sensor, and almost every owner has had the
experience of the machine glowing a contented green while the outdoor AQI is 350 and the
external monitor on the shelf reads red. Three things are going on.
- It measures its own exhaust neighbourhood. The sensor sits on the
machine, which, once running, is surrounded by the cleanest air in the room. It is honestly
reporting a pocket. - The thresholds are generous. Brands choose where green ends, and many
choose a level that a health-based scale would call moderate. Green means acceptable to the
manufacturer, not clean by a standard you picked. - The lens is dirty. Dust on the optical sensor reads as less scattering,
which the machine interprets as cleaner air. It drops to low speed and does less, and the
green light stays on.
Sensor type matters somewhat: laser-scattering sensors resolve finer
particles and react faster than the cheaper infrared kind, which drift more
and respond mainly to larger dust. But the feature that matters more than the sensor is the
display. A numerical PM2.5 readout lets you check the machine; a colour band asks you
to trust it. Given the choice, take the number.
Put an external PM2.5 monitor at the far side of the bed from the purifier for a night in
season. If it tracks the purifier’s reading within reason, trust auto mode. If the purifier
reads green while the monitor reads high, clean the sensor and, if that does not fix it, run
the purifier on a fixed speed and ignore auto.
Auto Mode, Sleep Mode and Timers
Auto mode lets the sensor set the speed. It is only as good as the sensor,
which is the point of the previous section: on a machine that reads green too readily, auto
mode is a way of running on low all winter. On a machine with a decent sensor and honest
thresholds, it is genuinely useful — it ramps up after cooking or a door opening and
settles when the room is clear.
Sleep mode is usually a fixed low speed with a dimmed display. Check what
it does on the specific model: on some it disables the sensor entirely, which means a smoky
evening goes unanswered until morning.
Timers are useful for one thing: pre-cleaning the bedroom an hour before
you go in, so you are not waiting for it to catch up. Schedule it for the evening; leave it
running overnight.
Apps, Voice Assistants and Remotes
These change how it feels to own, not what it does. In descending order of use:
- The app is worth having for history — seeing the PM2.5 trace over
a week tells you when the room gets dirty and why — and for scheduling. It is also how
most brands surface filter-life estimates. Do not pay a size class for it. - Voice assistants let you turn it up without getting out of bed. Nice.
Not a filtration feature. - The remote is the app for people who do not want the app. If the model
has one, fine; if it has an app, the remote is redundant. - Multi-device automation — running the purifier when the AC comes
on, or when the front door opens — is the one genuinely useful smart-home trick, and
only if you already run that ecosystem.
Child Lock, Filter Indicators and Power Draw
Child lock is a real feature in a home with a toddler: a purifier at floor
level with a row of touch buttons on top is an invitation. Look for it if that describes your
house; ignore it otherwise.
Filter indicators on most units are timers. They count hours against an
assumed load and tell you to replace on schedule, which in an Indian winter is late. Treat the
indicator as a reminder to look, and go by airflow and the state of the filter face.
Power draw is the feature buyers over-weight. On the low and sleep speeds
you run overnight, a purifier draws on the order of a bedside lamp; at full speed, somewhat
more than a ceiling fan. Over a season, filter replacements cost several times the
electricity. Efficiency badges are welcome but they should not decide anything; the
running-cost guide has the arithmetic.
The Worksheet
| Feature | Changes what it does? | What to check |
|---|---|---|
| CADR for your room | Yes — everything |
Against 4–5 air changes an hour, not the box area |
| Noise at sleep speed | Yes — whether you run it |
dB on low, and whether the display dims |
| Numerical display | Yes — whether you can verify it |
A PM2.5 number, not a colour |
| Laser sensor | Somewhat | Better than infrared; still needs cleaning |
| Auto mode | Only if the sensor is honest | Run the one-night test |
| App / voice / remote | No | Convenience; do not pay a size class |
| Child lock | No | Yes if you have a toddler |
| Energy rating | Marginally | Filters cost more than power |
Undersizing is the most common reason air purifiers disappoint. Work out the CADR your room actually needs — the coverage area on the box assumes a lower air-change rate than an Indian winter requires.
Frequently Asked Questions
What is the most important feature in an air purifier?
Adequate CADR for the room, which is not a feature so much as the whole point. After that, low noise at the speed you will actually run overnight, because a purifier you switch off at night does nothing for eight hours. Everything else on the spec sheet comes after those two.
Why does my air purifier show green when the outdoor AQI is red?
Three reasons, often together. The sensor samples air at the machine’s own intake, which is the cleanest spot in the room once it has been running. Many brands set the green threshold generously, so a level an external monitor would call moderate shows as good. And the sensor lens collects dust and reads low. Clean the sensor, compare with an external monitor for a night, and use a model with a numerical readout.
Is a laser sensor better than an infrared one?
Generally yes. Laser-scattering sensors resolve smaller particles and respond faster; infrared sensors are cheaper, cruder and drift more. Either needs cleaning, and either can be set to show green too readily. The sensor type matters less than whether the machine shows you a number you can check.
Do I need an app-connected air purifier?
No. The app is convenient for scheduling and for seeing history, and useful if you want the purifier to run with other devices. It does not change what the filter does. If the choice is between a connected model and a larger unconnected one for the same money, the larger one is the better purifier.
How much electricity does an air purifier use?
Less than most people expect. On the low and sleep speeds you run overnight, typical units draw on the order of a bedside lamp; at maximum speed, somewhat more than a ceiling fan. Over a season the filter replacements cost far more than the power. The running-cost guide on this site sets out the arithmetic.