Air Purifier for Smog: Worth Buying? HEPA vs Ioniser vs Nothing
Is an air purifier worth buying for smog? Yes, with three conditions: it must use a true HEPA filter, it must be big enough for the room, and the room must be closed. Under those conditions it really does bring down PM2.5 indoors. Ionisers and “ozone” purifiers are a skip, and a unit whose filter is never replaced is just a noisy fan. This is the device decision only; masks, plants and ventilation habits have their own pages, linked below.
In brief
- Buy: a true HEPA unit with a stated CADR that suits your room size
- Skip: ionisers, ozone generators, “plasma” add-ons you can’t switch off
- Works only: with windows and doors shut
- Best room: the bedroom, run through the night
- Hidden cost: replacement filters, roughly once or twice a year in heavy smog cities
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Is an air purifier worth it in smog season? The verdict step by step
Here’s the typical smog-season routine people copy from each other, taken apart one step at a time.
| Step | Verdict | Why |
|---|---|---|
| Buy a HEPA purifier | Worth it | Traps fine particles well |
| Buy an ioniser or ozone unit | Skip | Ozone irritates lungs |
| Put it in the lounge | Depends | Bedroom usually matters more |
| Run it with windows open | Skip | Outside air wins every time |
| Rely on houseplants instead | Skip | Far too slow for smog |
| Change filter when the light says | Worth it | A clogged filter stops working |
The plants question has its own honest answer in air-purifying plants for smog. Masks, cooking fumes, incense and airing the house are covered in indoor air pollution at home. This page stays on the machine.
HEPA vs ioniser vs nothing: how each one works
True HEPA
A HEPA filter is a dense mat of very fine glass fibres folded like an accordion. A fan pushes room air through it, and particles get caught as they bump into, stick to or wander into the fibres. The standard for “true HEPA” requires it to catch at least 99.97% of particles at 0.3 microns, the size that’s hardest to catch. Smog’s PM2.5 is well within its range. Watch the wording: “HEPA-type” or “HEPA-like” isn’t the same thing.
Most good units also have a carbon layer for smells. It helps with cooking odours and some gases, but a thin carbon sheet does little against heavy smoke smell for long.
Ionisers and ozone generators
An ioniser charges particles so they stick to walls, curtains and furniture, or to a plate inside the unit. They don’t disappear; they collect on your surfaces. Many ionisers also give off ozone as a by-product. Ozone is a lung irritant, the same gas that’s part of summer smog outdoors, and it can set off coughing and tightness in people with asthma. Units sold as “ozone purifiers” produce it on purpose. Skip them. And if a HEPA unit has an ioniser switch, keep it off.
Nothing (a closed room, a damp cloth, no machine)
Doing nothing isn’t useless. A closed room with a wet-mopped floor keeps out some outdoor dust, and on days when the reading outside is moderate rather than hazardous, a shut window and a damp duster do more than most people expect. But PM2.5 is so fine it slips in through window gaps and door frames, and without a filter it has nowhere to go. For someone healthy, doing nothing on most days is a fair budget choice. For an asthmatic child, an elderly parent with heart or lung disease, or a newborn in a heavy smog city, a purifier in the bedroom is one of the few buys that clearly does something.
A purifier cleans a room. It can’t clean a city, and it can’t clean a room with the window open.
How to match CADR to room size, in plain words
CADR (clean air delivery rate) tells you how many cubic metres of clean air the machine delivers per hour. It’s the most useful number on the box. Read it first. Aim for enough CADR to swap the room’s air about 5 times an hour.
- Measure the room: length times width times ceiling height, in metres. A 4 m by 3.5 m bedroom with a 3 m ceiling is 42 cubic metres.
- Multiply by 5. That room needs about 210 m³/hour of CADR.
- If the box gives CADR in CFM (cubic feet per minute), multiply CFM by 1.7 to get m³/hour.
- Pick a unit at or above that number, then round up rather than down. You can run a bigger unit on a quieter setting; a small one on max is loud and still falls short.
- Check the running noise at the level you’ll use at night. If it’s too loud to sleep with, it’ll get switched off.
Where to put it: on the floor or a low table, a hand’s breadth or more from walls and curtains, not tucked behind the bed. Close the door. Then wait. Run it for 30 to 60 minutes before bedtime, then leave it on through the night. An air quality meter or a unit with a PM2.5 display lets you see whether the number falls; if it doesn’t, check for open gaps.
Air purifier for a baby room: safe or not?
A true HEPA purifier is safe in a baby’s room and makes sense in a heavy smog city. Place it out of reach, with the cord tucked away, and not blowing directly on the cot. Don’t use one with an ioniser or ozone mode on. Don’t add essential oils, camphor or scented trays to it; those are unsafe near babies. And safe sleep doesn’t change: flat on the back in a bare cot, whatever the air.
If a baby has fast or effortful breathing, chest sucking in, grunting or blue lips, that’s emergency care now, not a device problem.
Running cost, filter changes and the cough some people blame on purifiers
Electricity use is usually modest, similar to a ceiling fan on a medium setting for most home units. The real cost is filters. In a city with months of heavy smog, the maker’s replacement indicator will come on sooner than the box suggests; budget for at least one new filter a year and check the price before you buy the machine. Some cheap units become expensive once you price the filter. Ask the shop.
Can an air purifier cause a cough? A HEPA unit shouldn’t. A cough or headache that starts with a new machine usually points to ozone from an ioniser mode, a new-plastic smell in the first days, or a dusty, overdue filter blowing stale air. Switch off any ioniser, air the new unit out on day one, and change a clogged filter.
Did you know? PM2.5 particles are around one-thirtieth the width of a human hair. That’s why they slip deep into the lungs, and also why a cloth or a curtain can’t catch them while HEPA fibres can.
A purifier doesn’t replace asthma or heart medicines, and nobody should cut back on an inhaler because the room feels cleaner. See a doctor soon for a cough lasting more than 3 weeks, wheezing, or breathlessness on stairs. Chest pain, lips turning blue, or breathlessness at rest needs emergency care now. The warning signs are laid out in smog cough vs asthma vs chest infection.
More verdicts on popular habits and buys: Routine Autopsy.
Air purifier questions people search
Are air purifiers really effective for smog?
True HEPA units with enough CADR clearly reduce PM2.5 in a closed room. They do nothing for the air outside or in rooms they aren’t in. Size and a shut door matter as much as the brand.
What are the side effects of an air purifier?
HEPA units have almost none beyond noise and a dry-ish breeze. Ionisers and ozone units can irritate lungs and throat. A neglected filter can smell stale.
Can an air purifier cause a cough or headache?
Usually only when it gives off ozone or when the filter is overdue. Turn off any ionising mode, air a new unit out, and replace a clogged filter.
Is an air purifier good for a baby’s room?
Yes, a HEPA one without ozone, placed out of reach and not blowing on the cot. Keep safe sleep rules exactly the same.
Air purifier vs humidifier: which is needed?
They do different jobs. A purifier removes particles; a humidifier adds moisture for dry noses in winter. In smog season, the purifier is the one that addresses smog.
Should an air purifier run all night?
Yes, that’s when it’s most useful, in the bedroom with the door shut. Use a quiet setting that still matches the room size.
Do air purifiers work with the windows open?
Not in any useful way. Outdoor air replaces what the machine cleaned faster than it can keep up. Air the room when the outdoor reading is lowest, then shut up and switch on.
How often does a HEPA filter need changing?
Follow the maker and the unit’s indicator. In heavy smog cities it’s often sooner than the box suggests, commonly once or twice a year. Pre-filters can usually be vacuumed monthly.
Is an ozone air purifier safe?
No. Ozone is a lung irritant, especially for children and people with asthma. Choose units that don’t produce it.
Air purifier vs indoor plants: which is better?
For smog, a purifier by a very wide margin. Plants are lovely but clean far too little air to matter against PM2.5.
Can a washable filter replace HEPA?
Washable pre-filters catch hair and big dust. They don’t do HEPA’s job. Water also damages true HEPA media, so never wash the HEPA layer.