What's in the air inside your home, and what actually fixes it
Most of us spend around 90% of our time indoors, and on an ordinary day the air inside can be worse than the air outside. Most indoor problems have cheap fixes. A purifier is one of them, but it isn't always the right one.
Source, ventilation, filter: in that order
Health and environment agencies give the same advice, in the same order, and it's the right order:
- Deal with the source. Stop the pollution being made, or capture it where it's made. A rangehood over the cooktop beats a purifier in the next room.
- Ventilate. Swap stale indoor air for fresh outdoor air, when the outdoor air really is fresh.
- Filter. A purifier mops up the particles that are left. It's the last step, not the first.
A purifier is excellent at particles. It does little or nothing for gases, carbon dioxide or damp. That's why a site that reviews purifiers has a page telling you when not to rely on one.
The usual suspects in a Perth home
Gas cooktops and unflued gas heaters
Burning gas releases nitrogen dioxide, a lung irritant, along with fine particles. Australian research links about 12% of the childhood asthma burden to gas cooking. Unflued gas heaters put the same combustion gases straight into the room.
What helps: a rangehood that vents outside, not one that recirculates, switched on before you start and left running for a while after. Cook on the back burners, which sit closer to the hood. Open a window if there's no hood. If you're renovating, induction removes the problem.
Wood heaters
Common in the Hills and outer suburbs. Even a well-run heater lets smoke into the room each time you open the door to reload, and a fire damped down to smoulder overnight fills the street with smoke.
What helps: burn only dry, seasoned wood, split and stored for months. Keep the fire hot and bright rather than turning it right down overnight. Have the flue cleaned each year.
Mould and damp
Perth's winters are mild but wet. Mould turns up in bathrooms, laundries and wardrobes on cold outside walls, and its spores are a common asthma and allergy trigger. About one in four Australian homes has a damp problem, and damp housing has been linked to around 8% of the childhood asthma burden.
What helps: fix leaks, run exhaust fans during and after showers and washing, and keep indoor humidity under about 60%. Small patches can be cleaned off; mould that keeps coming back means the moisture source hasn't been fixed.
Fumes from new furniture, paint and sprays
Flat-pack and pressed-wood furniture, fresh paint, new carpet, air fresheners and cleaning sprays give off volatile organic compounds (VOCs), including formaldehyde. Levels are highest when things are new.
What helps: unpack new furniture in a well-aired room or the garage for a few days. Choose low-VOC paint. Go easy on sprays and plug-in fresheners, which add chemicals to cover a smell rather than removing it.
Candles, incense, frying and toast
Some of the highest indoor particle readings come from ordinary things: a scented candle, a stick of incense, a pan of bacon, a slice of burnt toast. A home particle monitor shows the spike plainly.
What helps: rangehood on whenever you cook. Fewer candles and less incense, especially in bedrooms. Open a window when the outdoor air is good.
Dust mites, pets and tracked-in dust
Dust mites live in bedding, mattresses and carpet, and their droppings are one of the most common indoor allergens. Pets add dander, and Perth's sandy soil comes in on shoes and paws.
What helps: wash bedding weekly in hot water, 60°C or above. Use a vacuum with a HEPA filter. Keep pets out of the bedroom if anyone's allergic, and leave shoes at the door.
Stale air and carbon dioxide
Shut the bedroom door and window on a winter night and carbon dioxide from your own breathing builds up. Outdoors it sits around 420 parts per million; a closed bedroom with two people in it can pass 1,500 by morning. That isn't dangerous, but high levels are linked to stuffiness, headaches and, in some studies, poorer sleep.
What helps: a window open a crack, or the door left ajar. A carbon dioxide monitor tells you when it matters.
What a purifier can and can't fix
| Problem | HEPA filter | Carbon filter | What really fixes it |
|---|---|---|---|
| Bushfire and wood smoke particles | Yes | Helps with smell | Close up, purifier on |
| Pollen and pet dander | Yes | No | Purifier plus regular cleaning |
| Airborne mould spores | Yes | No | Fix the damp |
| Cooking particles | Yes | Some odour | Rangehood venting outside |
| Nitrogen dioxide from gas | No | Very little | Rangehood, or induction |
| VOCs and formaldehyde | No | Some, if it's a thick filter | Ventilation, low-VOC products |
| Carbon dioxide | No | No | Fresh air |
| Humidity and damp | No | No | Exhaust fans, a dehumidifier |
Airing the house, Perth-style
For most of the year Perth's outdoor air is cleaner than the air indoors, so opening up is the cheapest air treatment you have. The trick is timing.
- Open up when the outdoor reading is good. On a hazy or smoky day, check the live reading first.
- Use the sea breeze. Summer afternoons often bring cooler air in off the ocean. Close up again if there's a burn-off or bushfire alert.
- Mind the pollen. On windy spring days, and around thunderstorms, keep windows shut if hay fever is a problem.
- Know what your evaporative cooler is doing. It pushes outdoor air through the house and out through open windows. That's good ventilation on a clean day and a smoke pump on a bad one.
- Check where the exhaust fans go. Some bathroom fans, particularly in older homes, vent into the roof space rather than outside. That moves the damp without getting rid of it.
Cheap ways to measure
A few inexpensive gadgets take the guesswork out of it.
- A particle (PM2.5) monitor with a laser sensor shows the cooking and candle spikes described above, and whether your purifier is keeping up on smoke days. Treat the exact numbers loosely; the trends are what matter.
- A carbon dioxide monitor with an NDIR sensor. Cheaper “eCO2” sensors estimate carbon dioxide from other gases and aren't reliable.
- A hygrometer, or humidity gauge, tells you whether a room is heading for mould. Humidity sitting above about 60% for long stretches is the warning sign.
If you rent
- Report leaks, broken exhaust fans and mould to the property manager in writing, and keep a copy.
- If the kitchen has a gas cooktop without a rangehood that vents outside, use the back burners and open a window while you cook. A plug-in induction hotplate is an inexpensive way to do most everyday cooking without gas.
- A portable purifier and a humidity gauge move with you when you do.
Sources
- Knibbs L.D. et al. (2018). Damp housing, gas stoves, and the burden of childhood asthma in Australia. Medical Journal of Australia. Source of the 12.3%, 7.9%, 26.1% and 3.4% figures.
- National Asthma Council Australia: Gas stoves and asthma in children.
- The mortality burden attributable to wood heater smoke particulate matter (PM2.5) in Australia (2024). Science of the Total Environment.
- US Environmental Protection Agency: Guide to air cleaners in the home, on the source control, ventilation and filtration order and what filters can and can't remove.
- HealthyWA: Smoke hazards from bushfires.
- World Health Organization (2021). WHO global air quality guidelines.
This guide is general information, not medical or building advice. For persistent mould or damp, talk to a qualified tradesperson; for health concerns, your GP.