Air quality

The order that works

1. Remove the source. Nothing else on this list competes with simply not introducing the pollutant. Fragranced products, aerosols, scented candles, smoking indoors, idling a car in an attached garage, a gas hob without extraction — each is a source you can reduce at zero cost.

2. Ventilate. Opening windows for a few minutes is startlingly effective and completely free. Use the extractor over the hob every time you cook, and run the bathroom fan during and after showers. Ventilation dilutes everything at once, including the things no filter captures.

3. Then filter. A purifier is genuinely useful — for particulates, allergens, wildfire smoke — and it works far better once you have done the first two steps. It is a last step, not a first one.

Most people run this list backwards, buy a purifier, and are disappointed. A machine cannot out-filter a source you are still feeding.

What a purifier does and does not do

A HEPA-class filter — one the maker states captures 99.97% of 0.3-micron particles — is very good at particulates: dust, pollen, dander, smoke particles. That is a real and worthwhile job, especially for a child with allergies or asthma, or during wildfire season.

It does considerably less about gases and VOCs unless it carries a meaningful quantity of activated carbon — and most consumer units carry a thin carbon layer that saturates quickly. Ventilation and source removal do more here.

It does nothing about humidity, mold behind a wall, or a combustion problem. Those are different jobs with different equipment, and one of them is an emergency.

And avoid anything that generates ozone to do its work, however it is marketed. Ozone is a lung irritant. "Ionizer" and "activated oxygen" are the phrases to look at carefully.

There is a way to check that is not the maker's say-so. California requires every portable air cleaner sold there to be certified, and the state's public list files each model as either "Mechanical" (classified as physical filtration only, so not an ozone- or ion-generating design) or "Electronic" (may generate ozone, but tested to no more than 50 parts per billion; the category includes ionizers). The same brand can appear in both: Levoit's Core 300 is listed as Mechanical, while some other Levoit model numbers are listed as Electronic. So look up the model number on the box, not the brand. We only recommend models listed as Mechanical. CARB is explicit that its list does not say a unit works well or is safe to use; it tells you what kind of technology the model is. We read the list on 2026-10-23.

One thing that is not optional

A working carbon monoxide alarm on every level of the home and outside each sleeping area, and a smoke alarm alongside it. That is CPSC's advice (read 2026-10-26), and NFPA's is the same: a central location outside each sleeping area and on every level. CDC calls carbon monoxide an odorless, colorless gas that kills without warning, and says people who are asleep can die before they have symptoms.

We picked five alarms from First Alert and Kidde, and the best carbon monoxide alarm page has what each maker says about UL 2034, the standard for CO alarms, and what we could and could not check. We read that on the makers' own pages, data sheets and manuals, not in UL's directory. CPSC's warning of August 20, 2026 says to buy only CO detectors that meet UL 2034, and that they should be battery operated or have a battery backup. Whichever you buy, replace it when its end-of-life signal sounds or its replace-by date passes, whichever comes first.

Nothing else on this page is urgent. That one is.