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Indoor air quality: air purifiers, dehumidifiers and what really matters

City pollution, mold, wildfire smoke and dust after an earthquake. When you need a HEPA air purifier or a dehumidifier, and how to choose one.

Updated , 2 min read . Contains affiliate links (what this means)

In short

  • Short, intense airing two or three times a day is still the first step, except on days of heavy outdoor pollution or smoke.
  • A true HEPA air purifier traps fine particles (PM2.5), pollen and smoke. Size it to the room (CADR).
  • Ideal indoor humidity is 40 to 60%; above 60%, mold appears.
  • For dust after an earthquake, wildfire smoke or extreme pollution, you need FFP2 or N95 masks, not surgical masks.

We spend over 80% of our time indoors, and indoor air can be more polluted than outdoor air: fine particles from traffic, smoke from cooking and smoking, compounds from furniture and cleaning products, mold and dust mites. Many cities regularly exceed the World Health Organization’s guideline values for fine particles.

In a crisis, air becomes an even bigger problem: dust from damaged buildings after an earthquake, smoke from wildfires or industrial fires, damp and mold after a flood.

1. Airing your home properly

  • Short and intense: open windows wide for 5 to 10 minutes, two or three times a day, with a cross-draft. It’s more effective than a window left ajar for hours.
  • Don’t air during rush hour if you live next to a busy road, or on days with heavy pollution or wildfire smoke. Check your local air quality index.
  • Kitchen hoods and bathroom fans remove moisture and smoke at the source.

2. Humidity: not too much, not too little

Ideal indoor humidity is 40 to 60%. Measure it with a cheap hygrometer.

  • Above 60%: condensation on windows and mold, especially in corners and behind furniture. Mold worsens asthma and allergies. Solutions: airing, steady heating, a dehumidifier.
  • Below 30 to 35% (common in winter with the heating on): dry eyes and throat. A humidifier or a few bowls of water on radiators helps.

After a flood, drying the house fast with dehumidifiers and ventilation is essential to prevent mold.

3. The air purifier

A purifier makes sense if you live in a polluted area, have allergy or asthma sufferers at home, have a baby or want protection on smoke days.

What matters:

  • A true HEPA filter (class H13 or H14), which traps over 99.9% of very fine particles. Watch out for “HEPA-type” or “HEPA-like”.
  • An activated carbon filter for odors and some gases.
  • The right CADR for the room. A purifier that’s too small does almost nothing.
  • No ozone: avoid ionizers that can’t be switched off.
  • Filter costs: they’re usually replaced every 6 to 12 months.

4. Masks for emergencies

For dust after an earthquake, wildfire smoke or extreme pollution, you need FFP2 or N95 masks (or FFP3), well fitted to the face. Surgical and cloth masks don’t filter fine particles effectively. Put a few FFP2/N95 masks in every go bag.

In case of smoke or chemical pollution

If authorities warn of a smoke plume or chemical pollution (for example after an industrial fire):

  1. Close windows and doors, and switch off ventilation and air conditioning that draws in outside air.
  2. Seal large gaps with damp towels.
  3. Run your purifier, if you have one.
  4. Follow instructions from emergency alerts and the radio.

Recommended equipment

What to look for when buying. Links go to partner shops; if you buy through them we may earn a small commission at no extra cost to you.Details.

  1. True HEPA air purifier

    Traps fine particles, pollen, dust and smoke. Useful in polluted cities, for allergy sufferers and on wildfire smoke days.

    • H13 HEPA filter (not 'HEPA-type') plus activated carbon for odors
    • CADR suited to the room (at least 5 × floor area in m²)
    • No ionizer, or one that can be switched off
    • Quiet on low speed for the bedroom
    See offers

  2. Dehumidifier

    Prevents mold in damp homes, basements and after floods.

    • Capacity suited to the space (10 to 20 liters a day for a typical home)
    • Humidistat with auto shut-off
    • Continuous drainage hose option
    See offers

  3. FFP2 or N95 masks

    For dust after an earthquake, wildfire smoke and extreme pollution. A box at home and a few in every go bag.

    See offers

  4. Digital hygrometer

    A small, cheap device that tells you the humidity in your home.

    See offers

Frequently asked questions

How do I find out how polluted the air is where I live?

Most countries publish real-time air quality data from official monitoring stations (for example AirNow in the US or the European Environment Agency's air quality index in Europe). There are also independent sensor networks and apps that use this data.

What does CADR mean on an air purifier?

CADR (Clean Air Delivery Rate) shows how much clean air the purifier delivers per hour. A practical rule: the CADR in m³/h should be at least 5 times the room's floor area in m² (for a 20 m² room, at least 100 m³/h, ideally more).

Are ionizers and UV lamps useful?

Ionizers can produce ozone, which irritates the lungs. UV-C lamps can reduce some microorganisms but must not be used around people if the light is exposed. For a home, a true HEPA purifier without ozone is the safe choice.

Sources and further reading

Preper is for general education and does not replace instructions from your local authorities or advice from a doctor. In an emergency, call your local emergency number.

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