When a dust storm pushes outdoor PM2.5 above 150 micrograms per cubic metre, close the windows within the first hour, run the purifier on level 3, and wet-wipe surfaces once a day. Indoors, a closed room reaches 50 to 70 percent of the outdoor concentration within 1 to 2 hours, so the closing action has to happen before the peak, not after it. Cats with asthma or a history of bronchitis need this at 150, not at 300.
The numbers a dust storm brings
A spring dust event typically runs 6 to 12 hours and lifts PM2.5 from a 20 to 40 microgram baseline to 150 to 800 micrograms, with PM10 often 500 to 2,000 micrograms. Relative humidity drops to 15 to 30 percent at the same time, which is dry enough to crack the nose and double the static in a coat. Wind runs 8 to 15 m/s with gusts above 20 m/s, and fine dust seeps through window frames, door seals and range hoods rather than through open windows alone.
Why the first 2 hours set the indoor number
Indoor air does not track outdoor air instantly: with windows shut, a typical room climbs to 40 to 50 percent of the outdoor PM2.5 in 60 to 90 minutes and levels off near 60 to 70 percent after 2 hours. That means the most useful window is the 60 minutes before the front arrives. Close windows, seal the gap under the door with a folded towel, turn off the range hood and bathroom fan (both pull outdoor air in), and keep interior doors open so the purifier can work on the whole flat.
What to do at each threshold
| Outdoor PM2.5 | Indoor target | Action |
|---|---|---|
| under 75 | under 35 | windows open 2 x 15 minutes |
| 75 to 150 | under 35 | close 1 window, purifier level 1 |
| 150 to 300 | under 50 | all windows shut, level 3, wipe 1 x daily |
| 300 to 500 | under 50 | level 3 continuous, door gaps sealed |
| over 500 | under 75 | level 3, 1 room only, litter box moved inside |
Purifier on level 3: size it before you trust it
Match CADR to the room: for a 20 square metre room with a 2.5 m ceiling you want 250 to 350 cubic metres per hour, which is roughly 5 air changes. Level 3 on most domestic units delivers that at 45 to 55 dB, so put it in the room the cat actually uses, 30 cm off the wall and clear of curtains. Use a HEPA H13 filter, vacuum the prefilter every 2 weeks during dust season, and replace the main filter every 6 to 12 months or when the indicator turns. A purifier running next to an open window is a 300 USD fan.
Humidity: 40 to 50 percent, not 60
Dust events drop indoor humidity to 20 to 30 percent. Bring it back to 40 to 50 percent: a 2 to 3 litre a day cool-mist unit for a 20 square metre room, filled with filtered water and cleaned every 48 hours. Humid air makes 2.5 micrometre particles settle out of the breathing zone within 30 to 60 minutes. Stop at 50 percent; above 60 you trade dust for mold and dust mites, which is a worse problem for an asthmatic cat.
One wet wipe a day, in this order
Work top down: shelf tops, windowsills, then the table, then the floor. Use a damp microfiber cloth, not a wet one, and rinse it every 2 to 3 square metres. Dry dusting and a broom both lift the fine fraction back into the air, where it stays for 3 to 6 hours. Wet-wipe once a day during the event and for 2 days after it, and vacuum with a sealed HEPA unit rather than a bagless cyclonic, which returns a visible plume at the exhaust.
What the cat shows first
Watch for sneezing more than 3 times an hour, clear or brown discharge at the inner eye, a resting respiratory rate above 40 breaths per minute, and a cat that sits with its neck extended. Grooming is the other route: a dusty coat means the cat swallows the load, so brush it outdoors or in a closed bathroom for 5 minutes a day. Move the litter box away from the window and switch to a low-dust litter during the event, because clay dust adds to the same particle load the purifier is fighting.
When to call the vet
Call the same day for open-mouth breathing, a rate above 40 at rest, coughing that lasts more than 48 hours, or a cat that stops eating for 24 hours. Cats with feline asthma can tip into an attack on a 300 microgram day, and the signs to know are a hunched posture, rapid shallow breathing and a wheeze audible from 1 m away. Do not give any human cough medicine, and do not wait for the dust to clear before getting an asthmatic cat seen.
Five steps for a dust day
- Close all windows in the hour before the front hits, seal the door gap.
- Run the purifier on level 3 in the room the cat uses.
- Hold indoor humidity at 40 to 50%, cleaning the humidifier every 48 hours.
- Wet-wipe once a day, top down, and vacuum with a sealed HEPA unit.
- Check the resting breathing rate twice a day; above 40 means a vet call.
Indoor mold in wet autumn weather is the opposite failure mode, covered in autumn indoor mold. How pressure changes move a cat's behavior is in storm pressure and behavior, and anything the cat may have licked off a dusty surface should be run through the toxic product checker.
Frequently asked
How long should I keep the windows closed during a dust storm?
For the whole event and 2 hours after it ends, which usually means 6 to 12 hours plus the tail. Indoors climbs to 40 to 50 percent of the outdoor PM2.5 in 60 to 90 minutes and levels near 60 to 70 percent by 2 hours, so closing early matters more than closing often. Once outdoor PM2.5 falls under 75, ventilate twice for 15 minutes.
Is level 3 on a purifier enough for a 20 square metre room?
It is, if the CADR is 250 to 350 cubic metres per hour, which gives about 5 air changes. Put the unit in the room the cat uses, 30 cm from the wall, with a HEPA H13 filter, and vacuum the prefilter every 2 weeks in dust season. A purifier beside an open window is a 300 USD fan; seal the door gap as well.
Should I run a humidifier during a dust storm?
Yes, to 40 to 50 percent, not higher. Indoor humidity often drops to 20 to 30 percent during an event, and bringing it to 40 to 50 percent makes 2.5 micrometre particles settle out of the breathing zone within 30 to 60 minutes. Above 60 percent mold and dust mites take over, which is worse for an asthmatic cat than the dust.