Smoke reaches indoor air within minutes and lingers for days after the flames are out. A cat with asthma, or any cat past 11 years old, does not need a visible haze to start coughing. Three decisions carry most of the risk: when you shut the house, whether the filter is sized to the room, and what goes into the carrier if you have to leave.
Where the AQI line actually sits
AQI is calculated for a healthy adult walking outdoors, so for a cat it works as a floor, not a ceiling. Veterinarians who manage feline respiratory cases tend to start advising closed windows, no balcony, and a running filter at AQI 100 for a cat with diagnosed asthma, and at 150 for a healthy adult cat. Past 200, the better move is one sealed room instead of the whole house. Past 300, leave if you have a car and a destination.
| AQI | What the air is doing | Action for a cat |
|---|---|---|
| 0 to 50 | Baseline | Normal routine, windows open if you like |
| 51 to 100 | Faint haze, smell of burning | Asthmatic cat stays in, filter on low |
| 101 to 150 | Visible smoke at distance | Windows shut, no balcony, filter on medium |
| 151 to 200 | Smoke layer at ground level | One sealed room, litter box moved inside |
| 201 to 300 | Heavy smoke, ash fall | Sealed room plus plan to leave within 24 hours |
| Over 300 | Hazardous | Leave, cat in carrier, kit packed |
One detail gets skipped: AQI tracks PM2.5, particles 2.5 micrometers and smaller. Cat airways also react to the 0.3 to 1 micrometer fraction, and that fraction passes straight through a standard furnace filter. A MERV 8 furnace filter catches roughly 20 percent of it. A true HEPA filter is rated 99.97 percent at 0.3 micrometers. That gap is the difference between a smoky smell in the house and a cat coughing at 3 a.m.
Why cats lose this fight earlier than people do
A cat's lung volume is about one fifteenth of a human's, and a cat breathes 20 to 30 times a minute at rest against 12 to 16 for an adult person. Per kilogram of body weight, a cat moves more air through a smaller surface. Cats are also obligate nasal breathers, so every particle that reaches the nose gets filtered by a narrow passage that inflames fast.
Asthma affects an estimated 1 to 5 percent of cats, and smoke exposure is one of the recognized triggers for a flare in a cat that already has it. Age multiplies the effect: cats over 11 are more likely to carry chronic kidney disease, heart disease, or a low-grade airway inflammation that smoke pushes into a visible problem. A 4-year-old may sneeze for a day. A 14-year-old may stop eating for 48 hours.
Sizing a HEPA unit to the room, not to the house
Ignore the marketing square footage on the box. Use CADR, the clean air delivery rate in cubic feet per minute, and apply the two-thirds rule: CADR should be at least two-thirds of the room's floor area in square feet. A 150 square foot bedroom needs CADR 100. A 300 square foot living room needs CADR 200.
That ratio delivers about 5 air changes per hour with an 8 foot ceiling, which is the minimum useful turnover for smoke. Two smaller units beat one large unit in an L-shaped room, because air does not turn corners. Place the intake 30 to 60 cm off the floor, away from the wall and away from the litter box. Run it on high for the first 2 hours after you close the windows, then drop to medium.
Building the smoke room in 20 minutes
- Pick the smallest room with a door and one window. A bathroom works if it has a window; a bedroom is better.
- Seal the door gap with a rolled towel and tape the window frame if the sash is loose.
- Move in a litter box, water bowl, and a bed the cat already uses. Novelty causes hiding, not settling.
- Keep the filter intake clear by 30 cm on all sides and vacuum the prefilter every 2 weeks.
- Skip candles, incense, and spray air fresheners. They add particles and volatile compounds the filter was never meant to handle.
- Hold the room at 20 to 22 degrees Celsius and 40 to 50 percent humidity. Dry air thickens airway mucus.
Medication and monitoring on smoke days
For a cat already on inhaled therapy, most protocols use fluticasone at 44 to 110 micrograms twice daily through a spacer mask, with albuterol as a rescue bronchodilator rather than a daily drug. Do not start or change a steroid dose on your own during a smoke event; call the vet, because smoke can unmask a heart condition that steroids will make worse.
Count the sleeping respiratory rate once a day. Watch the chest for 30 seconds, multiply by 2, and write it down. Under 30 breaths per minute is normal. A jump of 10 or more above the cat's own baseline, sustained over 2 readings, is a reason to call. Open-mouth breathing, a rate over 60, or gums that look gray-blue is an emergency, not a monitoring question.
The 30-minute evacuation grab list
- Hard-sided carrier, one per cat, labelled with your phone number.
- 7 days of food, in the cat's normal formula, plus a folding bowl.
- 14 days of medication, in the original packaging, plus a copy of the prescription.
- One week of litter and a disposable tray.
- Vaccination record and microchip number, printed and on your phone.
- A towel that smells like home, and a pheromone wipe for the carrier.
Practice the load once, timed. Most people find that catching a cat that does not want to be caught eats 15 of the 30 minutes. If your cat hides, know the three places it goes before you need to know.
Five steps for tonight
- Check the AQI for your zip code and write down the number.
- Close the windows on the side the wind comes from, and check the weatherstrip.
- Confirm your filter's CADR against the room's square footage using the two-thirds rule.
- Set out the carrier with the door open, so it stops being a signal for a vet visit.
- Pack the medication and the food into one bag that lives by the door until the smoke clears.
If you have a long-haired cat, expect more swallowed hair during weeks when the house is sealed and grooming replaces outdoor self-cleaning; the shedding season guide at spring shedding and hairballs covers the brush schedule. Indoor confinement also changes eating and activity, which the autumn weight gain piece covers from the other direction. If the smoke season turns into a filter, medication, and boarding bill, the cost calculator is the place to see the annual total before you commit to a unit.
Frequently asked
At what AQI should I keep my cat indoors?
Most veterinarians suggest shutting windows and stopping balcony access at AQI 100 if the cat has diagnosed asthma, and at AQI 150 for a healthy adult cat. Above 200, move the cat into one sealed room with its litter box, water, and bed, and run the filter on high for the first two hours. Above 300, leave if you have a car and somewhere to go.
How big a HEPA filter do I need for one room?
Use the two-thirds rule on CADR: the filter's clean air delivery rate in cubic feet per minute should be at least two-thirds of the room's floor area in square feet. A 150 square foot bedroom needs CADR 100, a 300 square foot living room needs CADR 200. That gives about five air changes per hour at a standard ceiling height.
When does smoke become an emergency for a cat?
Open-mouth breathing, a sleeping respiratory rate above 60 breaths per minute, gums that look gray or blue, or a cat that stops eating for 24 hours are all reasons to call an emergency clinic rather than wait. A rise of 10 breaths per minute over the cat's own baseline, sustained across two readings, deserves a same-day call to your regular vet.