Why wildfire smoke response planning needs more than a CADR sticker
Wildfire smoke is no longer a seasonal inconvenience confined to the American West. Communities from Vancouver to New York to Atlanta have endured days or weeks of hazardous fine particulate matter blown in from distant blazes. Portable HEPA air cleaners are among the fastest ways to make a single room safer, but most households own just one device and need to decide how to run it efficiently. Manufacturer packaging usually lists a Clean Air Delivery Rate (CADR) number and a square footage recommendation. Unfortunately, those snapshots assume continuous operation, standard 8-foot ceilings, and moderate contaminant loads. They do not explain how to ration runtime when electricity prices, noise tolerance, and filter life are real constraints. The Portable Air Cleaner Smoke Event Runtime Planner translates CADR ratings into hourly airflow, compares that against your room volume, and shows how much runtime each fan speed requires to reach a chosen air changes per hour (ACH) target.
Public health agencies often suggest aiming for five to eight air changes per hour during wildfire smoke intrusions. That guidance is easy to quote but hard to apply because homes vary in size and sealing. A 180 CADR unit might deliver six ACH in a small bedroom yet struggle to hit three ACH in a large living room. Meanwhile, running a purifier at maximum speed all day can be loud and expensive, and it may burn through filters faster than supply chains can replenish them during regional smoke emergencies. This calculator provides a holistic view by combining room geometry, daily exposure duration, and multiple fan speed profiles. Instead of guessing whether to run the purifier all night or only during the worst hours, you can plan a duty cycle that balances clean air with energy use and consumable costs.
The planner accepts dimensions in either feet or meters, converting automatically to the cubic feet used in CADR ratings. It also accounts for the reality that many purifiers have multiple fan speeds with wildly different CADR and power draw values. Enter the CADR for smoke (which is typically lower than for dust or pollen) along with the wattage at each speed. If you do not know the noise level, you can leave that field blank; the table will simply omit decibel data. When you click “Plan runtime,” the tool evaluates each speed and produces a row showing actual ACH, runtime required to maintain the target on a daily basis, the resulting energy consumption, and how quickly the filter will reach its rated life.
Formulas translating CADR into runtime and filter wear
CADR expresses how many cubic feet of clean air a purifier delivers per minute. To convert that to air changes per hour, you multiply by 60 (minutes per hour) and divide by the room volume \(V\). In MathML:
When you set a target ACH \(A_t\) and the purifier can only run for part of the day, the required runtime fraction \(F\) is \(F = A_t / ACH\). If \(F\) is less than or equal to 1, running the purifier for \(F\) times the daily exposure window will meet the target. If \(F\) exceeds 1, the purifier must run the entire exposure window and still falls short by \(A_t - ACH\). Energy consumption follows from the power draw \(P\) in watts: dividing by 1,000 converts to kilowatts, and multiplying by runtime gives daily kWh. Filter life in days is the rated filter life in hours divided by the daily runtime; dividing the replacement cost by that day count yields a filter cost per day estimate. These relationships create a transparent performance picture so you can make informed choices rather than relying on generic room size charts.
Input choices and validation
The room dimension fields accept decimals to accommodate irregular spaces or metric conversions. Ceiling height defaults to eight feet because many U.S. homes fall near that value, but taller rooms will immediately show how much harder it is to sustain high ACH targets. The exposure duration field captures how many hours per day you expect heavy smoke infiltration; some households only need the purifier during late afternoon when windows are open, while others face 24-hour events. Target ACH defaults to five, aligning with consensus recommendations for wildfire smoke mitigation. Filter life and cost help translate operating plans into tangible maintenance schedules. Manufacturers often quote filter life in months, but those figures assume moderate use, so entering the rated hours yields more accurate projections.
Fan speed entries allow up to three profiles because many purifiers offer low, medium, and high settings. If your unit only has one speed, simply fill the first row and leave the others at zero CADR. The planner automatically ignores speeds without CADR data. Because CADR ratings differ for smoke, pollen, and dust, focus on the smoke value from the AHAM Verifide label or the product manual. Power draw can come from a Kill A Watt meter, manufacturer specifications, or utility data. Noise levels are optional but useful for comparing trade-offs between acoustic comfort and filtration effectiveness. The calculator validates that CADR, power, and runtime inputs are nonnegative; if the room volume is zero or any critical field is missing, it highlights the issue before computing results.
Worked example: Preparing a living room for a week of wildfire smoke
Imagine a family in Portland with a 16-by-18-foot living room and a nine-foot ceiling. They expect to shelter indoors for 14 smoky hours per day and want at least six ACH. Their purifier offers three speeds: low (150 CADR, 45 watts), medium (250 CADR, 75 watts), and high (400 CADR, 130 watts). Entering these figures shows that low speed delivers about 3.1 ACH. To reach six ACH, the planner calculates a runtime fraction of 1.94, meaning even continuous operation at low speed falls short by roughly 2.9 ACH. Medium speed reaches 5.2 ACH, requiring 11.5 hours of runtime to meet the target during a 14-hour smoke window. High speed delivers 8.3 ACH, so it only needs 10.1 hours per day to achieve the target; running longer provides a safety margin.
The table also reports energy and filter impacts. Medium speed consumes about 0.86 kWh per day, translating to $0.17 at an $0.20/kWh rate. High speed uses 1.32 kWh daily ($0.26). If the filter is rated for 250 hours and costs $45, medium speed requires a replacement every 21.7 days of smoke usage, while high speed shortens that to 17.3 days. Seeing those numbers helps the family plan to buy extra filters before wildfire season and to rotate between medium and high speeds depending on real-time air quality readings. The summary at the top flags high speed as the only setting that comfortably meets the target, while medium speed is highlighted as a viable compromise when noise sensitivity is high.
Comparison of common purifier sizes in a 200 square-foot room
| CADR (smoke) | ACH at full runtime | Runtime needed for 5 ACH | Notes |
|---|---|---|---|
| 120 CFM | 4.5 ACH | Full 12-hour smoke window still leaves 0.5 ACH shortfall | Better suited for small bedrooms or to supplement central filtration. |
| 220 CFM | 8.3 ACH | Requires 7.2 hours of runtime per 12-hour smoke window | Good balance of noise and filter wear for most living spaces. |
| 360 CFM | 13.6 ACH | Needs only 4.4 hours per 12-hour smoke window | High-capacity unit that can cover open floor plans or multiple rooms. |
Limitations and assumptions
This planner models a single room with closed doors and assumes the purifier is the primary filtration device. In real homes, infiltration, open windows, and central HVAC systems can add or remove particulates, changing the effective ACH. The tool does not account for filter loading, which gradually reduces CADR as filters accumulate smoke particles. Always monitor your purifier’s filter indicator and indoor air quality sensor data if available. Additionally, the planner treats filter life as a linear function of runtime, while some filters clog more rapidly under heavy smoke loads. If your purifier uses multiple filters (pre-filter, HEPA, activated carbon), consider replacing them as a set to maintain performance. Finally, remember that wildfire smoke contains gases as well as particulates; pairing HEPA filtration with activated carbon and minimizing indoor combustion will deliver the best results. Use this planner as a starting point, then adjust based on local air quality reports, comfort, and noise tolerance.