Air Purifier Smoke Clearance Time Calculator
How the clearance calculation works
Wildfire smoke drives particulate concentrations sharply upward. HEPA purifiers remove these particles following exponential decay. Once you enter the room volume and Clean Air Delivery Rate (CADR), the calculator derives the air changes per hour (ACH) and solves for the time to reach the target level. If the ACH is too low to meet your goal, increase CADR, add another purifier, or tighten the room envelope to reduce infiltration.
The underlying math models well-mixed air. The concentration over time obeys
Rearranging for time when the concentration equals a desired target gives
The script ensures the target concentration is lower than the starting level, otherwise the log becomes invalid. It also converts metric dimensions to cubic feet to match CADR units. If you prefer metric CADR, use the Air Changes per Hour Calculator first and enter the resulting ACH.
Scenario table: minutes to reach 12 µg/m³
Use the table to compare common purifier and room combinations. Values assume one purifier and no new smoke sources.
| Room size (ft³) | CADR (CFM) | ACH | Minutes to 12 µg/m³ |
|---|---|---|---|
| 2,000 | 250 | 7.5 | 18 |
| 2,400 | 300 | 7.5 | 16 |
| 3,000 | 350 | 7.0 | 19 |
| 3,000 | 500 | 10.0 | 13 |
Practical steps for faster clearance
Close windows, seal obvious gaps, and run the purifier on its highest setting until the calculator shows you have reached your target. Ceiling fans or box fans facing upward promote mixing so more air passes through the HEPA filter. Replace clogged filters promptly; a loaded filter reduces CADR and extends the run time. For very large rooms, consider supplementing with a DIY Corsi-Rosenthal box and evaluate runtime with the Portable Air Cleaner Smoke Runtime Planner.
Once the room stabilizes near your goal, switch to a quieter maintenance speed. Monitor particulate levels with a sensor and cross-check overall health risk in the Air Quality Index Calculator. If your building continually pulls in smoke, explore the Wildfire Smoke Infiltration Risk Calculator to evaluate long-term sealing projects.
Health guidelines and assumptions
Public health agencies flag 12 µg/m³ as an annual average benchmark and 35 µg/m³ for 24-hour exposure. During wildfire events, levels can exceed 300 µg/m³, so the difference between initial and target concentrations may be dramatic. The calculator assumes no additional smoke is introduced mid-cycle and that the purifier maintains its rated CADR. If you open doors frequently or the filter clogs, rerun the calculation with a higher starting concentration or lower CADR to see how clearance time changes.
Remember that HEPA filtration targets particles. If odors linger, choose a purifier with activated carbon media or use a secondary carbon filter. People with respiratory conditions should consult medical guidance and prioritize rooms with the cleanest air for rest and recovery.