What comes in with the air?: sources & model

Open a room, plan when to exchange its air, and lift the cover off a particle filter. Follow the mass, inspect real observations, and discover what an AQI number can—and cannot—tell you.

What supports the explanation?

What this model assumes

1. The original furnished room and generic cleaner are illustrative geometry, not a measured home or a reverse-engineered product. The chosen room volume is 30 m³.
2. The model is one perfectly mixed zone with fixed outdoor concentration, full penetration, no surface deposition, no resuspension and no chemical reactions. Real airflow and particle composition can differ.
3. Window position represents an assumed balanced flow, not a calibrated opening/ventilation relationship. The fan motion and particle paths are slowed, enlarged and qualitative.
4. Gold markers and visible deposits are representative. Numerical removal follows CADR, not simulated collisions. No particle count or personal risk is inferred.
5. The instantaneous room value is not converted to daily AQI. NowCast and daily calculations use separate constructed records and stated U.S. rules.
6. The measured dataset contains two selected ambient 24-hour samples. Missing days stay gaps. Satellite context is not a ground-level concentration map.
7. The home activity uses supplied records and paper. Do not generate smoke or dust, dismantle filters, or change appliances or building ventilation for the activity. For real conditions, use a responsible adult and current local guidance.
8. What PM categories mean: Particulate matter is a mixture of airborne solid particles and liquid droplets. PM2.5 is included in the larger PM10 size fraction; adding their mass readings can double-count the shared material. The size conventions are operational and aerodynamic, not exact geometric boundaries for every shape.
9. Mass is not particle count: Concentration in µg/m³ is particle mass per air volume. Many tiny particles and fewer larger particles can contribute mass differently. Our markers do not encode a measured size distribution, chemistry or particle count.
10. The room equation: V dC/dt = E + Q C_out − Q C − D C. V is volume, E is an assumed emission rate, Q is balanced outdoor exchange and D is particle CADR. Time is in hours. This original specialization of NIST’s single-zone mass balance fixes penetration at 1 and surface deposition at 0.
11. A segment has an exact solution: For constant inputs, λ = (Q + D)/V and B = (E + Q C_out)/V. When λ is positive, C tends toward B/λ exponentially. With no removal, C increases linearly at rate B. Each scheduled action begins a new segment from the concentration already present.
12. Integrate to keep the ledger: The concentration integral J over a segment gives outgoing mass QJ and captured mass DJ. Source addition is EΔt and outdoor entry is Q C_out Δt. J has units µg·h/m³; it is not a deposited dose or a complete measure of a person’s exposure.
13. CADR connects airflow and collection: In an ideal representation, clean-air delivery rate equals processed airflow multiplied by removal efficiency for a specified particle test. Whole-room change also depends on room volume, ongoing inputs and mixing. Neither a high-efficiency label nor a pictured fan implies an instant room-wide reduction.
14. Why smaller does not always mean harder to capture: Fibrous media can collect through interception, inertial impaction, diffusion and electrostatic effects. The nominal 0.3 µm HEPA test description does not mean all smaller particles pass. Actual performance depends on the medium and test conditions.
15. Particle cleaning and gas removal differ: The particle CADR in this lesson is not a CO₂-removal rate. Gas sorbents and chemical removal have different mechanisms and limits. A selected pollutant monitor does not represent all indoor-air hazards or comfort conditions.
16. A U.S. PM2.5 sub-index has a convention: The separate calculator uses the EPA May 2026 table. Eligible PM2.5 concentration is truncated to one decimal place, interpolated within the appropriate interval and rounded to an integer index. A completed daily calculation uses a 24-hour concentration. Our implemented table ends at 325.4 µg/m³.
17. A NowCast responds to recent changes: For PM, the previous twelve hourly positions are weighted. The base weight is at least 0.5 and approaches 1 when concentrations are steady. Older observations receive powers of that weight. At least two of the latest three positions need valid data. Gaps retain their original hour positions.
18. An index is not a percentage: A pollutant sub-index uses a defined scale. Overall AQI takes the highest applicable pollutant sub-index; it is not their sum or mean. An index of 100 is neither 100 µg/m³ nor a percentage of personal danger. Other countries may use different systems.
19. A record has a sampling history: The selected AQS site uses a 24-hour gravimetric sample. Its June 5 and June 8 records do not determine June 7, an hourly peak, indoor concentration or a person’s dose. Historical archive indices can be reprocessed; our displayed indices explicitly use the May 2026 method.

What has been checked

Analytical reference cases, conservation or transition invariants, finite drawing commands, bounded setup parsing, discovery and route integrity are checked automatically. These checks do not establish anatomical fidelity, learner outcomes or browser/device compatibility. Independent subject review, learner trials, comprehensive accessibility review and browser video encoding checks remain pending.

Each source supports the associated claim. Sources do not certify this implementation or its visuals.

About the cover illustration

Original furnished room and generic particle-cleaner model. Actual teaching scene, with illustrative proportions and enlarged representative markers; not measured building airflow or manufacturer CAD.