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INTERACTIVE EXPLANATION

What is hiding in the air you breathe?

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.

Enable JavaScript to change the conditions and run the interactive experiment.

Make a discovery

Exchanging air both brings particles in and carries particles out. The same opening can lower the room’s concentration when outside air is cleaner, or raise it when outside air is more concentrated. A recirculating filter has a different job: it collects particles as room air passes through.

  • Predict how outdoor exchange depends on the entering concentration.
  • Separate stopping a source, exchanging air and recirculating filtration.
  • Account for particles still airborne, captured or carried out.
  • Compare particle size, particle count and mass concentration.
  • Explain why fibrous filters can capture particles smaller than their apparent openings.
  • Distinguish an instantaneous calculation, a daily sample, a NowCast and a forecast.
  • Read actual monitoring data without inventing missing measurements.
  • Explain why a pollutant reading or AQI is not a personal diagnosis.

Make a prediction

Opening a window always lowers room particle concentration. Is that enough information?

  • Yes, outdoor exchange can only remove particles
  • No, we also need to know what enters
Read the explanation

Outdoor exchange has both incoming and outgoing terms. Their concentrations may differ.

Understand it

Begin with a room that looks ordinary

Air can carry particles too small to distinguish by eye. Our small gold markers are enlarged symbols. The concentration readout, rather than the number of drawn dots, describes the modeled particle mass.

Make a prediction

Will exchanging air lower the room concentration? Compare what is inside with what is entering. Try both the cleaner-outside and smokier-outside cases before turning on a filter.

Put an action on the timeline

Move outdoor exchange, filter startup or source stopping to a chosen minute. Scrubbing time recalculates the same plan. Moving an action changes the future pathway without creating a jump in the room’s stored particle mass.

Follow two sides of the window

Equal air volumes enter and leave this model. Their particle concentrations can differ, so incoming and outgoing particle mass rates can differ too. The selected flow rate is an assumption, not a measurement of window opening.

Open the cleaner

A fan moves room air through pleated filter material and back into the room. Some particle mass collects on that material. The cleaner does not add an equal flow of outdoor air.

Think beyond a simple sieve

Air bends around fibers. A passing particle can touch a fiber, continue into it through inertia, or wander into contact through diffusion. These mechanisms do not require every apparent opening to be smaller than the particle.

Audit the result

Open the mass budget. What started in the air plus what entered or was generated must match what remains, what left, and what collected on the filter. Captured material changed location; the model did not destroy its mass.

Ask what a measurement represents

A reading needs units, a place, a time interval and a method. The selected real record has two 24-hour samples. The separate hourly cards teach NowCast weighting. Neither comes from the fictional room.

Look closer at the science

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Where this is used

Understand an air-cleaner specification

Distinguish media efficiency, particle CADR, processed airflow and room assumptions. Do not treat a product picture as a measured performance curve.

Read a smoke headline carefully

Check whether a number is a concentration, daily index, NowCast or forecast—and where and when it applies.

Ask a better sensor question

Identify the pollutant and measurement limits. One reading cannot answer every question about indoor air.

Try it yourself: Read a week of air without inventing a day

Supplies

  • Paper
  • Pencil
  • The two supplied EPA observations
  • Optional calculator
  1. Make a seven-day strip

    Write June 4–10, 2023. Put 10.5 µg/m³ on June 5 and 60.7 µg/m³ on June 8. Each is a selected 24-hour sample. Mark all other days “no selected sample.”

  2. Compare what was measured

    Circle the higher concentration. Subtract the lower from the higher. Optional: calculate their ratio, then label it a concentration ratio rather than a health-risk ratio.

  3. Keep the index convention attached

    Use the supplied May 2026 U.S. PM2.5 table. The two sub-indices are 54 and 155. The measurements are from 2023; the stated calculation convention is from 2026.

  4. Say what the gaps leave unknown

    Write one thing the record cannot show: the June 7 concentration, an hourly maximum, indoor conditions or anyone’s dose. Do not draw a line that pretends to measure the gaps.

  5. Draw a familiar room from memory

    Mark possible air entry, air exit and recirculating filtration on paper. Use dashed lines for uncertain routes. Keep the drawing private; no address or uploaded room plan is needed.

  6. Compare two predictions

    Predict what outdoor exchange might do with cleaner entering air, then with smokier entering air. Compare in the digital model. The observation is complete with the supplied data and paper.

Which parts of your answer came from measurements, a calculation, or an assumption?

No smoke, incense, candles, sprays, dust release, filter disassembly or appliance changes. Children should not alter windows or building equipment for this activity. Use current local guidance and a responsible adult for real air-quality decisions.

Check your understanding

The air looks clear. Does that prove fine particles are absent?

  • Yes, particles always make visible haze
  • No, many particles cannot be seen individually
Answer and explanation

No, many particles cannot be seen individually Visibility is not a complete particle concentration measurement.

PM2.5 is 8 µg/m³ and PM10 is 12 µg/m³. Is their combined particle total automatically 20?

  • No, the size fractions overlap
  • Yes, add the two readings
Answer and explanation

No, the size fractions overlap PM2.5 is included within the PM10 fraction, so addition can double-count mass.

What determines whether outdoor exchange lowers this room concentration?

  • Only the shape of the window
  • Only whether the room looks hazy
  • The incoming concentration and the other sources/removals
Answer and explanation

The incoming concentration and the other sources/removals Compare all inputs and removals in the room budget.

The particle filter lowers PM. Must it remove CO₂ at the same rate?

  • No, gas removal uses different mechanisms
  • Yes, all air ingredients are filtered identically
Answer and explanation

No, gas removal uses different mechanisms Particle CADR is not an equivalent removal rate for every gas.

A medium catches 99.97% of a specified test aerosol passing through. What happens to a whole room?

  • The whole room instantly loses 99.97%
  • Room change also depends on processed airflow, time and inputs
Answer and explanation

Room change also depends on processed airflow, time and inputs Media efficiency is different from whole-room clearance.

A brief indoor reading is 60.7 µg/m³. Can you call it a daily U.S. AQI of 155?

  • Yes, the number alone is enough
  • No, the required averaging method and context are missing
Answer and explanation

No, the required averaging method and context are missing A brief reading is not an eligible completed daily average.

Our selected monitor has June 5 and June 8 samples. What do we know about June 7?

  • This subset does not give its value
  • It must be halfway between the two samples
Answer and explanation

This subset does not give its value Connecting observations does not create a missing measurement.

Particles collect on the filter in this model. What happened to their mass?

  • It was destroyed
  • Its chemical danger was fully measured
  • It changed location
Answer and explanation

It changed location Captured mass leaves the room air and appears in the filter ledger.

Sources and model limits

  • 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.
  • 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.
  • Window position represents an assumed balanced flow, not a calibrated opening/ventilation relationship. The fan motion and particle paths are slowed, enlarged and qualitative.
  • Gold markers and visible deposits are representative. Numerical removal follows CADR, not simulated collisions. No particle count or personal risk is inferred.
  • The instantaneous room value is not converted to daily AQI. NowCast and daily calculations use separate constructed records and stated U.S. rules.
  • The measured dataset contains two selected ambient 24-hour samples. Missing days stay gaps. Satellite context is not a ground-level concentration map.
  • 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.

Single-zone mass balance and mixing assumptions

Original simplified model derived from equations 1–2, with penetration 1 and deposition 0. This page does not execute FaTIMA or CONTAM.

NIST · Technical Note 2095, FaTIMA

Indoor sources and outdoor entry

Context for an assumed indoor source and ambient entry; the model does not estimate a real household source rate.

EPA · Indoor particulate matter

Different roles of source reduction, cleaner outdoor air and filtration

Particle filters and gas-removal media have distinct roles and limits.

EPA · Air cleaners and filters

HEPA test terminology

Nominal 0.3 µm test description is not an absolute lower particle cutoff or whole-room removal fraction.

EPA · What is a HEPA filter?

Actual selected outdoor monitoring observations

2023 daily parameter 88101 archive, retrieved September 8, 2026. Exact site, POC, method and two-row selection are retained in downloadable metadata.

EPA AQS · Daily PM2.5 data archive

Monitoring-data reuse

AQS data are public domain. Source and participating monitoring agencies remain credited.

EPA · AQS data reuse

Actual satellite context for regional smoke

GOES-16 image for June 7, 2023. NASA Earth Observatory / Lauren Dauphin, NOAA and NESDIS. Separately credited third-party ground photographs were not reused.

NASA · Smoke Smothers the Northeast

Independent subject review is pending.

Read the sources and model assumptions