Pollutant guide
What is PM2.5? A practical guide to fine-particle forecasts
PM2.5 is particulate matter with an aerodynamic diameter of 2.5 micrometres or less. AirAgora uses the provider’s modeled concentration and matching concentration-band context as decision-support inputs, while keeping the grid resolution, timestamp, and uncertainty visible.
Source → method → decision
What the PM2.5 label describes
“PM” means particulate matter, and the 2.5 label refers to an aerodynamic size boundary rather than a single chemical substance. Particles within that size class can have different compositions and origins. A forecast value therefore reports a modeled mass concentration for a size fraction; it does not identify every particle, prove a local source, or describe what one person inhaled.
OpenWeather exposes hourly PM2.5 concentration, normally in micrograms per cubic metre, together with its official 1–5 provider value. AirAgora keeps the provider category visible and derives PM2.5 severity separately from the documented concentration bands.
Why concentration and AQI are not interchangeable
A concentration is a physical quantity supplied for a modeled grid cell. An AQI is a communication scale that applies breakpoints, pollutant rules, and averaging conventions. A PM2.5 concentration should not be compared numerically with a nitrogen-dioxide concentration, and a European AQI number should not be treated as a direct conversion of a U.S. AQI number.
AirAgora preserves the provider’s official 1–5 value and derives continuous severity separately for each supported pollutant. The main pollutant is selected from those comparable severities, not by choosing whichever raw concentration has the largest number. This prevents unlike units from being ranked as though they shared one scale.
What a modeled PM2.5 forecast can tell you
The forecast can show how the provider expects the grid-cell concentration and index contribution to change by hour. That is useful for comparing complete periods, checking whether one practical window is modeled as relatively less polluted, and seeing the provider update time. The information is directional and time-specific rather than a guarantee.
OpenWeather documents OpenWeather European forecasts on a broad forecast grid. A cell can contain many roads, buildings, terrain features, and microenvironments. Conditions beside a street, indoors, or near a local source can differ from the grid value.
How AirAgora uses PM2.5 in a daily plan
AirAgora does not optimize the day from PM2.5 alone. It evaluates complete consecutive two-hour windows from the strongest derived pollutant severity and uses the supported component bands to explain what most affected the result. The score gives more weight to the window mean while retaining a penalty for the worst hour.
If the cleanest-scoring window is only marginally different from typical windows, the result says conditions remain similar. If all practical periods are poor, the wording changes to “least polluted available window.” Neither phrase means that the period is risk-free.
- Check the named index and timestamp before comparing screenshots.
- Use the hourly pattern, not one isolated number, for planning.
- Treat activity guidance as general caution, not individual medical advice.
- Follow official alerts during severe events even when an app forecast is available.
Health guidance without a universal-safe claim
The WHO 2021 air-quality guidelines summarize evidence linking fine-particle exposure with health effects at concentrations lower than older recommendations recognized. That is one reason AirAgora does not present an AQI category as “safe for everyone” and does not convert a better forecast period into a medical recommendation.
People with questions about personal risk, symptoms, treatment, or an existing condition need appropriate professional or official guidance. AirAgora’s role is narrower: organize modeled public data into a transparent comparison and make its limitations easy to find.
Questions to ask when another service differs
First compare timestamps, timezone, index scheme, averaging period, and whether the other value is modeled or observed. A nearby monitor may represent a point measurement, while a forecast represents a grid. Services can also refresh at different times or choose different model domains.
A disagreement does not automatically prove that one service is fraudulent or broken. The useful response is to inspect provenance, prefer official warnings for urgent decisions, and avoid combining numbers from incompatible scales. AirAgora publishes the source and method so that comparison remains possible.
Direct answers
Frequently asked questions
Does PM2.5 mean every particle is exactly 2.5 micrometres?
No. It names a particulate size fraction defined by aerodynamic diameter at or below the boundary. The particles can vary in exact size and composition; the forecast reports modeled mass concentration for that fraction.
What unit does AirAgora show for PM2.5?
The provider concentration is normally shown in micrograms per cubic metre (μg/m³). concentration-band severity is separate from the provider value and should not be read as concentration units.
Is a PM2.5 forecast the same as a nearby sensor reading?
No. The OpenWeather forecast value is modeled for a grid cell. A station or sensor is a point observation with its own siting, calibration, and update time, so the two may differ.
Does the lowest PM2.5 hour mean it is safe to exercise?
No. “Lower” is relative to the available forecast and does not remove risk or account for individual health. AirAgora compares complete windows and provides general caution, not medical clearance.
Why might PM2.5 be the main pollutant?
AirAgora compares derived severity from the documented pollutant bands. PM2.5 is named when its component severity is highest under the stable tie rules, not because its raw concentration has the largest numeral.
What should I do during an official smoke or pollution warning?
Use the official warning and public-health instructions as the primary source. AirAgora does not paywall urgent guidance and should not replace emergency or local-authority information.
Evidence register
Sources and further reading
The links below identify the primary references used on this page. Selection, summary, and presentation are AirAgora modifications.
Keep exploring
AirAgora provides informational planning guidance, not medical advice. Local conditions can differ from modeled forecasts; follow official guidance during severe events.