How Polly works

Where the air-quality data comes from

All air-quality readings are fetched live from the Open-Meteo Air Quality API, which aggregates open data from the Copernicus Atmosphere Monitoring Service (CAMS). It's free and open — Polly stores no air-quality data of its own.

City search and reverse geocoding use Open-Meteo's geocoding API and BigDataCloud's free reverse-geocoder.

What the balls mean

Each pollutant has its own colour. Rather than showing raw concentrations, Polly measures every reading against the WHO 2021 Air Quality Guidelines — the levels considered safe to breathe. A pollutant at its limit shows a baseline cloud; at 3× the limit it shows roughly three times the balls. So a hazy city looks unmistakably denser than a clean one, and the colours are directly comparable to each other.

• PM2.5 — Fine particles. Tiny inhalable particles (≤2.5µm) from combustion — the pollutant most linked to health harm. (WHO guideline: 15 µg/m³)
• PM10 — Coarse particles. Dust, pollen and larger particles up to 10µm across. (WHO guideline: 45 µg/m³)
• NO₂ — Nitrogen dioxide. Mostly from traffic and combustion engines. (WHO guideline: 25 µg/m³)
• O₃ — Ozone. Ground-level ozone formed by sunlight reacting with pollutants. (WHO guideline: 100 µg/m³)
• SO₂ — Sulphur dioxide. From burning fossil fuels, especially industry and shipping. (WHO guideline: 40 µg/m³)
• CO — Carbon monoxide. Colourless gas from incomplete combustion. (WHO guideline: 4000 µg/m³)

Pollen (for allergy sufferers)

Where it's available, you can switch the visualisation from air quality to pollen using the toggle. Pollen forecasts come from the same Open-Meteo / CAMS source but only cover Europe, so the toggle is disabled elsewhere. Each species is normalised against the approximate onset of “high” levels, and the orbs work exactly like the pollutant view.

• Grass — Grass pollen. The most common hay-fever trigger; peaks late spring to summer. (high ≈ 50 grains/m³)
• Birch — Birch pollen. A major spring tree-pollen allergen across northern Europe. (high ≈ 90 grains/m³)
• Alder — Alder pollen. Early-season tree pollen, often the first of the year. (high ≈ 90 grains/m³)
• Mugwort — Mugwort pollen. A late-summer weed pollen that affects many hay-fever sufferers. (high ≈ 25 grains/m³)
• Olive — Olive pollen. A key allergen around the Mediterranean in late spring. (high ≈ 90 grains/m³)
• Ragweed — Ragweed pollen. Highly allergenic late-summer/autumn weed pollen. (high ≈ 20 grains/m³)

Hover or tap any orb to inspect it, and click an orb (or a colour in the key) to isolate that pollutant. For PM2.5 we also show a rough cigarette equivalence — the Berkeley Earth rule of thumb that breathing ~22 µg/m³ of PM2.5 for a day is about as harmful as smoking one cigarette. It's a heuristic for fine combustion particles only, so it doesn't apply to dust, pollen or the gaseous pollutants.

The sneeze tracker & your privacy

Your personal sneeze log is stored only in your browser (localStorage). It never leaves your device unless you choose to record a sneeze, which also sends:

• an anonymous random device id (no account, no name, no email);
• your approximate city and country;
• coordinates rounded to roughly 1 km — never your precise location.

The map only ever shows counts grouped by city, and cities below a minimum count are hidden so no individual can be identified. Raw records are never readable by anyone — the map is built from an aggregate-only database function.

Open data, open source

Polly is a small project that visualises public data. Air-quality data © Open-Meteo / CAMS; map tiles © OpenStreetMap contributors & CARTO.