What is the difference between PM2.5, PM10, NO2 and ozone?

**Different pollutants, different sources, different damage profiles**. [[PM2.5||fine particulate matter smaller than 2.5 micrometres]] is the worst overall: tiny soot particles from wood burning, diesel engines, brake dust, wildfires. So small they cross from your lungs straight into your blood, linked to heart attacks, strokes and lung cancer. [[PM10||coarser particles up to 10 micrometres]] is the bigger sibling: road dust, construction, pollen and mould. Irritating but mostly trapped in the upper airways. [[NO2||nitrogen dioxide]] is a brown gas from diesel cars and gas boilers; high near busy roads, triggers asthma. [[O3||ozone]] is a tricky one: stratospheric ozone (10 to 50 km up) protects us from UV, but **ground-level ozone** is harmful, formed when sunlight cooks car exhaust on hot summer afternoons. SO2 (sulphur dioxide) comes from coal plants and ship fuel; CO (carbon monoxide) from indoor stoves and broken gas appliances.

What are the WHO 2021 air quality guidelines and how do they differ from EU limits?

In 2021 the **World Health Organization** revised its global air quality guidelines, dropping the recommended annual PM2.5 limit from 10 to **5 ug/m3** based on a decade of new health research. The 24-hour limits are tougher too: **15 ug/m3 for PM2.5** (down from 25), **45 ug/m3 for PM10** (down from 50), **25 ug/m3 for NO2** (down from no daily limit), **100 ug/m3 for O3** (8-hour peak season). The EU is slowly catching up: the **EU Ambient Air Quality Directive 2024/2881** sets binding limits that are roughly **2x stricter than the current 2008 directive** but still **2x looser than WHO**. Full EU compliance is required by 2030. The US EPA NAAQS standards sit somewhere between WHO and EU. We show WHO limits because they are the health-driven gold standard.

What is a "healthy" range and where does it cross into "dangerous"?

Rule of thumb for **PM2.5** (the most consequential single number): **under 5 ug/m3** is the WHO annual target, achievable in rural Norway or Iceland on most days. **5 to 15** is what most of central and northern Europe sees on clean days. **15 to 35** is the daily background in most European cities. **35 to 75** is a smog episode (Krakow winter, Po Valley inversion, wildfire smoke). **75 to 150** is unhealthy for everyone; cancel outdoor exercise. **Above 150** is hazardous (Delhi winter, megacity stagnation events). For European AQI, **stay under 50** if you can; **50 to 100** is fine for most people; **100+** means asthmatics and heart-disease patients should stay indoors; **150+** is when everyone should reduce exertion outdoors.

When should I wear an N95 / FFP2 mask outdoors?

A proper-fit **N95 or FFP2 respirator** filters out roughly **95% of PM2.5** and is the right tool when AQI goes red or higher. Surgical masks and cloth masks are **useless against PM2.5** because the particles slip around the loose seal. Practical thresholds: **PM2.5 under 35** no mask needed for healthy people; **35 to 75** mask only if you have asthma or are very sensitive; **75 to 150** mask recommended for everyone outside; **above 150** mandatory for outside exposure, also try to reduce outdoor time. Beard? Get a hooded PAPR or shave for the day. The mask seal matters more than the rating: an FFP3 with a leaky seal is worse than an FFP2 that fits.

Indoor vs outdoor air: is it really better inside?

It depends. **With windows closed and an air purifier running, indoor air is typically 5 to 10x cleaner than outdoor air** during a smog episode. **Without filtration, indoor PM2.5 follows outdoor PM2.5 within 1 to 2 hours** because particles squeeze in through vents and gaps. Cooking, candles, fireplace, vacuuming without a HEPA bag, and **gas stoves** all spike indoor pollution well above outdoor levels, sometimes hitting 200+ ug/m3 PM2.5 right above the pan. A single 30 to 50 W HEPA purifier rated for your room size will pull indoor PM2.5 down to 1 to 5 ug/m3 within an hour, and is the single most cost-effective air health investment if you live in a polluted area.

Where does air pollution come from? Why is winter worse in Poland?

Three big sources: **traffic** (diesel cars dominate NO2, also brake/tyre dust = PM2.5), **heating** (winter coal and wood stoves are huge PM2.5 sources in Poland, Czechia, the Balkans, parts of Italy), and **industry** (coal power plants, cement, steel = SO2 + PM). **Why Poland is bad in winter**: roughly 3 million homes still burn coal or wood, mostly in old stoves. On cold still nights without wind, this smoke just sits in the city like a lid (a "**temperature inversion**"). Krakow, Nowy Sacz and Zakopane regularly hit PM2.5 over 150 in January and February. Western Europe escaped this around 1980 by switching to gas and district heating; eastern Europe is finishing the transition now (Polish "Czyste Powietrze" subsidy programme has so far replaced about 800k boilers since 2018).

Why is summer worse for ozone but winter worse for PM2.5?

Two different chemistries. **PM2.5 peaks in winter** because (1) people burn coal and wood for heat, (2) cold air sits still under temperature inversions, (3) low winter sun means atmospheric mixing is weak. **Ozone peaks in summer** because ozone is **made by sunlight**: UV photons split NO2 from car exhaust, freeing oxygen atoms that grab onto O2 to form O3. Hot, sunny, low-wind afternoons are perfect ozone-cookers. So a Polish winter day might have PM2.5 = 100 + O3 = 30, while a Greek summer afternoon might have PM2.5 = 15 + O3 = 200. Different threats, different mitigation: PM2.5 needs masks and filters, O3 needs you to stay out of strong sunlight in cities (and not exercise outdoors between 14:00 and 18:00 on heatwave days).

How accurate is a 24-hour air quality forecast?

**Reasonably good for "general level" but not for "exact number at 14:00 tomorrow"**. CAMS forecasts capture the overall trend (smog building up, clearing out) within about +/- 20% on typical days. They struggle with **sudden events**: wildfire smoke that arrives in 6 hours, traffic surges, a freight ship parking near port. Wind direction errors of even 10 degrees can move a plume by 5 km and ruin a local forecast. Treat the forecast as **"is tomorrow going to be roughly clean or roughly dirty"** rather than **"the AQI at 11:00 will be 87"**. For exact street-level numbers you need a local sensor (Airly, PurpleAir, Plume Labs); for "should I cycle tomorrow morning" the forecast is plenty.

How is this different from a sensor reading? Can I trust the data quality?

CAMS is a **model + assimilation** product: it takes real measurements from ~3000 ground stations, ~10 satellites, and weather data, then computes a **physics-based estimate** for every point on a 10 km grid. So a CAMS reading for downtown Warsaw is **an interpolated estimate**, not a real sensor measurement of that exact corner. For most points more than a few hundred metres from a road, the CAMS number is within +/- 30% of a local sensor on average. **For street-level precision** check official networks: **EPA AirNow** (US), **GIOS** (Poland, gios.gov.pl), **EEA European Air Quality Index** (eea.europa.eu), or low-cost dense networks like **Airly**, **PurpleAir**, **AirNow Fire & Smoke** for wildfires. CAMS is best for "what is the air like in this region right now" and 24-hour forecasts.

Air Quality Snapshot - free

What this shows you

Real-time air quality at any point on the map. Pick a location, see the current AQI number, the six pollutants behind it, what it means for your health, and how it is forecast to change in the next 24 hours.

Data comes from Open-Meteo's air quality service, which serves up the CAMS model output (the official European atmospheric monitoring system, run by the EU's Copernicus programme). CAMS combines satellite measurements, ground sensors, and weather models to estimate pollution everywhere on Earth at a roughly 10 km resolution. No API key, no signup, free for everyone.

Why you care: when you know whether the air is "Good" or "Very unhealthy" before you head out for a run, you can time it right (or skip it), close the windows, turn on a HEPA filter, wear an FFP2 mask, or just decide to drive instead of cycle. Long-term exposure to PM2.5 is associated with heart disease, lung cancer, and shorter lifespan, so even a 30-second check has a real payoff.

How to use it

Pick a location: click anywhere on the map, drag the marker, type lat/lng into the inputs, or tap one of the sample cities (Warsaw, Berlin, Delhi, Los Angeles).
The big number at the top is the European AQI for your spot. The colored band tells you the health category: green (Good), yellow (Moderate), orange (Unhealthy for sensitive), red (Unhealthy), purple (Very unhealthy), maroon (Hazardous).
Read the advisory paragraph for plain-language guidance: should you go for that run, open the windows, wear a mask, or stay inside.
The six pollutant cards show current concentrations of PM2.5, PM10, NO2, O3, SO2 and CO. Each is compared to the WHO 2021 short-term guideline, so you instantly see "I am 3x over the limit".
The 24-hour forecast chart plots predicted AQI for the next 24 hours. Spot a dirty window? Plan around it.
All concentrations are in ug/m3 (sometimes written as ug/m3 or microgrammes per cubic metre).
Data is cached for 1 hour, so refreshing rapidly is harmless. The CAMS model itself updates every 3 hours.

When this is useful

Real situations where the snapshot changes a decision:

Before a run, bike ride, or outdoor workout: A 60-minute hard run at AQI 150 forces the same lung dose as living for a full day at AQI 30. Check the colour, then either go now, push the workout to a cleaner window, or move it indoors.
Opening windows for ventilation: In winter many homes have stale indoor air with a CO2 build-up, so the instinct is to throw the windows open. If outdoor PM2.5 is 80 ug/m3, you are pumping pollution into your bedroom. Better: run a HEPA purifier with windows closed, or wait for the cleaner overnight window.
Asthma and COPD management: People with asthma or chronic obstructive pulmonary disease feel the difference between AQI 40 and AQI 120. Check the snapshot, take a preventer puff if needed, and keep the rescue inhaler in your pocket on bad days.
Pregnancy and small children: Long-term PM2.5 exposure is linked to lower birth weight and impaired lung development. Knowing whether your neighbourhood is consistently above WHO limits helps you weigh up a HEPA purifier in the nursery or a stroll in a cleaner park instead.
Choosing a stroller-walk park: Two parks 500 metres apart can have wildly different air, especially if one borders a busy road. Drop the pin on each, compare the NO2 numbers, pick the cleaner one.
Cycling to work: Morning rush-hour NO2 near big roads is typically 3 to 5x higher than residential streets 200 metres away. Plan a side-street route on bad-air days.
Air purifier and HVAC decisions: If your snapshot regularly shows PM2.5 above 25 ug/m3 outdoors, an indoor HEPA filter that brings indoor PM2.5 below 5 ug/m3 is one of the highest-return health purchases you can make.
Travel planning: Heading to Delhi, Beijing, Krakow in winter, or any wildfire-prone area? Check the snapshot for your hotel and the airport. Pack an FFP2 mask if numbers are above 100.
Sanity-checking news headlines: When the media says "air pollution alert", you can see for yourself whether the alert applies to your specific block or only the city-wide average.

Questions and answers

The AQI is a single 0 to 500+ score that translates several pollutant concentrations (PM2.5, PM10, NO2, O3, SO2, CO) into one health-risk number. Higher means worse. We surface two flavours: European AQI (used by the European Environment Agency, scale roughly 0 to 100+) and US AQI (used by the US EPA, scale 0 to 500). Both use the same colour bands: 0-50 Good, 51-100 Moderate, 101-150 Unhealthy for sensitive groups, 151-200 Unhealthy, 201-300 Very unhealthy, 300+ Hazardous. The number is computed from whichever pollutant is currently worst, so an AQI of 80 might mean "moderate PM2.5" or "moderate ozone" depending on the day.

What this shows you

How to use it

Pick a location: click anywhere on the map, drag the marker, type lat/lng into the inputs, or tap one of the sample cities (Warsaw, Berlin, Delhi, Los Angeles).

The big number at the top is the European AQI for your spot. The colored band tells you the health category: green (Good), yellow (Moderate), orange (Unhealthy for sensitive), red (Unhealthy), purple (Very unhealthy), maroon (Hazardous).

Read the advisory paragraph for plain-language guidance: should you go for that run, open the windows, wear a mask, or stay inside.

The six pollutant cards show current concentrations of PM2.5, PM10, NO2, O3, SO2 and CO. Each is compared to the WHO 2021 short-term guideline, so you instantly see "I am 3x over the limit".

The 24-hour forecast chart plots predicted AQI for the next 24 hours. Spot a dirty window? Plan around it.

All concentrations are in ug/m3 (sometimes written as ug/m3 or microgrammes per cubic metre).

Data is cached for 1 hour, so refreshing rapidly is harmless. The CAMS model itself updates every 3 hours.

When this is useful

Real situations where the snapshot changes a decision:

Before a run, bike ride, or outdoor workout: A 60-minute hard run at AQI 150 forces the same lung dose as living for a full day at AQI 30. Check the colour, then either go now, push the workout to a cleaner window, or move it indoors.
Opening windows for ventilation: In winter many homes have stale indoor air with a CO2 build-up, so the instinct is to throw the windows open. If outdoor PM2.5 is 80 ug/m3, you are pumping pollution into your bedroom. Better: run a HEPA purifier with windows closed, or wait for the cleaner overnight window.
Asthma and COPD management: People with asthma or chronic obstructive pulmonary disease feel the difference between AQI 40 and AQI 120. Check the snapshot, take a preventer puff if needed, and keep the rescue inhaler in your pocket on bad days.
Pregnancy and small children: Long-term PM2.5 exposure is linked to lower birth weight and impaired lung development. Knowing whether your neighbourhood is consistently above WHO limits helps you weigh up a HEPA purifier in the nursery or a stroll in a cleaner park instead.
Choosing a stroller-walk park: Two parks 500 metres apart can have wildly different air, especially if one borders a busy road. Drop the pin on each, compare the NO2 numbers, pick the cleaner one.
Cycling to work: Morning rush-hour NO2 near big roads is typically 3 to 5x higher than residential streets 200 metres away. Plan a side-street route on bad-air days.
Air purifier and HVAC decisions: If your snapshot regularly shows PM2.5 above 25 ug/m3 outdoors, an indoor HEPA filter that brings indoor PM2.5 below 5 ug/m3 is one of the highest-return health purchases you can make.
Travel planning: Heading to Delhi, Beijing, Krakow in winter, or any wildfire-prone area? Check the snapshot for your hotel and the airport. Pack an FFP2 mask if numbers are above 100.
Sanity-checking news headlines: When the media says "air pollution alert", you can see for yourself whether the alert applies to your specific block or only the city-wide average.

Questions and answers

Air Quality Snapshot

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