About the air quality index

How station measurements become five clear levels, which thresholds apply, and how the national index differs from the WHO scale.

Good
Satisfactory
Fair
Poor
Very poor

Operational five-level index

Step 1

Measurement

For each pollutant the latest hourly mean is taken (8-hour mean for CO). Only measurements that passed quality control are used.

Step 2

Pollutant sub-index

The value is compared with the threshold table and gets a level from 1 (good) to 5 (very poor). The position within the band is kept, so the index can also be shown as a number.

Step 3

Station index

The station index is the worst sub-index. The pollutant with the highest sub-index is called dominant and is shown next to the level.

This approach is used by operational indices in Europe (for example FMI/HSY): it shows the current situation without smoothing peaks. The thresholds below are edited by the Agency without a software release.

Level thresholds · NATIONAL_OPERATIONAL

Concentration thresholds by pollutant and level
Pollutant	Averaging	Good	Satisfactory	Fair	Poor	Very poor
PM2.5 particlesPM2.5, µg/m³	1 hour	< 10	10 – 25	25 – 50	50 – 75	≥ 75
PM10 particlesPM10, µg/m³	1 hour	< 20	20 – 50	50 – 100	100 – 200	≥ 200
Nitrogen dioxideNO₂, µg/m³	1 hour	< 40	40 – 70	70 – 150	150 – 200	≥ 200
Sulphur dioxideSO₂, µg/m³	1 hour	< 20	20 – 80	80 – 250	250 – 350	≥ 350
Carbon monoxideCO, mg/m³	8 hours	< 4	4 – 8	8 – 20	20 – 30	≥ 30
OzoneO₃, µg/m³	1 hour	< 60	60 – 100	100 – 140	140 – 180	≥ 180

Intervals are half-open: the lower bound is included, the upper is not. Units are µg/m³; mg/m³ for CO.

Agency for Hydrometeorology method

The Agency's daily air pollution bulletins assess air quality by the ratio of concentrations to maximum permissible concentrations (MPC). The portal calculates the same indicators from the valid measurements of each station (the “Ratio to limit values” scale, scale=RATIO_TO_LIMIT in the API).

Ratio to MPC

The daily mean concentration is divided by the daily MPC and the maximum single value of the day by the single MPC. A ratio up to 1 means the standard is met; the scale levels correspond to ratios up to 1, 1–2, 2–3, 3–5 and above 5.

NO₂ + SO₂ summation

Nitrogen dioxide and sulphur dioxide have a summation effect: their daily-MPC ratios are added and the sum should not exceed 1. The indicator is calculated when the station measures both pollutants.

High and extremely high pollution

High pollution (HP) means the maximum single concentration reached 10 single MPCs, extremely high pollution (EHP) — 50 single MPCs. The flags are shown on the station page and returned by the API.

Bulletins assess the change against the previous day; on the portal the measurement chart shows the dynamics, and the ratios are recalculated every 10 minutes over a sliding 24-hour window.

Bulletins publish concentrations in mg/m³; the display unit of the portal (µg/m³ or mg/m³) is chosen by the administrator, and the API and CSV state the unit in the unit field.

How to read the colours

Good
Clean air: no restrictions.
Satisfactory
Acceptable quality; very sensitive people may notice slight discomfort.
Fair
Sensitive groups should reduce prolonged outdoor exertion.
Poor
Pollution is noticeable for everyone: limit time outdoors, especially for children and the elderly.
Very poor
Very high pollution: stay indoors if possible and keep windows closed.
No data
No data, or the data is stale (measurement older than 6 hours).

Colour is never the only carrier of meaning: a level label is always shown next to it and a number on the map. Colours are chosen for a text contrast of at least 4.5:1 in light and dark themes.

National index: ratio to limit values and the IZA

The ratio to limit value compares a concentration with the maximum permissible concentration (MPC): the daily mean with the daily MPC, the maximum single value with the single MPC. Up to 1 means the standard is met; levels 2–5 correspond to ratios of 1–2, 2–3, 3–5 and above 5.

The composite air pollution index (IZA, RD 52.04.186-89) sums normalised monthly mean concentrations of the five pollutants with the largest contribution, weighted by hazard class: IZA = Σ (q̄ᵢ / MPCᵢ)^Cᵢ. Grades: low < 5, elevated 5–6, high 7–13, very high ≥ 14.

Low 0 – 5
Elevated 5 – 7
High 7 – 14
Very high ≥ 14

Maximum permissible concentrations by pollutant
Pollutant	Daily MPC (daily mean)	Single MPC (maximum single)	Hazard class
PM2.5 particlesPM2.5, µg/m³	35	160	3
PM10 particlesPM10, µg/m³	60	300	3
Nitrogen dioxideNO₂, µg/m³	40	85	3
Sulphur dioxideSO₂, µg/m³	50	500	3
Carbon monoxideCO, mg/m³	3	5	4
OzoneO₃, µg/m³	30	160	1
Nitric oxideNO, µg/m³	60	400	3
FormaldehydeCH₂O, µg/m³	3	35	2
PhenolC₆H₅OH, µg/m³	3	10	2
Hydrogen fluorideHF, µg/m³	5	20	2
Suspended particulates (dust)TSP, µg/m³	150	500	3
TVOCTVOC, ppb	300	900	—

Limit values and hazard classes are tabular data of the Agency for Hydrometeorology (daily air pollution bulletins, limits in mg/m³) and are edited in the admin panel.

WHO guideline scale

An optional scale based on the WHO Air Quality Guidelines 2021 and interim targets (24-hour means). Available in the API with scale=WHO_24H.

Concentration thresholds by pollutant and level
Pollutant	Averaging	Good	Satisfactory	Fair	Poor	Very poor
PM2.5 particlesPM2.5, µg/m³	24 hours	< 15	15 – 25	25 – 37.5	37.5 – 75	≥ 75
PM10 particlesPM10, µg/m³	24 hours	< 45	45 – 75	75 – 100	100 – 150	≥ 150
Nitrogen dioxideNO₂, µg/m³	24 hours	< 25	25 – 50	50 – 120	120 – 200	≥ 200
Sulphur dioxideSO₂, µg/m³	24 hours	< 40	40 – 125	125 – 250	250 – 350	≥ 350
Carbon monoxideCO, mg/m³	24 hours	< 4	4 – 7	7 – 10	10 – 20	≥ 20
OzoneO₃, µg/m³	8 hours	< 100	100 – 120	120 – 160	160 – 240	≥ 240

Data quality

Every measurement is checked: physical range, spikes, stuck sensors and outliers relative to the station's history. Suspect values are reviewed by an operator; only valid and corrected values are published.

If the measurement is older than 6 hours the index is considered stale and shown as grey “no data” — an honest gap is better than an outdated number.