UISH

This page collects the values against which measured and estimated concentrations are compared: the European Union limits, the decree transposing them in Italy, and the World Health Organization guidelines. They are not written into the page: they come from the database, and every value carries the act that establishes it.

A limit is not a number

This is the misunderstanding to clear away first. Saying that “the nitrogen dioxide limit is 40” is incomplete to the point of being unusable: 40 µg/m³ is the annual mean, while for a single hour the limit is 200 µg/m³, and it may be exceeded up to eighteen times a year without being in breach.

A reference value is therefore always a set of four things:

• the value, with its unit;
• the averaging period — ten minutes, one hour, eight hours, a day, the calendar year — because comparing an annual mean with an hourly measurement means nothing;
• the permitted exceedances per calendar year, which separate “must never be exceeded” from “may be exceeded eighteen times”;
• the legal nature: a limit value is binding, a target value is to be pursued where possible, an information threshold triggers a public warning, a critical level protects vegetation rather than people. Putting them on the same colour scale would be misleading.

The four references

Browse the values

The filters are the three that make comparable what otherwise is not — species, reference, averaging period — plus the nature of the value and what is being protected. The WHO interim targets are hidden by default: they are five rows per species and they bury the guideline level, which is the one that matters.

The colour scale that follows

UISH maps, isolines and coloured areas, use a fixed scale for each species: ten thresholds that divide concentrations into eleven classes, from grade 0 to grade 10. The thresholds are in the database table air_quality.tbl_limits, the same one used by the programs that draw the isolines, and the table below reads them from there; the references beside them come from the regulatory values of the table further up. The spacing is roughly logarithmic: dense at the bottom, where most values fall, wide at the top, where episodes are rare.

Only the species the model produces are listed: benzene, carbon monoxide and nitrogen oxides are measured at the stations but have no maps, and therefore no scale.

For comparison, here is the scale you would get using as thresholds only the values of the species and reference chosen in the filters. Change the reference and it rebuilds itself, which is the reason it is worth keeping the thresholds in a table rather than in the code. Regulatory values, however, are few and far apart, and on a map they would use only a few classes: this is why the map scale has eleven.

One choice remains that the data cannot make for us: for some species the reference defines no annual value. Ozone, for instance, is assessed on the maximum daily 8-hour mean and on the peak season, not on the year. When a single characteristic concentration per species is needed — as in the validation page, where it serves as the yardstick for errors expressed in µg/m³ — for ozone we adopt the 60 µg/m³ of the WHO peak season: it is the value that best describes the level around which ozone moves in the season that matters.

The official texts

Every value on this page was read from the text of the act, not from a third-party summary table. The documents are here:

• Directive (EU) 2024/2881 — English text from the Official Journal of the European Union of 20 November 2024 (Italian version). The values are in Annex I, sections 1 to 4.
• Italian Legislative Decree 155 of 13 August 2010 — Italian only. Annexes XI (limit values and critical levels), XII (information and alert thresholds) and XIII (target values for metals).
• WHO global air quality guidelines 2021. The guideline levels and interim targets are in tables 0.1 and 0.2 of the executive summary.

The text of Directive 2008/50/EC is not published here: its values coincide with table 2 of Annex I of the new directive, which restates them as the target for 11 December 2026, and that coincidence was used as a cross-check. The full text is on EUR-Lex.