NO₂ — Nitrogen dioxide

Nitrogen dioxide is the most local of the five: it is made by engines, it falls away within tens of metres of a road, and which station you look at matters more than for anything else.

Measured at 5 of the 5 monitoring stations in Malta and Gozo · averaged over hourly · µg/m³

What it is

A reddish-brown gas formed when fuel burns at high temperature. Concentrations are highest beside busy roads and drop away sharply within tens of metres.

Nitrogen dioxide is a reddish-brown gas formed whenever fuel burns hot enough to make nitrogen and oxygen in ordinary air react with each other. It is therefore not a property of the fuel so much as of the temperature of the flame, which is why it comes out of well-maintained modern engines as well as badly maintained old ones.

Its defining characteristic for anyone reading a monitor is how fast it disperses. Concentrations are highest within a few metres of the exhaust and drop sharply over tens of metres. A roadside figure and a figure from a park two streets away are both true and can differ by a factor of several.

It is also chemically short-lived in sunlight, where it takes part in the reactions that make ozone. That relationship is the reason ozone and nitrogen dioxide often move in opposite directions across a day.

Where it comes from

Road traffic, particularly diesel engines; shipping in harbour; power generation; and some industrial processes.

Road traffic is the dominant source, and diesel engines disproportionately so. Shipping in harbour, power generation and some industrial combustion contribute as well.

Because it is a combustion product that disperses quickly, the daily shape of a nitrogen dioxide series usually tracks traffic rather than weather — which makes it the pollutant where an hourly reading is most informative about what is happening right now, nearby.

NO₂ in Malta and Gozo

Malta’s monitoring network makes the siting question explicit. Two of the five stations — Msida and St Paul’s Bay — are classified as Traffic sites, and the other three as Background. For nitrogen dioxide that distinction is the single most important thing on the page: a Traffic station is deliberately placed where exposure is highest, and a Background station deliberately is not.

So the two are not rival estimates of one number. A Traffic reading answers "what is it like beside a busy road", and a Background reading answers "what is the general level away from one". Reading across them as though they disagreed would be a mistake; reading the gap between them tells you how much of the local figure is traffic.

Għarb, on Gozo, is the network’s only Rural-Regional site, and it is the closest thing available to a baseline for the islands. A nitrogen dioxide figure there that is as high as an urban one would be genuinely unusual, and would point at a regional air mass rather than local traffic.

Health effects

Raised levels can inflame the airways and make asthma symptoms more likely. People who live, work or travel beside heavy traffic have the greatest exposure.

Raised nitrogen dioxide can inflame the airways and make asthma symptoms more likely. People who live, work, cycle or walk beside heavy traffic have the greatest exposure, and the exposure is strongly determined by proximity rather than by the city-wide average.

The practical implication is unusually actionable for an air pollutant: moving a route one street back from a main road meaningfully reduces exposure, in a way that is not true for ozone or for dust.

Who is most affected

These are the groups for whom NO₂ specifically is most relevant, in rough order. The ordering is a matter of which mechanism this pollutant acts through, not a ranking of how much anyone matters.

People with asthma
People with lung conditions
Children
People who work outdoors
People exercising hard outdoors

maqua.app provides general environmental information and does not replace medical advice or official emergency guidance.

How a NO₂ reading becomes a band

The nitrogen dioxide band scale is the narrowest after fine particles — Good ends at 10 µg/m³, Fair at 25 — reflecting that it is a directly toxic gas rather than a mass of inert material.

Note the gap between the index and the law. The hourly EU limit value — set out in full further down this page, read from the same table the application computes against — sits above the top of the Very poor band, and it permits a number of exceedances each year before the limit is breached at all. An hour near it is a serious index reading and still establishes nothing legally, and this site will not describe it as a breach.

These are the European Air Quality Index bands for NO₂ alone. Each band is an inclusive range of whole µg/m³: a concentration is rounded to the nearest whole number first, then matched. A station takes the band of its worst pollutant, so a single figure here does not by itself decide what the station reports.

Index bands for NO₂, in µg/m³
Band	NO₂ (µg/m³)
Good	1–10
Fair	11–25
Moderate	26–60
Poor	61–100
Very poor	101–150
Extremely poor	151–1000

Source: European Air Quality Index (EEA), verified 2026-07-26.

Legal limits and health guidelines

The index is a communication scale. It is not the law, and it is not the World Health Organization's advice — all three describe NO₂ and none of them is a translation of the others.

EU limit values — legally binding

200 µg/m³ — 1 hour: Up to 18 exceedances are permitted per calendar year before the limit is breached. This limit is an average over a longer period, so no single hourly reading can establish anything about it. Directive 2008/50/EC Annex XI.
40 µg/m³ — Calendar year: This limit is an average over a longer period, so no single hourly reading can establish anything about it. Directive 2008/50/EC Annex XI.

WHO guidelines — health guidance, not law

25 µg/m³ — 24 hours: A health-based guideline rather than a legal threshold. Nobody is in breach of a WHO guideline; it describes where the evidence suggests harm begins. WHO 2021.
10 µg/m³ — Annual: A health-based guideline rather than a legal threshold. Nobody is in breach of a WHO guideline; it describes where the evidence suggests harm begins. WHO 2021.

A single hourly reading above any of these figures is not a breach of a limit, and this site will never describe one as such. Most are averages over 24 hours or a calendar year, and several permit a fixed number of exceedances before the limit is breached at all. Directive (EU) 2024/2881 tightens several of them from 1 January 2030; the figures above are the ones in force today.

Where NO₂ is measured

Not every station measures every pollutant, and the ones that do are not interchangeable — a Traffic site is deliberately placed where exposure is highest, a Background site deliberately is not, so they answer different questions rather than competing to answer the same one.

Attard, Malta — Background, Urban: Sited away from single sources to represent general exposure in the area. 86 m above sea level. See the current NO₂ reading.
Għarb, Gozo — Background, Rural-Regional: Sited away from single sources to represent general exposure in the area. 114 m above sea level. See the current NO₂ reading.
Msida, Malta — Traffic, Urban: Sited beside a busy road, so it records the higher end of local exposure. 2 m above sea level. See the current NO₂ reading.
St Paul's Bay, Malta — Traffic, Urban: Sited beside a busy road, so it records the higher end of local exposure. 7 m above sea level. See the current NO₂ reading.
Żejtun, Malta — Background, Urban: Sited away from single sources to represent general exposure in the area. 56 m above sea level. See the current NO₂ reading.