O₃ — Ozone

Ozone is the one pollutant nobody emits. It is built in the air by sunlight, which is why it peaks on hot afternoons, is often worse in the countryside than in town, and is the pollutant Malta’s climate most reliably produces.

Measured at 4 of the 5 monitoring stations in Malta and Gozo · averaged over hourly · µg/m³

What it is

Ozone is not emitted directly. It forms in the air when sunlight reacts with other pollutants, so it peaks on hot sunny afternoons and is often higher in rural areas than in town.

Ground-level ozone is a secondary pollutant: there is no ozone source to regulate, because nothing emits it. It forms when sunlight drives reactions between nitrogen oxides and volatile organic compounds already in the air. Sunlight is the ingredient in shortest supply elsewhere in Europe and in abundant supply here.

That photochemical origin gives it a daily shape unlike any of the others. It builds through the morning, peaks in the afternoon when sunlight has been working longest, and falls away in the evening. A morning reading tells you comparatively little about what the afternoon will bring.

It is also destroyed by fresh nitric oxide from vehicle exhaust, which produces a result that surprises people: ozone is frequently lower beside a busy road than in a quiet rural area downwind of one. Traffic suppresses ozone locally while supplying the ingredients that make it elsewhere.

Where it comes from

Formed from nitrogen oxides and volatile organic compounds under strong sunlight. Ozone also drifts to Malta from mainland Europe.

Formed from nitrogen oxides and volatile organic compounds under strong sunlight, from a mixture of local and continental precursors. Ozone and its ingredients also drift to Malta from mainland Europe, so an episode here can be the product of emissions hundreds of kilometres away.

There is nothing to switch off locally that would reliably lower an ozone peak within a day, which is what makes it a harder pollutant to manage than nitrogen dioxide.

O₃ in Malta and Gozo

Malta has the sunlight and the summer temperatures that ozone formation needs, so the islands see their highest ozone on hot, still, bright afternoons — typically late spring through summer. This is the pollutant where the season matters most.

The network reflects the chemistry in a way worth noticing: Msida, one of the two Traffic sites, is the only station that does not report ozone at all. The suppression effect above would explain that — but it is one case and not a rule, because St Paul’s Bay is the other Traffic site and does measure ozone. The registry records only that no ozone has been observed from Msida, not why.

Għarb matters especially here. A rural site on Gozo, away from the traffic that would destroy ozone locally, is where the regional photochemical signal should show up most clearly — that is what the chemistry predicts. Whether it carries the highest figure on the islands on any given day is a question for the readings rather than for this page, and each station page now shows its own measured daily ozone pattern, so the prediction can be checked rather than taken on trust.

Health effects

Higher ozone can cause throat irritation, coughing and reduced lung function during exercise. Symptoms typically ease as levels fall in the evening.

Higher ozone can cause throat irritation, coughing and measurably reduced lung function during exercise. Symptoms typically ease as levels fall in the evening.

Ozone is unusual in that exertion is the main risk multiplier: the dose depends on how hard you are breathing. The useful response is to move strenuous outdoor exercise to the morning or evening on a hot bright day, rather than to avoid being outdoors at all.

Who is most affected

These are the groups for whom O₃ 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 exercising hard outdoors
People with asthma
People with lung conditions
Children
People who work outdoors

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

How a O₃ reading becomes a band

The ozone scale is shaped quite differently from the others. Good extends all the way to 60 µg/m³ — far higher than for any other pollutant — because low background ozone is normal everywhere, while the upper bands are compressed: Moderate ends at 120 and Very poor at 180. Most of the scale’s resolution sits in a narrow range, so a modest-looking rise can move the band.

The EU figures for ozone are also structured unlike the rest: a target value on the maximum daily eight-hour mean rather than an hourly limit, plus information and alert thresholds that do apply to a single hour. Only the latter can be judged from one reading.

These are the European Air Quality Index bands for O₃ 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 O₃, in µg/m³
Band	O₃ (µg/m³)
Good	1–60
Fair	61–100
Moderate	101–120
Poor	121–160
Very poor	161–180
Extremely poor	181–600

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 O₃ and none of them is a translation of the others.

EU limit values — legally binding

120 µg/m³ — Maximum daily 8-hour mean: Up to 25 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 VII (target value).
180 µg/m³ — 1 hour: This limit is defined over a single hour, so one reading can be compared against it — though a comparison is not a finding of breach while exceedances remain permitted. Directive 2008/50/EC Annex XII (information threshold).
240 µg/m³ — 1 hour: This limit is defined over a single hour, so one reading can be compared against it — though a comparison is not a finding of breach while exceedances remain permitted. Directive 2008/50/EC Annex XII (alert threshold).

WHO guidelines — health guidance, not law

100 µg/m³ — Peak season 8-hour: 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 O₃ 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 O₃ 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 O₃ 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 O₃ 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 O₃ reading.

Msida does not report O₃. An absent value there is the instrument not reporting — not a reading of zero, and not evidence of clean air. The station page says so plainly rather than leaving a blank.