SO₂ — Sulphur dioxide

Sulphur dioxide is normally the quietest of the five in Malta — low for long stretches, then briefly not. It is the pollutant where an episode means more than an average.

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

What it is

A sharp-smelling gas produced when fuels containing sulphur are burnt. In Malta it is normally low and appears in short episodes.

Sulphur dioxide is a sharp-smelling gas produced when fuel containing sulphur is burnt. Unlike nitrogen dioxide, it is a property of the fuel rather than of the flame temperature, so it responds directly to what is being burnt — which is why fuel sulphur regulation has driven it down across Europe over decades.

It dissolves readily in water, which is why it is absorbed in the upper airways rather than reaching deep into the lung — the opposite of fine particles, and the reason its effects appear as airway tightening rather than as long-term cardiovascular risk.

Its behaviour is episodic rather than cyclical. Where nitrogen dioxide follows traffic and ozone follows the sun, sulphur dioxide tends to sit near the bottom of its scale and then spike when a specific source is upwind.

Where it comes from

Shipping and port activity, industrial combustion, and occasionally volcanic emissions carried across from Sicily.

Shipping and port activity, industrial combustion, and occasionally volcanic emissions carried across from Sicily. The volcanic contribution is a real and locally specific source: Malta is close enough to Etna that its plume can reach the islands when the wind is in the right quarter.

The episodic character follows from that list. Each of these is either intermittent or dependent on wind direction, so the figure is governed by whether a source happens to be upwind rather than by a daily rhythm.

SO₂ in Malta and Gozo

Levels recorded in Malta are usually far below the range where health effects are expected, and the honest summary of the standing situation is that sulphur dioxide is not currently a routine concern on the islands.

What makes it worth a page anyway is that the average is the wrong statistic for it. A pollutant that is near zero for weeks and then briefly elevated has an unremarkable mean and a meaningful maximum, so the figure to look at is the peak during an episode.

Coverage is also incomplete, and visibly so. Attard has been observed to report no sulphur dioxide at all across a sustained sampling window, so four of the five stations carry this pollutant. An absent value there is the instrument not reporting, not clean air — and this site renders it as unavailable rather than as zero, because zero is a measurement claim and a missing reading is the absence of one.

Health effects

Brief exposure to higher levels can tighten the airways, most noticeably in people with asthma. Levels recorded in Malta are usually far below that range.

Brief exposure to higher levels can tighten the airways, most noticeably in people with asthma, for whom the response can be rapid. Levels recorded in Malta are usually far below that range.

Because the effect is fast and concentrated in one group, the relevant guidance is narrower than for the other pollutants: during an episode, people with asthma should have reliever medication to hand and avoid strenuous outdoor exertion. For everyone else, a Maltese sulphur dioxide reading is rarely something to act on.

Who is most affected

These are the groups for whom SO₂ 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

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

How a SO₂ reading becomes a band

The sulphur dioxide scale is comparatively wide at the bottom — Good to 20 µg/m³, Fair to 40 — and the Moderate band jumps to 125, which is a large step. A reading moving from Fair to Moderate therefore represents a bigger absolute change than the same band move would for nitrogen dioxide.

The EU limits are stricter in structure than the index suggests, and both are set out in full further down this page: an hourly limit that permits a substantial number of exceedances a year, and a daily limit that permits very few. Neither can be settled by one hour, and the daily one cannot be assessed from an hourly series at all.

These are the European Air Quality Index bands for SO₂ 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 SO₂, in µg/m³
Band	SO₂ (µg/m³)
Good	1–20
Fair	21–40
Moderate	41–125
Poor	126–190
Very poor	191–275
Extremely poor	276–1000

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 SO₂ and none of them is a translation of the others.

EU limit values — legally binding

350 µg/m³ — 1 hour: Up to 24 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.
125 µg/m³ — 24 hours: Up to 3 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.

WHO guidelines — health guidance, not law

40 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.

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 SO₂ 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.

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 SO₂ 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 SO₂ 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 SO₂ 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 SO₂ reading.

Attard does not report SO₂. 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.