Sulphur dioxide

What sulphur dioxide is and its effects on health. Usual levels in New Zealand and standards and guidelines values to protect human health.

Chemical formula and description

The chemical formula for sulphur dioxide is SO2.

Sulphur dioxide is a colourless, soluble gas with a characteristic pungent smell which forms sulphuric acid when combined with water.


Sulphur dioxide is produced mainly from the combustion of fossil fuels that contain sulphur, such as coal and oil (eg, coal being burnt in a home fireplace for heating and diesel-powered vehicles). Sulphur dioxide is also produced from some industrial processes (eg fertiliser manufacturing, aluminium smelting and steel making).

Natural sources of sulphur dioxide include geothermal activity.

Effects on health

Sulphur dioxide can cause respiratory problems such as bronchitis, and can irritate your nose, throat and lungs. It may cause coughing, wheezing, phlegm and asthma attacks. The effects are worse when you are exercising.  Sulphur dioxide has been linked to cardiovascular disease.

Groups most sensitive to sulphur dioxide

  • children
  • adults with lung disease
  • asthmatics

Standards and guideline values to protect health

The national environmental standard for sulphur dioxide are 350 µg/m3 and 570 µg/m33 as a 1-hour average.  The average concentrations of sulphur dioxide should not exceed the 350 µg/m3 standard more than nine times a year and should not exceed the 570 µg/m3 standard at all.

The national ambient air quality guideline for sulphur dioxide is 120 µg/m3 as a 24-hour average.

New Zealand’s ambient standards and guidelines are generally consistent with World Health Organisation (WHO) recommendations.  In October 2006, the WHO released its first global air quality guidelines, which reduced the 24-hour average sulphur dioxide guideline from 120 µg/m3 to 20 µg/m3.  The Ministry for the Environment is currently investigating whether the ambient air quality guideline should be reviewed in light of this change.

Effects on ecosystems

Sulphur dioxide can cause acid rain which seriously affects ecosystems. Acid rain is a major problem in the northern hemisphere where trees and whole forests have been affected. Acid rain does not occur in New Zealand. However, sulphur dioxide deposition can affect vegetation around industrial discharges and in cities. Lichens are good bio-indicators of pollution as they do not like to grow where there is sulphur dioxide in the air.

Effects on visibility

Sulphur dioxide can form secondary particles (sulphates) that cause haze and reduce visibility.

Usual levels in New Zealand

Sulphur dioxide levels in urban areas have decreased significantly since the 1970s and are generally below the ambient standard and guideline levels. In 2013, the standard was breached at a busy industrial site in Christchurch and the WHO guideline value was exceeded at three industrial sites (two in Canterbury, one in Tauranga) and one port location (Auckland).  

Areas where sulphur dioxide may affect health and the environment

These are generally localised (eg, around industrial discharges).