4 Sulphur dioxide
Sulphur dioxide (SO2) is a colourless, water-soluble gas that is reactive and has a pungent odour. Sulphur dioxide is detectable to the human nose at concentrations of around 0.5–0.8 parts per million (1400–2240 µgm-3). Concentrations of SO2 in ambient air typically occur as a result of combustion processes, in particular the burning of high sulphur fuels, although specific industries such as manufacturing fertiliser also discharge SO2. Sulphur dioxide is subject to a series of transformation processes in the atmosphere, which can result in, sulphurous and sulphuric acids, sulphites and sulphates being formed.
4.1 Guideline values
The ambient air quality guideline values (Ministry for the Environment, 2002) for SO2 are:
- 350 µgm-3 (one-hour average)
- 120 µgm-3 (24-hour average).
Previously, the 1994 ambient air quality guideline values for New Zealand (Ministry for the Environment, 1994) included a 10-minute average SO2 guideline value of 500 µgm-3.
4.2 Concentrations
Concentrations of SO2 measured at ambient air quality monitoring sites in New Zealand indicate compliance with both 24-hour and one-hour average guideline values. It is possible that some locations within New Zealand have breached the short-term (10-minute) 1994 guideline value for SO2. For example, this value has been exceeded at an ambient air quality monitoring site in Hornby, Christchurch on a few occasions.
4.3 Health effects of sulphur dioxide
Sulphur dioxide causes its irritant effects by stimulating nerves in the lining of the nose and throat and the lung’s airways. This causes a reflex cough, irritation, and a feeling of chest tightness, which may lead to narrowing of the airways. This latter effect is particularly likely to occur in people suffering from asthma and chronic lung disease, whose airways are often inflamed and easily irritated (Department of Environment, 1995).
Asthmatics are generally considered the most sensitive group in the community to concentrations of SO2. Other sensitive groups include those exercising. This is because SO2 is very reactive and consequently the distribution of SO2 along the conductive airways of the respiratory tract is non-uniform, depending on breathing volumes and types. For nasal breathing with low to moderate volumes the penetration into the lungs is negligible. For oral inhalation and larger volumes, doses may reach the segmental bronchi (World Health Organisation, 2000).
The health effects of concentrations of SO2 have been studied in a number of ways including exposure of volunteers to sulphur dioxide in the air they are breathing in a laboratory situation and by examination of the effects on members of the population who have been exposed to episodes of atmospheric pollution. In the controlled laboratory situation, acute responses occur within the first few minutes of exposure and further inhalation does not increase effects.
Short-term (less than 24-hour exposure) guideline values for SO2 have been developed based on the minimum concentrations associated with adverse effects in asthmatic patients exercising in a laboratory situation (World Health Organisation, 2000). Thus the guideline values represent a protective level for vulnerable groups within the community.
Information on the effects of exposure for longer periods (e.g. 24-hour) is obtained from epidemiological studies, which show associations between contaminants such as SO2 and health impacts in communities and selected panels. In evaluating the health evidence relating to SO2 exposure for the New Zealand ambient air quality guideline values, Dennison et al (2002) concludes that because of the correlations between SO2 and other contaminants in the air it is difficult to confidently attribute the observed effects in the epidemiological studies to SO2 alone. Experimental studies were therefore used to derive the dose-response relationships underpinning the ambient air quality guideline values for SO2 for New Zealand.
4.3.1 Implications for New Zealand
Results of air quality monitoring would suggest that there are unlikely to be major health impacts associated with SO2 exposure in New Zealand as concentrations in most areas are well within the ambient air quality guideline values. Some exceptions may occur on occasion in localised areas if significant industrial SO2 concentrations exceed around 500 µgm-3 (10-minute average). These effects should be considered and addressed as a part of the resource consent process for industrial discharges.
