Sulphur dioxide is one of the most common gases released in volcanic eruptions (following water and carbon dioxide) and is of concern on the global scale due to its potential to influence climate. On the local scale SO2 is a hazard to humans in its gaseous form and also because it oxidises to form sulphate aerosol.
Sulphur dioxide (SO2) is a colourless gas with a characteristic and irritating smell. This odour is perceptible at different levels depending on the individual's sensitivity, but is generally perceived between 0.3-1.4 ppm and is easily noticeable at 3 ppm (Baxter, 2000; Wellburn, 1994). SO2 is non-flammable, not explosive and relatively stable. It is more than twice as dense as ambient air (2.62 g L-1 at 25°C and 1 atm (Lide, 2003)) and is highly soluble in water (85 g L-1 at 25°C (Gangolli, 1999)). On contact with moist membranes, SO2 forms sulphuric acid (H2SO4), which is responsible for its severe irritant effects on the eyes, mucous membranes and skin (Komarnisky et al., 2003).
Typically, the concentration of SO2 in dilute volcanic plumes is <10 ppm, as little as 10 km downwind of the source, compared to the tropospheric background of 0.00001-0.07 ppm (Brimblecombe, 1996; Oppenheimer et al., 1998). Assuming that the gas has a half-life of 6-24 hours, then only about 5% of the emitted gas is present in the lower atmosphere after 1-4 days (Brimblecombe, 1996; Finlayson-Pitts and Pitts, 1986; Porter et al., 2002).
Sulphur dioxide is irritating to the eyes, throat and respiratory tract. Short-term overexposure causes inflammation and irritation, resulting in burning of the eyes, coughing, difficulty in breathing and a feeling of chest tightness. Asthmatic individuals are especially sensitive to SO2 (Baxter, 2000) and may respond to concentrations as low as 0.2-0.5 ppm. Volcanologists suffering from asthma may notice adverse effects at concentrations substantially below those that affect their colleagues. Prolonged or repeated exposure to low concentrations (1-5 ppm) may be dangerous for persons with pre-existing heart and lung diseases. While health effects are documented at various concentrations by different researchers and organizations, a sampling of thresholds for health effects are outlined in the table.
Health effects of respiratory exposure to sulphur dioxide
High levels of ambient SO2 have been shown to cause various health problems in children (Ware et al., 1986). However, studies at Mt Sakurajima did not show a correlation between prevalence of asthma in children and prolonged exposure to volcanic gases (Uda et al., 1999).
In 1971, the USA EPA set the level of SO2 that could cause significant harm to the health of persons at 2620 µg m-3 (1 ppm) (24-hour average). When particulate matter or other trace components are also present, this level is reduced. International ambient and occupational guidelines for SO2, which vary significantly for different countries, are provided in the tables below.
Ambient air quality guidelines for SO2.
The ambient SO2 guidelines table above demonstrates the tremendous range of international guidelines that exist. Difference between country's guidelines may be explained by the age of the guideline, the practical achievement of a standard based on current and predicted pollution levels or the data from which the standard was set (e.g. epidemiological study versus actual pollution levels). Averaging times for guidelines range from 10 minutes (WHO) to annual. The table below summarises the range of guideline values for each averaging period.
Summary of the ranges of ambient SO2 guideline levels
Occupational guidelines for SO2
A number of volcano observatories have implemented their own SO2 guidelines. At Mt. Aso crater, Japan, for example, visitors are evacuated when SO2 levels exceed 0.2 ppm continuously for 1 minute or instantaneous levels exceed 5.0 ppm. These levels were reduced from >5 ppm for 5 minutes following gas related fatalities in the 1990's (Ng'Walali et al., 1999). In 2000, Hawaii Volcanoes National Park in collaboration with the USGS Hawaiian Volcano Observatory introduced a set of SO2 advisories to protect staff and visitors to the park (below).
Concentrations of sulphur dioxide (SO2) hazardous to human health have been recorded downwind of many volcanoes. The highest concentrations are often seen close to persistently degassing volcanoes:
Populations and cities can be seriously affected by SO2 emissions during more explosive volcanic activity:
Other examples of SO2 concentrations and effects at varying distances:
In other regions, people living and working close to volcanoes emitting SO2 may be unwittingly at risk from the gas. For example, mean SO2 levels by Lake Furnas in the caldera of the active Furnas volcano, Azores, have been measured at 0.115 ppm. This was recorded in an area where tourists and locals use the fumaroles for cooking and is several times higher than any listed annual guideline and higher than most 1- and 24-hour guideline levels. Levels in Furnas village centre (also in the caldera) had a range of 0.070-0.085 ppm (Baxter et al., 1999), also higher than any annual guideline levels.
Most known incidents related to SO2 poisoning have occurred at Aso volcano in Japan (see table). Here, 7 people have died from SO2 in the past 15 years and 59 people were hospitalised from inhalation of volcanic gas from January 1980 to October 1995. Over half of the fatalities had a history of asthma. Following autopsies of the dead, the SO2 evacuation criteria levels were reduced and strict warnings about the risks of exposure are given to visitors to protect those with asthma and respiratory diseases (Ng'Walali et al., 1999).
Mortality and morbidity incidents associated with volcanic SO2 emissions in the Twentieth Century
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Download our pamphlets on preparing for ashfall and on the health hazards of ash. They are designed for mass distribution at the onset of new eruptions. They are now avaiable in English, Japanese, French Spanish, Portuguese, Swahili, Indonesian and Icelandic with Italian versions being available shortly. Please see our Pamphlets page for further infomation.
FACE MASK USE
IVHHN has an article under the Guidelines tab which used to be called 'Recommended Face Masks'. This has now been updated to 'Information on face masks' and is an interim page whilst the Health Interventions in Volcanic Eruptions project investigates which types of respiratory protection are effective in protecting the general population from volcanic ash inhalation. Please note that the translations in Spanish, Japanese and Portuguese have not yet been updated.