Sulphur Dioxide (SO₂)

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.

Properties

Exposure Effects

Existing Guidelines

Volcanic Examples and Incidents

References

Volcanic Gases and Aerosols Index

Properties

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).

Exposure Effects

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
(Baxter, 2000; Nemery, 2001; NIOSH 1981; Wellburn, 1994)

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).

Existing Guidelines

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.
Values in brackets are approximate conversions of published guidelines.

1. The normal condition corresponds to the pressure of an atmosphere (1 atm.) and a temperature of 25ºC.
2. (i) Sensitive areas of special protection; (ii) typical urban and rural areas and (iii) special industrial areas.
3. Must be standardised to 293 K and 101.3 kPa
4. Measured at 0ºC and 1 atm pressure. This does not apply to sulphur acid mist
5. Based on evidence from epidemiological studies

1. http://www.cepis.ops-oms.org/bvsci/e/fulltext/normas/normas.html
2. http://www.conama.cl/portal/1255/propertyvalue-10316.html
3. European Commission Guidelines Website
4. http://www.env.go.jp/en/lar/regulation/aq.html
5. http://www.mfe.govt.nz/publications/air/ambient-air-quality-may02/index.html
6. http://www.defra.gov.uk/environment/airquality/airqual/index.htm
7. http://www.epa.gov/air/criteria.html
8. WHO, 2000. Guidelines for Air Quality, World Health Organisation, Geneva.

Ambient Guidelines Summary

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
Values in brackets are approximate conversions of published guidelines

1. ppm by volume at 25ºC and 760 torr.
2. http://www.cdc.gov/niosh/nmam/

1. HSE, 2002. Occupational Exposure Limits 2002. HSE Books, Sudbury.
2. OSHA Guidelines Website
3. NIOSH Manual of Analytical Methods (NMAM®), 1994, Cassinelli, M.E. and O'Connor, P.F. (Eds.). DHHS (NIOSH) Publication 94-113, 4th ed. and/or http://www.osha.gov/dts/chemicalsampling/data/CH_268500.html
4. AIHA Emergency Response Planning Guidelines Committee, 2002. Emergency Response Planning Guidelines 2002 Complete Set, American Industrial Hygiene Association, Fairfax.

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).

The Hawaii Volcanoes National Park and
Hawaiian Volcano Observatory's SO₂ advisory table.

Volcanic Examples and Incidents

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:

• Kilauea, Hawaii: Ambient concentrations of SO₂ in a tourist car park during an episodic increase in activity in 1996 rose to 4.0 ppm (BGVN 21:01), nearly ten times higher than the USA 3-hour concentration guideline. From 1987-2001, the ambient SO₂ concentration exceeded the US 24-hour primary health standard on more than 85 occasions at Hawaii Volcanoes National Park Headquarters (Elias, 2002). Such measurements at this popular tourist destination have prompted the introduction of the SO₂ guidelines for the park.
• Masaya, Nicaragua: Currently actively degassing and in the periods March-April 1998 and February-March 1999 mean concentrations of SO₂ measured at downwind sites up to 44 km away had a range of <0.002 - 0.23 ppm (~5-600 µg m^-3) (Delmelle et al., 2002). About 30 % of these measurements were above the World Health Organisation (WHO) 24-hour ambient guideline level. Maximum concentrations measured on the Llano Pacaya ridge 14 km away were 0.6 ppm (Horrocks, 2001). In May 2001, the maximum SO₂ abundance recorded in the Masaya plume on the edge of the Santiago crater was 3.1 ppm (7950 µg m^-3) (Allen et al., 2002). These concentrations indicate a potential risk to the health of the local population and complaints about eye sensitivity and inflammation, bronchitis, sore throats and headaches have been received from local people. It is estimated that ~ 50,000 people are at risk from SO₂ and plume induced water pollution in the Masaya region.
• Poas, Costa Rica: Residents and scientists in the vicinity of the volcano have complained of eye and throat irritation over time. Long-term measurements of SO₂ in populated downwind areas showed mean concentrations up to ~0.28 ppm (730 µg m^-3), with short-term measures up to 0.3-0.5 ppm (Nicholson et al., 1996). These levels, observed in 1991 and 1992, exceed the WHO 24-hour ambient guideline values and in some locations exceed the 15-minute level. The highest SO₂ levels measured at Poas crater rim were ~ 35 ppm, substantially above all guideline levels.
• Villarrica, Chile: SO₂ concentrations measured at the crater rim showed that a concentration of 13 ppm (equivalent to the NIOSH 15 min occupational limit for SO₂) was often exceeded (Witter and Delmelle, 2004). At the height of the summer tourist season, about 100 tourists climb to the summit of Villarrica volcano per day. A large number of these people are exposed to the noxious gases.
• White Island, New Zealand: A pilot health study reported time-averaged measurements of personal exposure to SO₂ for a 20 minute period spent downwind of fumaroles of ~6-75 ppm (Durand et al., 2004). These concentrations exceed short-term occupational exposure limits by up to 15 times.

Populations and cities can be seriously affected by SO₂ emissions during more explosive volcanic activity:

• Soufrière, Guadeloupe: During the 1976 eruption, the population complained of headaches associated with a strong SO₂ odour (Le Guern et al., 1980).
• Popocatepetl, Mexico: In Mexico City, directly downwind of the persistently active volcano, SO₂ concentrations have exceeded 0.08 ppm (160 µg m^-3) under the influence of volcanic emissions (Raga et al., 1999). This is more than four times the city's typical monthly average and above most of the recognised annual and 24 hour exposure guidelines.
• Sakurajima, Japan: This volcano has been very active in recent history, fumigating a wide region downwind. Maximum hourly SO₂ levels in Sakurajima city (~5 km from Sakurajima volcano) in 1980 were 0.84 ppm, exceeding Japanese ambient air quality standards (Yano et al., 1986). From September 1985 to February 1986, monthly average SO₂ concentrations measured at the base of Sakurajima ranged from 0.015 ppm to 0.138 ppm, with an average of 0.079 ppm for the period (Kawaratani and Fujita, 1990). Epidemiological investigations into health in the region surrounding the volcano have shown positive associations between SO₂ concentrations and adult mortality from bronchitis and neonatal mortality (Shinkuro et al., 1999; Wakisaka et al., 1988).
• Miyakejima, Japan: In autumn 2000, southerly and southwesterly winds brought the volcanic gases emitted by Miyakejima to the main island and caused high concentrations of SO₂ at many surface stations 100-400 km downwind (e.g. Naoe et al., 2003). At 88 km distance, maximum SO₂ surface levels were ~0.114 ppm, compared to 0.0028 ppm at the same time in the previous year (An et al., 2003). At 4.5 km, the maximum recorded hourly concentration was 0.945 ppm. This is more than nine times the Japanese air-quality hourly value. The eruption influenced the air quality of the Tokyo metropolitan area, which has more than 30 million residents, some of whom reported smelling malodorous gas in the city (Fujita et al., 2003). From August to November 2000, SO₂ levels at 623 air monitoring stations across Japan exceeded hourly air quality values (Fujita et al., 2003).

Other examples of SO₂ concentrations and effects at varying distances:

• Concepcion, Nicaragua: SO₂ emissions from the crater in 1986 and 1993 measured 8-10 km downwind were sufficient to cause mild fumigation of populated areas (SEAN 11:05; BGVN 18:03).
• Cerro Hudson, Chile: Sulphurous fumes on 11 October 1991 were so intense in the Huemules valley on the west flank of the volcano that some inhabitants became sick, resulting in vomiting and loss of consciousness (BGVN 16:09). (It is unclear what the composition of these fumes was and there may have been sulphate aerosol and/or hydrogen sulphide present).
• St Augustine, Alaska: The plume from the 1 February 1976 eruption contained concentrations of gaseous sulphur (assumed by the investigators to be all sulphur dioxide) up to 10 ppm close to the volcano and 1 ppm 10 km downwind that caused minor throat irritation (Stith et al., 1978).
• Yasur, Vanuatu: Hazardous levels of SO₂ have been found in the plume at the crater rim. In September 1988, plume concentrations here were between 3 and 9 ppm (SEAN 13:12), exceeding many occupational air-quality standards.
• Popocatepetl, Mexico: Near-vent concentrations of SO₂ in February 1997 were ~3.8 ppm (10,000 µg m^-3), which is double the NIOSH recommended time-weighted average (Goff et al., 1998).
• Telica, Nicaragua: In March-June 1994, sulphur-rich steam from the crater moved down the slopes of the volcano and filled a valley with high concentrations of SO₂. A sulphur odour was also reported on the NE slope (BGVN 19:07).
• Taal, Philippines: Strong smells of SO₂ were observed during the 1911 eruption and it has been suggested (Baxter 1990) that this may have contributed to the mortality caused by the eruption.

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
(after BGVN 16:09; Hayakawa, 1999; Ng'Walali et al., 1999)

References

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