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Sulphur Dioxide (SO2)


Sulphur dioxide (SO2) 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.

Exposure Effects
Existing Guidelines
Volcanic Examples and Incidents
Volcanic Gases and Aerosols Index


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)

Exposure limits (ppm) Health Effects
1-5 Threshold for respiratory response in healthy individuals upon exercise or deep breathing
3-5 Gas is easily noticeable. Fall in lung function at rest and increased airway resistance
5 Increased airway resistance in healthy individuals
6 Immediate irritation of eyes, nose and throat
10 Worsening irritation of eyes, nose and throat
10-15 Threshold of toxicity for prolonged exposure
20+ Paralysis or death occurs after extended exposure
150 Maximum concentration that can be withstood for a few minutes by healthy individuals

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.

(µg m-3)
Averaging period Guideline type Date of Implem- 
Relevant Law Notes Ref.
Argentina 1 2620 1 hour   16 April 1973 Ley 20.284   a
0.3 780 8 hours   16 April 1973 Ley 20.284   a
(0.027) 70 1 month   16 April 1973 Ley 20.284   a
Chile 0.096 250 24 hours Primary 6 March 2003 D.S. Nº 113/02 1 b
0.031 80 Annual Primary 6 March 2003 D.S. Nº 113/02 1 b
China (0.057),
150 (i),
500 (ii),
700 (iii)
1 hour   January 1996 GB 3095-1996 2 a
50 (i),
150 (ii),
250 (iii)
24 hours   January 1996 GB 3095-1996 2 a
20 (i),
60 (ii),
100 (iii)
Annual   January 1996 GB 3095-1996 2 a
Colombia (0.573) 1500 3 hours not to be exceeded more than once per year 11 January 1982 Decreto No. 2   a
(0.153) 400 24 hours not to be exceeded more than once per year 11 January 1982 Decreto No. 2   a
(0.038) 100 Annual   11 January 1982 Decreto No. 2   a
Costa Rica (0.573) 1500 3 hours not to be exceeded more than once per year   Reglamento sobre inmisión de contami- 
nantes atmosféricos
(0.139) 365 24 hours not to be exceeded more than once per year   Reglamento sobre inmisión de contami- 
nantes atmosféricos
(0.031) 80 Annual     Reglamento sobre inmisión de contami- 
nantes atmosféricos
Ecuador (0.573) 1500 3 hours not to be exceeded more than once per year 15 July 1991 Registro Oficial No. 726   a
(0.153) 400 24 hours not to be exceeded more than once per year 15 July 1991 Registro Oficial No. 726   a
(0.031) 80 Annual   15 July 1991 Registro Oficial No. 726   a
EU (0.134) 350 1 hour not to be exceeded more than 24 times in a calendar year 1 January 2005 COUNCIL DIRECTIVE 1999/30/EC 3 c
(0.048) 125 24 hours not to be exceeded more than 3 times in a calendar year 1 January 2005 COUNCIL DIRECTIVE 1999/30/EC 3 c
(0.008) 20 Annual   19 July 2001 COUNCIL DIRECTIVE 1999/30/EC 3 c
Japan 0.1 260 1 hour   16 May 1973     d
0.04 110 24 hours   16 May 1973     d
Mexico (0.130) 341 24 hours not to be exceeded more than once per year 23 December 1994 NOM-022- 
(0.030) 79 Annual   23 December 1994     a
New Zealand (0.134) 350 1 hour   May 2002   4 e
(0.046) 120 24 hours   May 2002   4 e
UK (0.102) 266 15 min not to be exceeded more than 35 times a calendar year 31 December 2004 The Air Quality (England) Regulations 2000   f
(0.134) 350 1 hour not to be exceeded more than 24 times in a calendar year 31 December 2004 The Air Quality (England) Regulations 2000   f
(0.048) 125 24 hours not to be exceeded more than 3 times in a calendar year 31 December 2004 The Air Quality (England) Regulations 2000   f
USA 0.14 365 24 hours Primary 1990 NAAQS   g
0.50 1300 3 hours Secondary 1990 NAAQS   g
0.030 80 Annual Primary 1990 NAAQS   g
WHO 0.175 500 10 minutes   2000 WHO 2000 5 h
(0.048) 125 24 hours   2000 WHO 2000   h
(0.019) 50 Annual   2000 WHO 2000   h
  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 Organization, 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

Averaging Period Min (ppm) Max (ppm)
10-15 min 0.102 0.175
1 hour 0.057 1
24 hr 0.019 0.153
Annual 0.008 0.038

Occupational guidelines for SO2
Values in brackets are approximate conversions of published guidelines

(µg m-3)
Averaging period Guideline type Date of Implem- 
Relevant Law Notes Ref.
UK 5 13000 15 min MEL       a
2 5300 8 hour TWA MEL       a
USA 5 13000 15 min STEL 1994 NIOSH/ ACGIH 2 c
5 13000 8 hour TWA PEL   OSHA Regulations (Standards - 29 CFR) 1 b
2 5000 8 hour TWA   1994 NIOSH/ ACGIH 2 c
0.3 (800) 1 hour ERPG-1 1989 Emergency Response Planning Guideline   d
3 (7900) 1 hour ERPG-2 1989 Emergency Response Planning Guideline   d
15 (39300) 1 hour ERPG-3 1989 Emergency Response Planning Guideline   d
  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 1990s (Ng'Walali et al., 1999). In 2000, Hawaii Volcanoes National Park in collaboration with the USGS Hawaiian Volcano Observatory introduced an SO2 advisory to protect staff and visitors to the park. In 2008, with the onset of the summit eruption, all agencies on the Island of Hawai'i, in consultation with EPA, adopted a single advisory system: http://hiso2index.info/assets/FinalSO2Exposurelevels.pdf.


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)

Volcano Date Mortality/Morbidity Further detail
Aso, Japan 12 Feb 1989 1 death 66 yr old male tourist
Aso, Japan 26 Mar 1990 1 death Tourist
Aso, Japan 18 Apr 1990 1 death 78 yr old male tourist
Aso, Japan 19 Oct 1990 1 death 54 yr old female tourist
Hudson, Chile 11 Oct 1991 Some inhabitants became sick,
vomiting and losing consciousness
Intense sulphurous fumes in one valley
Kilauea, Hawaii 1993 1 death Tourist with a sulphur sensitivity died in the Halemaumau crater parking lot.
Aso, Japan 29 May 1994 1 death Female 69 yr old tourist
Aso, Japan 23 Nov 1997 2 deaths 62 and 51 yr old male tourists. Levels had reached 5 ppm just prior to their collapse.


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