Volcanic Gases and Aerosols
Volcanic gas includes any gas-phase substance that is emitted by volcanic or volcanic-geothermal activity. Volcanic aerosols include liquid or solid particles that are small enough to be suspended in the air, and that are emitted by volcanic or volcanic-geothermal activity (adapted from Baxter and Horwell, 2015, Fischer and Chiodini, 2015, and Williams-Jones and Rymer ,2015).
Primary reference(s)
Baxter, P.J. and C.J. Horwell, 2015. Impacts of eruptions on human health. In: Sigurdsson, H., B. Houghton,
S. McNutt et al (eds.), The Encyclopedia of Volcanoes, 2nd Ed. Academic Press, pp. 1035-1047. Accessed 15 May 2025.
Fischer, T.P. and G. Chiodini, 2015. Volcanic, magmatic and hydrothermal gases. In: Sigurdsson, H., B. Houghton, S. McNutt et al (eds.), The Encyclopedia of Volcanoes, 2nd Ed. Academic Press, pp. 779-797. Accessed 15 May 2025.
Williams-Jones, G. and H. Rymer, 2015. Hazards of volcanic gases. In: Sigurdsson, H., B. Houghton, S. McNutt et al (eds.), The Encyclopedia of Volcanoes, 2nd Ed. Academic Press, pp. 985-992. Accessed 15 May 2025.
Annotations
Additional scientific description
Volcanic gases can be emitted directly into the atmosphere from magma or by magma interacting with crustal rocks. They can be observed with spectroscopic instruments from ground and space, and their future dispersion can be modelled, allowing forecasts of gas and aerosol concentrations to be made. Volcanic gases can also be measured by direct sampling.
The chemical composition of volcanic gas and aerosol emissions is highly heterogeneous. The composition changes continuously as the emissions drift away from their source and react with the atmosphere and sunlight. Typically, the most abundant volcanic gas is water vapour (usually 80% or more of the gas mass). Other common gases are carbon dioxide (CO2), sulphur dioxide (SO2), hydrogen sulphide (H2S), hydrogen (H2) and hydrogen halides (hydrogen chloride [HCl] and hydrogen fluoride [HF]). Radon (222Rn) and carbon monoxide (CO) are also emitted in trace amounts. Volcanic gases can be classified as magmatic or hydrothermal. Magmatic gases, usually rich in sulphur dioxide (SO2) and halogens and are associated with a relatively shallow magmatic source. Hydrothermal gases result essentially from magmatic gas-heated boiling aquifers, and are richer in steam, methane (CH4) and hydrogen sulphide (H2S), and are virtually hydrogen chloride (HCl) and hydrogen fluoride (HF) free (Stix et al., 2025).
Volcanic aerosol sizes range from a few nanometres (nm) to several hundred micrometres (µm). Volcanic aerosol refers to particles formed through condensation of volcanic gases, or through reaction of the gases with the atmosphere and sunlight and is thereby distinct from 'ash' or 'tephra' that is formed through fragmentation of magma or lava (see GH0010). Aerosols can be in liquid or solid form and evolve between these states with time.
Volcanic gases and aerosols are emitted by almost any type of volcanic activity and from a variety of sources:
- Emissions from explosive eruptions: Depending on the explosive power, emissions can be injected into the stratosphere or stay in the troposphere and spread around the globe in the most powerful events. Typical emission duration is hours to days.
- Emissions from effusive lava eruptions, open vents and lava lakes: Emission durations can last from days up to several decades or longer. Emissions are typically confined to the troposphere and have been instrumentally detected up to thousands of kilometres from the source.
- Emissions from medium (100 - 400 ºC) and high (> 400 ºC) temperature fumaroles located in volcanic craters or on flanks of volcanoes. Gases are usually confined to the immediate vicinity of the source and may be sampled remotely or through direct sampling (Stix et al., 2025).
- Emissions from volcanic-geothermal systems: These low-energy and relatively low-temperature emissions (typically <100°C) are usually confined to the immediate vicinity of the source, and are usually detected through direct sampling (Stix et al., 2025).
- Emissions from soil diffuse degassing and lakes: Gases diffusely released from soils are dominated by carbon dioxide (CO2) and commonly occur in specific areas, such as faults, small vents and steaming ground (Stix et al., 2025). However, large and highly hazardous emissions can occur if gases accumulate in the bottom of a deep lake and then rapidly release (Schmid et al., 2005). These gases may accumulate far from their source and flow down valleys as a gravity flow (Edmonds et al., 2015).
Aerosol forms by condensation of volcanic gases, both near-instantaneously after emission, and on the timescale of hours to days. Sulphate, a common aerosol component, forms through conversion of SO2 gas. Aerosol contains a variety of trace components, including elements collectively classified as metal pollutants by environmental and health protection agencies.
Metrics and numeric limits
The abundance of emitted volcanic gases and aerosol varies greatly among eruptions. Recent large eruptions of Holuhraun in Iceland 2014-2015 and Kīlauea (Hawaii) in 2018, emitted as much SO2 per day as anthropogenic activities in China (50-200 kt/day) over several months (Pfeffer et al., 2018; Kern et al., 2020). A larger-scale emission scenario, which may occur in the coming decades or centuries, includes a 'Laki-type' eruption in Iceland, which can emit ten times more SO2 than the recent eruptions described above. There are tens, or potentially hundreds, of volcanoes worldwide, which emit smaller amounts of SO2 (0.5-5 kt/day) (Carn et al., 2016), but sustain the emissions over years-to-decades (e.g., Mt Etna).
Key relevant UN convention / multilateral treaty
Sendai Framework for Disaster Risk Reduction 2015–2030 (UNDRR, 2015).
Drivers
Volcanic gases are emitted by volcanic activity as described above; however, their release may also be triggered by surface rupture and fissuring (see GH0311). Volcanic gases and aerosols can in turn trigger hazards including (but not limited to) acid rain (see EN0105), soil pollution (see TL0303), soil degradation (see GH0402), air pollution (EN0102), critical infrastructure failure (TL0207).
For the specific cases of diffuse degassing, changes in the meteorological conditions may interfere with the flux of the gas. For instance, usually decreases in barometric pressure can increase the gas flow thereby increasing the hazard. Low barometric pressure reduces natural ventilation, which can result in increase of indoor air pollution.
Impacts
Volcanic gas and aerosol exposure are listed as the cause of 1% of total volcanic hazard fatalities (2283 people; Brown et al., 2017). This estimate includes only fatalities due to extreme direct exposure and does not include premature mortality caused by long-term air and environmental pollution. It has been estimated that 800 million people live within 100 km of a volcano that has erupted in the last 10,000 years, a range within which they could be exposed to this hazard.
Multiple chemical species in volcanic gases and aerosols may cause human and/or environmental impact (Viveiros et al., 2025)(see also Toxic gases CH0300 and Asphyxiant gases CH0400 for more details).
- Human health impacts and animals: The common effects of volcanic gases, in particular SO2, H2S, HCl and HF, which are poisonous, are: (i) irritation to the respiratory tract, eyes and skin; (ii) chest tightness, shortness of breath, and headaches; and (iii) asthma aggravation. SO2 is the greatest respiratory hazard, causing health impacts, especially for asthmatics, up to thousands of kilometres from the source. High concentrations of fluoride (from HF) causes damage to teeth and bones; it is especially dangerous to grazing animals. Some gases, such as the CO2, are dangerous only if present in such a high concentration that blocks oxygen respiration. All of the listed gas species, as well as CO, can cause death in high concentrations to humans and any other animals. Volcanic aerosol is typically PM2.5, an air pollutant with no known safe exposure limits (WHO, 2021). Both acute and chronic exposure to PM2.5 causes respiratory and cardiovascular morbidity and premature mortality. More information on the health hazards and impacts of volcanic gases and aerosols can be found on the International Volcanic Health Hazard Network website (IVHHN, 2020a).
- Environmental impacts: Acid rain (see MH0033) is commonly caused by mixing of atmospheric water with volcanic gas and aerosol and leads to degradation of plant health and diversity, crop damage and damage to infrastructure. Metal pollutants can contaminate rainfall and accumulate in soils, surface waters and plants. CO2 diffuse degassing areas may reduce plant health and diversity, and result in bare soils areas.
- Climate impacts: Large explosive eruptions can form an aerosol blanket in the stratosphere, leading to cooling at the surface which may last for months to years. For example, the 1991 Mt Pinatubo eruption lowered the surface temperature by 0.2-0.5°C, which lasted for about two years (McCormick et al., 1995).
Owing to the multiple impacts of volcanic gases, agencies in Hawaii provided a public dashboard, which summarizes the various impacts as well as providing access to monitoring and forecasting data (IVHHN, 2020b). The dashboard was accessed more than 50,000 times per week during the 2018 Kīlauea volcanic crisis. Hawaii Volcano Observatory also have a dedicated vog (volcanic gas haze) warning system.
Multi-hazard context
The figure below summarises common interactions between volcanic gases & aerosols and other hazards. This information should be used with caution and not be solely relied upon in Disaster Risk Management, particularly as some interactions may not have been included. Note that hazardous events occurring together or locally in space or time may not necessarily cause, amplify or be otherwise related to each other. Specific examples of multi-hazard context can be found in the ‘Hazard drivers’ and ‘Impacts’ sections above.
Multi-hazard diagram
Risk Management
SO2, particulate matter <2.5 µm in diameter (PM2.5) and, in some cases, H2S, are volcanic gas and aerosol pollutants that are monitored and forecasted operationally by volcano observatories. Volcano observatories combine gas measurements with meteorological data and simulations to forecast gas pollution, for example this is routinely done in Iceland. Gas pollution hazard maps are used to plan and prepare for risk management such as evacuations, road closures, and diversions.
Monitoring
The section and the table below offer an overview of monitoring volcanic gases & aerosols. This information can be used for forecasting within a national early warning system (EWS). Since EWS capacities and processes differ across countries, the most current and specific information regarding EWS should be obtained from the appropriate national or regional agency/authority responsible for disaster management.
| Which institution(s) produce(s) Disaster Risk Data/Information? | Volcano Observatories monitor precursory and ongoing activity that could lead to hazards associated with volcanic unrest or eruption (including but not limited to ground shaking, ground fracturing, subsidence and uplift, and volcanic gas emissions). Volcano Observatories issue warnings and, in association with civil protection and emergency management organizations, recommendations. |
| How is the Hazard Observed/Monitored/Forecast? | Volcanic gas emissions can be monitored using both ground-based gas sensors and satellite data meaning that gases can be detected and tracked from space. . CO2 in diffuse degassing areas is also permanently monitored in some diffuse degassing areas, for example in Italy, Spain (Canaries) and Portugal (Azores). |
References
Brown, S.K., S.F. Jenkins, R.S.J. Sparks, H. Odbert and M.R. Auker, 2017. Volcanic fatalities database: analysis of volcanic threat with distance and victim classification. Journal of Applied Volcanology, 6:15. doi.org/10.1186/s13617-017-0067-4.
Carn, S.A., L. Clarisse and A.J. Prata, 2016. Multi-decadal satellite measurements of global volcanic degassing. Journal of Volcanology and Geothermal Research, 311:99-134.
Edmonds, M., Grattan, J., Michnowicz, S. (2015). Volcanic Gases: Silent Killers. In: Fearnley, C.J., Bird, D.K., Haynes, K., McGuire, W.J., Jolly, G. (eds) Observing the Volcano World. Advances in Volcanology. Springer, Cham.
IVHHN, 2020a. Health impact of volcanic gases; Information on different volcanic gases. International Volcanic Health Hazard Network (IVHHN). Accessed 15 October 2020.
IVHHN, 2020b. Hawaii Interagency Vog Information Dashboard. International Volcanic Health Hazard Network (IVHHN). Accessed 15 October 2020.
Kern, K., A.H. Lerner, T. Elias, P.A. Nadeau, L. Holland, P.J. Kelly, C.A. Werner, L.E. Clor and M. Cappos, 2020. Quantifying gas emissions associated with the 2018 rift eruption of Kīlauea Volcano using ground-based DOAS measurements. Bulletin of Volcanology, 82:55. doi.org/10.1007/s00445-020-01390-8.
McCormick MP, Thomason LW, Trepte CR (1995) Atmospheric effects of the Mt Pinatubo eruption. Nature 373(6513):399–404.
Pfeffer, M.A., B. Bergsson, S. Barsotti and 30 others, 2018. Ground-based measurements of the 2014–2015 Holuhraun volcanic cloud (Iceland). Geosciences, 8:29.
Schmid, M, M. Halbwachs, B. Wehrli and A. Wuest, 2005. Weak mixing in Lake Kivu: New insights indicate increasing risk of uncontrolled gas eruption. Geochemistry, Geophysics, Geosystems, 6: Q07009.
Stix et al. Volcanic gases and volcano degassing, Editor(s): C. Bonadonna et al., The Encyclopedia of Volcanoes (Third Edition) [Manuscript in preparation], Academic Press, 2025
Viveiros et al. Volcanic gas impact, Editor(s): C. Bonadonna et al., The Encyclopedia of Volcanoes (Third Edition) [Manuscript in preparation], Academic Press, 2025
WHO global air quality guidelines. Particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide. Geneva: World Health Organization; 2021. Accessed 26 July 2024