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Volcanic gases can be harmful to health, vegetation and infrastructure.
Magma contains dissolved gases, which are the driving force behind most volcanic eruptions. As the magma rises to the surface and the pressure decreases, the gases are released from the liquid part of the magma (melt) and continue to move upwards, eventually being released into the atmosphere. Large eruptions can release huge amounts of gas in a short period of time. The 1991 eruption of Mount Pinatubo is thought to have injected more than 250 megatonnes of gas into the upper atmosphere in a single day. However, even if the magma never reaches the surface, gases can often escape continuously into the atmosphere from the ground, volcanic vents, fumaroles and hydrothermal systems.
By far the most abundant volcanic gas is water vapor, which is harmless. However, significant amounts of carbon dioxide, sulfur dioxide, hydrogen sulfide and hydrogen halides can also be emitted from volcanoes. Depending on their concentrations, these gases are all potentially hazardous to people, animals, agriculture, and property.
Carbon dioxide gas can collect in low-lying volcanic areas, posing a lethal risk to humans and animals. A burning torch lowered into a CO2 pocket (top) causes the flame to go out (bottom).
Carbon dioxide (CO2) trapped in low-lying areas can be lethal to people and animals.
Carbon dioxide makes up about 0.04% of the air in the Earth’s atmosphere. When this colourless, odourless gas is emitted from volcanoes, it usually dilutes very quickly to low concentrations and is not life-threatening. However, because cold carbon dioxide is heavier than air, it can flow into low-lying areas where it can reach much higher concentrations under certain very stable atmospheric conditions. This can pose serious risks to people and animals. Breathing air containing more than 3% CO2 can quickly lead to headaches, dizziness, increased heart rate and breathing difficulties. At mixing levels of more than about 15%, carbon dioxide quickly causes unconsciousness and death.
Volcanic mudflows (lahars and debris flows) occur more commonly after a landscape has been covered by loose volcanic material. Sign on the slopes of Rabaul, Papua New Guinea.
In volcanic areas where CO2 emissions occur, it is important to avoid small depressions and low-lying areas that could act as CO2 traps. The line between healthy air and deadly gas can be extremely sharp; just one step. High concentrations of CO2 gas in soils can also damage or destroy vegetation.
In addition to their direct hazard, volcanic CO2 emissions also have the capacity to affect the global climate, but scientific studies indicate that the average global volcanic output is insignificant when compared to emissions from human activity.
Sulfur dioxide (SO2) is irritating to eyes, skin and respiratory system.
Sulphur dioxide is a colourless gas with a pungent smell that irritates the skin, tissues and mucous membranes of the eyes, nose and throat. SO2 emissions can cause acid rain and air pollution, high concentrations of sulphur dioxide produce volcanic smog (VOG) which causes persistent health problems for people. During very large eruptions, SO2 can be injected into the stratosphere at altitudes above 10 km. There, the SO2 is transformed into sulphate aerosols that reflect sunlight and thus have a cooling effect on the Earth’s climate. They also play a role in ozone depletion, as many ozone-destroying reactions occur on the surface of these aerosols.
Gas Plume during the 1984 eruption of Mauna Loa, Hawaii blocks out the sun.
Hydrogen sulfide (H2S) is very toxic in high concentrations
Hydrogen sulphide is a colourless, flammable gas with a strong, foul smell. It is sometimes called sewer gas. Interestingly, the human nose is more sensitive to H2S than any gas monitoring instrument we have today: the smell is associated with a rotten egg smell. Exposure to 500 ppm can render a human unconscious within 5 minutes and dead within an hour or less.
Hydrogen halides (HF, HCl, HBr) are toxic acids
When magma ascends close to the surface, volcanoes can emit the halogens fluorine, chlorine and bromine in the form of hydrogen halides (HF, HCl and HBr). These species have high solubility; therefore they rapidly dissolve in water droplets within volcanic plumes or the atmosphere where they can potentially cause acid rain. In an ash-producing eruption, ash particles are also often coated with hydrogen halides. Once deposited, these coated ash particles can poison drinking water supplies, agricultural crops, and grazing land.
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