Volcanic eruptions are one of the most impressive, violent and dramatic natural agents of change on our planet. Images of an eruption can include a vivid display of yellow and red lava bubbling from fissures, orange fires, and white smoke.

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Along with the visible dangers erupting from a volcano, there is something unseen but no less dangerous: toxic gases sulfur dioxide, carbon dioxide, hydrogen sulfide, hydrochloric acid and carbon monoxide. The gases are released from magma when the pressure of the earth is no longer able to contain them, similar to opening a bottle of champagne or a can of soda.

Being prepared includes knowing volcano and eruption types, what gases and other debris can be released, and how far the smoke, ash and gases can travel.

Volcano types

Geologists generally group volcanoes into four main types – cinder cones, composite volcanoes, shield volcanoes, and lava domes.

1. Cinder or scoria cones are the most common volcano type. They feature straight, steep slopes, have a giant crater on top, and rarely exceed 300 meters (1,000 feet) in height.
2. Composite volcanoes, sometimes called stratovolcanoes, often stand more than 3,000 m (10,000 ft). Towards the base, they have gentle slopes that become steep toward the top and feature a small crater on top. These are the most picturesque volcano type – and the deadliest.
3. Shield volcanoes are massive and broad, typically 20 times wider than they are high. The world’s tallest volcanoes are shield volcanoes and rise more than 9,500m (31,000ft).
4. Volcanic or lava domes are formed by relatively small, bulbous masses of lava too viscous to flow any great distance.¹

Types of volcanic eruptions

The type of volcanic eruption is determined by the crystal and gas content as well as the temperature of the magma:

• Crystals affect magma viscosity (thickness of a liquid). Increasing crystal content creates correspondingly thicker magma with higher viscosity.
• Gases trapped in magma that’s more viscous have a more difficult time escaping. This makes an explosion more likely.
• Temperature: Higher temperature magmas allow gases to escape more easily, while lower temperature magmas are more viscous and increase the likelihood of an explosion.

Volcano eruptions are classified into several types. Some are named after a particular volcanoes and others are named by the shape of the eruption debris or the place where they occur.

1. Strombolian eruptions are distinct bursts of fluid lava coming from the mouth of a magma-filled summit conduit. The explosions usually occur every few minutes at regular or irregular intervals.
2. Vulcanian eruptions are short, violent, relatively small explosions of viscous magma. Vulcanian eruptions create powerful explosions in which material can travel faster than 350 meters per second (800 mph) and rise several kilometers into the air.
3. Lava dome eruptions form when very viscous, rubbly lava is squeezed out of a vent without exploding.
4. Surtseyan eruptions are when magma or lava interacts explosively with water, usually from an undersea volcano.
5. Hawaiian: Fluid lava is thrown into the air in jets from a vent or line of vents (a fissure) at the summit. The jets can last for hours or even days, called “fire fountaining.” Because the magma has a low viscosity, the lava can travel miles before cooling and hardening.
6. Plinian are the largest and most violent of all the types. They are extremely destructive and can even obliterate the entire top of a mountain, as occurred at Mount St. Helens in 1980.

What is volcanic ash?

Volcanic ash is a term commonly used to refer to all “tephra” or “pyroclastics,” which are particles of igneous rock material of various sizes that have been ejected from volcanoes. Tephra / pyroclastic terms are classified by size:

• Blocks or bombs: more than 64 millimeters (2.5 inches)
• Lapilli: less than 64 mm
• Volcanic ash: less than 2 mm (0.079 in.)
• Fine volcanic ash or volcanic dust: less than 0.063 mm (0.0025 in.)

All explosive eruptions produce tephra. Volcanic and fine volcanic ash are then dispersed by prevailing winds and can fall hundreds or even thousands of kilometers away. Volcanic ash suspended in the atmosphere is a hazard for aviation.

Tephra can also create considerable impacts on the ground. Relatively thin falls (less than 10 mm) may have adverse health effects for vulnerable individuals and can disrupt critical infrastructure services, aviation, agriculture and other socio-economic activities over potentially huge areas.

Thick ash falls (more than 100 mm) may damage crops, vegetation and infrastructure, cause structural damage to buildings and create major clean-up requirements. However, they are typically confined to within tens of kilometers of the vent and, as they occur with large eruptions, are relatively rare.

Short-term effects commonly include irritation of the eyes and upper airways and exacerbation of pre-existing asthma. Affected communities can also experience other direct and indirect social impacts, including disruption of livelihoods and the anxiety it may cause.²

What is vog?

The term "vog" is generic, with the actual proportion of gases and particles dependent on how much time it has had to react in the atmosphere. Vog is a hazy mixture of SO₂ gas and PM2.5, which are primarily composed of sulfuric acid droplets and other sulfate (SO₄) compounds.

Aerosols are created when SO₂ and other volcanic gases combine in the atmosphere and interact chemically with oxygen, moisture, dust, and sunlight over periods of minutes to days.

The exact composition of vog depends on how much time the volcanic plume has had to react in the atmosphere. Far from the erupting vent, aerosols are the main component of vog. Closer to the volcano, vog contains both aerosols and unreacted SO₂ gas.

Close to the gas emission sources, vog can contain significant amounts of unreacted SO₂ gas. The longer SO₂ gas has to react in the atmosphere, the more complete the SO₂ gas to particle conversion is. The fine particles scatter sunlight, causing the visible haze that is observed downwind. So, farther away from the volcano, vog is composed primarily of PM2.5.

People with pre-existing medical conditions are the primary group at risk of experiencing health effects from exposure to vog, but healthy people may also experience symptoms.

Agricultural crops and other plants are subject to injury by exposure to the pollutants. Farmers and gardeners in the path of the pollutants (SO₂ and acid rain) have reported significant damage to plants caused by winds blowing SO₂ gas and acid particles.

What is laze?

When molten lava flows into the ocean, it reacts vigorously with seawater to create a different type of gas plume that results in hazy and noxious conditions downwind of an ocean entry. Referred to as a "laze" plume (for a blending of the words 'lava' and 'haze'), it forms through a series of chemical reactions as hot lava boils seawater to dryness.

Laze is often an irritating mixture of hydrochloric acid gas (HCl), steam, and tiny volcanic glass particles. This hot, corrosive gas mixture has caused deaths, so laze should be taken seriously. Winds can carry laze and even the wispy edges can cause skin and eye irritation and breathing difficulties. Laze can also result in acid rain with corrosive properties.³

What makes up volcanic gas?

Magma contains dissolved gases, which provide the driving force that causes most volcanic eruptions. 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.

Volcanic carbon dioxide

When this colorless, odorless gas is emitted from volcanoes, it typically becomes diluted to low concentrations very quickly and is not life-threatening. However, because cold carbon dioxide gas is heavier than, air it can flow into in low-lying areas where it can reach much higher concentrations in certain, very stable atmospheric conditions. This can pose serious risks to people and animals.

Breathing air with more than 3% CO₂ can quickly lead to headaches, dizziness, increased heart rate and difficulty breathing. At concentrations exceeding about 15%, CO₂ quickly causes unconsciousness and death.

Carbon dioxide emissions from a volcano tend to become diluted quickly, and therefore do not typically pose a direct threat to people. However, carbon dioxide is heavier than air, so it can accumulate in low-lying areas. If a person were to enter a low area where the gas settled after a volcanic eruption, then breathing this concentrated air could prove fatal.³

Volcanic sulfur dioxide (SO₂)

Sulfur dioxide (SO₂) is irritating to eyes, skin and respiratory system. People with cardiovascular disease or respiratory ailments, such as asthma, are especially vulnerable. Older adults, infants and pregnant women are also particularly sensitive. No one knows the long-term health effects of exposure to volcanic sulfur dioxide, officials say.

Sulfur dioxide is a colorless gas with a pungent odor that irritates skin and the tissues and mucous membranes of the eyes, nose, and throat. SO₂ emissions can cause acid rain and air pollution downwind of a volcano high concentrations of sulfur dioxide produce volcanic smog, causing persistent health problems for downwind populations.

During enormous eruptions, SO₂ can be injected to altitudes of greater than 10km into the stratosphere. Here, SO₂ is converted to sulfate aerosols which reflect sunlight and therefore have a cooling effect on the Earth's climate. They also have a role in ozone depletion, as many of the reactions that destroy ozone occur on the surface of such aerosols.⁴

Volcanic hydrogen sulfide

Hydrogen sulfide is a colorless, flammable gas with a strong, offensive odor and is sometimes referred to as sewer gas. It is very toxic in high concentrations.

Interestingly, the human nose is more sensitive to H2S than any gas monitoring instrument we have today: air mixtures with as little as 0.000001% H2S are associated with a rotten egg smell. However, at mixing ratios above about 0.01%, H2S becomes odorless and very toxic, causing irritation of the upper respiratory tract and, during long exposure, pulmonary edema.

Exposure to over 500 ppm for more than five minutes can cause a person to fall. When exposed to this level for up to an hour, death can occur.

Volcanic Hydrogen halides (HF, HCl, HBr)

When magma ascends close to the surface, volcanoes can emit the halogens fluorine (HF), chlorine (HCl) and bromine in the form of hydrogen halides (HBr). These gases are acidic, have high solubility and can potentially cause acid rain. 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.⁵

At-risk populations and volcano emissions

Most healthy adults will recover from exposure. However, certain individuals are more at-risk for severe health consequences, including:

• Young children. Children whose lungs are still developing are considered more vulnerable, regardless of whether they have a pre-existing condition.
• Pregnant women. Vog inhalation puts pregnant women and their unborn children at a higher risk than the general population.
• Older adults. This population is considered to be at-risk due to an increased rate of pre-existing lung and heart diseases.
• Anyone with a respiratory disease. Individuals with emphysema, chronic bronchitis, COPD, asthma or another respiratory disease are at-risk.
• Individuals with a cardiovascular disease. Circulatory diseases include high blood pressure, vascular diseases, heart failure and cerebrovascular conditions. These conditions make sufferers susceptible to heart attacks, transient chest pain, heart failure, stroke, and sudden death from cardiac arrhythmia.

Volcano emissions preparation tips

• Remain indoors as much as possible. This is most useful in buildings that effectively stop outdoor air from getting inside.
• Wear a mask outdoors. Only use a respirator mask with an N95 or N100 rating to help protect against smoke particles.
• Check air quality monitoring sites, such as the AirVisual Air Quality Index.
• Keep medications handy. If you have asthma or other respiratory conditions, keep your medication available and use as prescribed. If you don't have medications, but feel you might need them, call your doctor.
• Wear an air pollution mask, such as the KN95-certified IQAir Mask, to protect yourself from inhaling airborne particulate pollutants in vog.
• Create a clean air sanctuary inside your home. Particles and gases can quickly build up inside your home. Keep windows closed and seal off any openings to the outside, including vents. When using an air-conditioner, be sure to set it to re-circulate and close the fresh-air intake. Filter the air when ventilating the space with a high-performance air purifier for volcanic smog (vog), such as the GC MultiGas.
• Avoid activities that further pollute the indoor air. Avoid burning candles, using the fireplace, or even vacuuming (unless you own a high-performance HEPA vacuum cleaner). All of these can otherwise become additional sources of indoor air pollutants

Natural events such as volcanic eruptions can’t be controlled. However, with knowledge and preparation, you can protect yourself and your family as best as possible.