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Ninety-nine percent of the gas molecules emitted during a volcanic eruption are water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2). The remaining one percent is comprised of small amounts of hydrogen sulfide, carbon monoxide, hydrogen chloride, hydrogen fluoride, and other minor gas species.
Learn more: Volcanic gases can be harmful to health, vegetation and infrastructure
The sulfuric acid droplets in vog have the corrosive properties of dilute battery acid. When vog mixes directly with moisture on the leaves of plants it can cause severe chemical burns, which can damage or kill the plants. Sulfur dioxide (SO 2) gas can also diffuse through leaves and dissolve to form acidic conditions within plant tissue. Farmers on Hawai`i Island, particularly in the Ka`u...
Predicting the vog levels that visitors might experience during a short stay in Hawai`i is as difficult as predicting the weather. Once volcanic emissions are in the atmosphere, they are distributed by prevailing winds. Where and how bad the vog is ultimately depends on several factors including wind direction, wind speed, air temperature, humidity, and rainfall, as well as the location of the...
Vog poses a health hazard by aggravating preexisting respiratory ailments. Sulfur dioxide (SO 2) gas can irritate skin and the tissues and mucous membranes of the eyes, nose, and throat, and can penetrate airways, producing respiratory distress in some individuals. Aerosol particles in vog can also penetrate deep into human lungs and, at elevated levels, can induce symptoms of asthma. Physical...
Vog (volcanic smog) is a visible haze comprised of gas and an aerosol of tiny particles and acidic droplets created when sulfur dioxide (SO 2) and other gases emitted from a volcano chemically interact with sunlight and atmospheric oxygen, moisture, and dust. Volcanic gas emissions can pose environmental and health risks to nearby communities. Vog is a hazard that's associated with Hawaiian...
The most critical factors that determine how much vog impacts an area are wind direction and speed. Air temperature, humidity, rainfall, location of the source, and the amount of sulfur dioxide (SO2) being emitted are also factors. During prevailing trade (from northeast) wind conditions, any SO2 emitted from Pu`u `Ō`ō is blown out to sea, while any SO2 from the summit vent often creates vog in Ka...
Instruments to measure sulfur dioxide and carbon dioxide can be mounted in aircraft to determine the quantity of gas being emitted on a daily basis. Such instruments can also be used in a ground-based mode. An instrument that detects carbon dioxide can be installed on a volcano and configured to send data continuously via radio to an observatory. Sulfur dioxide in volcanic clouds can also be...
The U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) determines the amount and composition of gases emitted by Kīlauea Volcano. Changes in gas emissions can reveal important clues about the inner workings of a volcano, so they are measured on a regular basis. HVO scientists use both remote and direct sampling techniques to measure compositions and emission rates of gas from Kīlauea...
There are 161 potentially active volcanoes in the United States. According to a 2018 USGS assessment, 57 volcanoes are a high threat or very high threat to public safety. Many of these volcanoes have erupted in the recent past and will erupt again in the foreseeable future. As populations increase, areas near volcanoes are being developed and aviation routes are increasing. As a result, more...
Kīlauea Volcano is renowned for its relatively benign eruptions of fluid lava flows. Therefore, many people were surprised by the small explosions that occurred in Halema`uma`u Crater in 2008 and 2018, and even more surprised to learn that volcanic ash was being erupted from a new gas vent. However, ash emissions from Halema`uma`u Crater are part of the volcano's legacy. Kīlauea's summit has...
Volcanoes are inherently beautiful places where forces of nature combine to produce awesome events and spectacular landscapes. For volcanologists, they're FUN to work on! Safety is, however, always the primary concern because volcanoes can be dangerous places. USGS scientists try hard to understand the risk inherent in any situation, then train and equip themselves with the tools and support...
This photograph shows the inside of the USGS Hawaiian Volcano Observatory (HVO) Multi-Gas site at Sulfur Cone high up on Mauna Loa's Southwest Rift Zone. Volcanic gas (Sulfur Dioxide, Hydrogen Sulfide, Carbon Dioxide, and Water vapor) concentrations are measured then telemetered back to HVO to be monitored and analyzed by HVO gas scientists. USGS photo by C.
This photograph shows the inside of the USGS Hawaiian Volcano Observatory (HVO) Multi-Gas site at Sulfur Cone high up on Mauna Loa's Southwest Rift Zone. Volcanic gas (Sulfur Dioxide, Hydrogen Sulfide, Carbon Dioxide, and Water vapor) concentrations are measured then telemetered back to HVO to be monitored and analyzed by HVO gas scientists. USGS photo by C.
On July 29, 2026, USGS Hawaiian Volcano Observatory gas scientists flew via helicopter to the Sulfur Cone Multi-Gas instrumentation site on Mauna Loa's Southwest Rift Zone, which is at an elevation of 3,430 meters (11,240 feet) above sea level.
On July 29, 2026, USGS Hawaiian Volcano Observatory gas scientists flew via helicopter to the Sulfur Cone Multi-Gas instrumentation site on Mauna Loa's Southwest Rift Zone, which is at an elevation of 3,430 meters (11,240 feet) above sea level.
A USGS Hawaiian Volcano Observatory volcanic gas specialist sets up a Fourier Transform Infrared spectrometer, or FTIR, near Kīlauea summit during episode 46 of lava fountaining on May 5, 2026.
A USGS Hawaiian Volcano Observatory volcanic gas specialist sets up a Fourier Transform Infrared spectrometer, or FTIR, near Kīlauea summit during episode 46 of lava fountaining on May 5, 2026.
USGS scientists use an FTIR (Fourier transform infrared spectrometer) to measure the chemical composition of volcanic gas during episode 28 of the ongoing Kīlauea summit on July 9, 2025. USGS photo by M. Cappos.
USGS scientists use an FTIR (Fourier transform infrared spectrometer) to measure the chemical composition of volcanic gas during episode 28 of the ongoing Kīlauea summit on July 9, 2025. USGS photo by M. Cappos.
This photograph shows the inside of the Hawaiian Volcano Observatory (HVO) Multi-Gas site at Sulfur Cone. Currently, HVO has 2 of these continuous gas monitoring sites in operation on the Island of Hawai‘i. This one at Sulfur Cone high on Mauna Loa's Southwest Rift Zone, along with one at the summit of Kīlauea.
This photograph shows the inside of the Hawaiian Volcano Observatory (HVO) Multi-Gas site at Sulfur Cone. Currently, HVO has 2 of these continuous gas monitoring sites in operation on the Island of Hawai‘i. This one at Sulfur Cone high on Mauna Loa's Southwest Rift Zone, along with one at the summit of Kīlauea.
Title: What on Earth is going on at Kilauea Volcano?
Title: What on Earth is going on at Kilauea Volcano?
USGS geologist Laura Clor (right) and Rachel Teasdale (California State University – Chico, left) collect gas samples from a thermal feature at Sulphur Works in Lassen Volcanic National Park.
USGS geologist Laura Clor (right) and Rachel Teasdale (California State University – Chico, left) collect gas samples from a thermal feature at Sulphur Works in Lassen Volcanic National Park.
USGS geologist Deborah Bergfeld collects a gas sample from a superheated (hotter than the boiling point) fumarole in Little Hot Springs Valley at Lassen Volcanic National Park.
USGS geologist Deborah Bergfeld collects a gas sample from a superheated (hotter than the boiling point) fumarole in Little Hot Springs Valley at Lassen Volcanic National Park.
Hawaiian Volcano Observatory Geochemist Jeff Sutton and CSAV international volcanology students visit a continuous gas monitoring site on Kilauea's east rift zone during field studies portion of the summer training course.
Hawaiian Volcano Observatory Geochemist Jeff Sutton and CSAV international volcanology students visit a continuous gas monitoring site on Kilauea's east rift zone during field studies portion of the summer training course.
Hawaiian Volcano Observatory Geochemist Jeff Sutton and CSAV international volcanology students visit a continuous gas monitoring site on Kilauea's east rift zone during field studies portion of the summer training course.
Hawaiian Volcano Observatory Geochemist Jeff Sutton and CSAV international volcanology students visit a continuous gas monitoring site on Kilauea's east rift zone during field studies portion of the summer training course.
Sulfur dioxide gas emissions from the crater of Pu‘u ‘Ō ‘ō on Kīlauea’s east rift zone and the vent within Halema‘uma‘u Crater at Kīlauea’s summit create volcanic pollution that affects the air quality of downwind communities. Here, a USGS Hawaiian Volcano Observatory gas geochemist measures Pu‘u ‘Ō‘ō gas emissions using an instrument that detects ga
Sulfur dioxide gas emissions from the crater of Pu‘u ‘Ō ‘ō on Kīlauea’s east rift zone and the vent within Halema‘uma‘u Crater at Kīlauea’s summit create volcanic pollution that affects the air quality of downwind communities. Here, a USGS Hawaiian Volcano Observatory gas geochemist measures Pu‘u ‘Ō‘ō gas emissions using an instrument that detects ga
This photo was taken from the lamp on the other side of the plume. The FTIR is the small dark silhouette on the rim across the crater gap.
This photo was taken from the lamp on the other side of the plume. The FTIR is the small dark silhouette on the rim across the crater gap.
HVO gas geochemists deployed a FTIR spectrometer on the east rim of Pu`u `Ō `ō crater. The FTIR measures the composition of the East Wall vent gases by "looking" through the plume at an infrared lamp (obscured by fume in this photo)
HVO gas geochemists deployed a FTIR spectrometer on the east rim of Pu`u `Ō `ō crater. The FTIR measures the composition of the East Wall vent gases by "looking" through the plume at an infrared lamp (obscured by fume in this photo)
A gas plume arising from Augustine Volcano during it's eruptive phase 2005-06. This photo was taken during a FLIR/maintenance flight on January 24, 2006.
A gas plume arising from Augustine Volcano during it's eruptive phase 2005-06. This photo was taken during a FLIR/maintenance flight on January 24, 2006.
USGS geochemist Bill Evans measures the temperature of a superheated (hotter than the boiling point) fumarole in Lassen Volcanic National Park.
USGS geochemist Bill Evans measures the temperature of a superheated (hotter than the boiling point) fumarole in Lassen Volcanic National Park.
USGS hydrologist Michael Sorey tries to locate the steam vent at a fumarole in the Bumpass Hell area in Lassen Volcanic National Park.
USGS hydrologist Michael Sorey tries to locate the steam vent at a fumarole in the Bumpass Hell area in Lassen Volcanic National Park.
USGS geochemist Cathy Janik (left) and Iceland Geosurvey chemist Jón Örn Bjarnason (right) collect a gas sample from a fumarole in Lassen Volcanic National Park.
USGS geochemist Cathy Janik (left) and Iceland Geosurvey chemist Jón Örn Bjarnason (right) collect a gas sample from a fumarole in Lassen Volcanic National Park.
USGS geologists gathered samples by hand from vents on the dome and crater floor. Additionally, sulfur dioxide gas was measured from a specially equipped airplane before, during, and after eruptions to determine "emission rates" for the volcano.
USGS geologists gathered samples by hand from vents on the dome and crater floor. Additionally, sulfur dioxide gas was measured from a specially equipped airplane before, during, and after eruptions to determine "emission rates" for the volcano.
ARRA-funded student Taryn Lopez (Univ. Alaska-Fairbanks) sampling gas emissions at fumarole next to dome at the summit of Augustine volcano.
ARRA-funded student Taryn Lopez (Univ. Alaska-Fairbanks) sampling gas emissions at fumarole next to dome at the summit of Augustine volcano.
Taryn Lopez measuring the temperature of gas emissions near summit of Augustine Volcano.
Taryn Lopez measuring the temperature of gas emissions near summit of Augustine Volcano.
The sulfuric acid droplets in vog have the corrosive properties of dilute battery acid. When vog mixes directly with moisture on the leaves of plants it can cause severe chemical burns, which can damage or kill the plants. Sulfur dioxide (SO 2) gas can also diffuse through leaves and dissolve to form acidic conditions within plant tissue. Farmers on Hawai`i Island, particularly in the Ka`u...
Predicting the vog levels that visitors might experience during a short stay in Hawai`i is as difficult as predicting the weather. Once volcanic emissions are in the atmosphere, they are distributed by prevailing winds. Where and how bad the vog is ultimately depends on several factors including wind direction, wind speed, air temperature, humidity, and rainfall, as well as the location of the...
Vog poses a health hazard by aggravating preexisting respiratory ailments. Sulfur dioxide (SO 2) gas can irritate skin and the tissues and mucous membranes of the eyes, nose, and throat, and can penetrate airways, producing respiratory distress in some individuals. Aerosol particles in vog can also penetrate deep into human lungs and, at elevated levels, can induce symptoms of asthma. Physical...
Vog (volcanic smog) is a visible haze comprised of gas and an aerosol of tiny particles and acidic droplets created when sulfur dioxide (SO 2) and other gases emitted from a volcano chemically interact with sunlight and atmospheric oxygen, moisture, and dust. Volcanic gas emissions can pose environmental and health risks to nearby communities. Vog is a hazard that's associated with Hawaiian...
The most critical factors that determine how much vog impacts an area are wind direction and speed. Air temperature, humidity, rainfall, location of the source, and the amount of sulfur dioxide (SO2) being emitted are also factors. During prevailing trade (from northeast) wind conditions, any SO2 emitted from Pu`u `Ō`ō is blown out to sea, while any SO2 from the summit vent often creates vog in Ka...
Instruments to measure sulfur dioxide and carbon dioxide can be mounted in aircraft to determine the quantity of gas being emitted on a daily basis. Such instruments can also be used in a ground-based mode. An instrument that detects carbon dioxide can be installed on a volcano and configured to send data continuously via radio to an observatory. Sulfur dioxide in volcanic clouds can also be...
The U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) determines the amount and composition of gases emitted by Kīlauea Volcano. Changes in gas emissions can reveal important clues about the inner workings of a volcano, so they are measured on a regular basis. HVO scientists use both remote and direct sampling techniques to measure compositions and emission rates of gas from Kīlauea...
There are 161 potentially active volcanoes in the United States. According to a 2018 USGS assessment, 57 volcanoes are a high threat or very high threat to public safety. Many of these volcanoes have erupted in the recent past and will erupt again in the foreseeable future. As populations increase, areas near volcanoes are being developed and aviation routes are increasing. As a result, more...
Kīlauea Volcano is renowned for its relatively benign eruptions of fluid lava flows. Therefore, many people were surprised by the small explosions that occurred in Halema`uma`u Crater in 2008 and 2018, and even more surprised to learn that volcanic ash was being erupted from a new gas vent. However, ash emissions from Halema`uma`u Crater are part of the volcano's legacy. Kīlauea's summit has...
Volcanoes are inherently beautiful places where forces of nature combine to produce awesome events and spectacular landscapes. For volcanologists, they're FUN to work on! Safety is, however, always the primary concern because volcanoes can be dangerous places. USGS scientists try hard to understand the risk inherent in any situation, then train and equip themselves with the tools and support...
This photograph shows the inside of the USGS Hawaiian Volcano Observatory (HVO) Multi-Gas site at Sulfur Cone high up on Mauna Loa's Southwest Rift Zone. Volcanic gas (Sulfur Dioxide, Hydrogen Sulfide, Carbon Dioxide, and Water vapor) concentrations are measured then telemetered back to HVO to be monitored and analyzed by HVO gas scientists. USGS photo by C.
This photograph shows the inside of the USGS Hawaiian Volcano Observatory (HVO) Multi-Gas site at Sulfur Cone high up on Mauna Loa's Southwest Rift Zone. Volcanic gas (Sulfur Dioxide, Hydrogen Sulfide, Carbon Dioxide, and Water vapor) concentrations are measured then telemetered back to HVO to be monitored and analyzed by HVO gas scientists. USGS photo by C.
On July 29, 2026, USGS Hawaiian Volcano Observatory gas scientists flew via helicopter to the Sulfur Cone Multi-Gas instrumentation site on Mauna Loa's Southwest Rift Zone, which is at an elevation of 3,430 meters (11,240 feet) above sea level.
On July 29, 2026, USGS Hawaiian Volcano Observatory gas scientists flew via helicopter to the Sulfur Cone Multi-Gas instrumentation site on Mauna Loa's Southwest Rift Zone, which is at an elevation of 3,430 meters (11,240 feet) above sea level.
A USGS Hawaiian Volcano Observatory volcanic gas specialist sets up a Fourier Transform Infrared spectrometer, or FTIR, near Kīlauea summit during episode 46 of lava fountaining on May 5, 2026.
A USGS Hawaiian Volcano Observatory volcanic gas specialist sets up a Fourier Transform Infrared spectrometer, or FTIR, near Kīlauea summit during episode 46 of lava fountaining on May 5, 2026.
USGS scientists use an FTIR (Fourier transform infrared spectrometer) to measure the chemical composition of volcanic gas during episode 28 of the ongoing Kīlauea summit on July 9, 2025. USGS photo by M. Cappos.
USGS scientists use an FTIR (Fourier transform infrared spectrometer) to measure the chemical composition of volcanic gas during episode 28 of the ongoing Kīlauea summit on July 9, 2025. USGS photo by M. Cappos.
This photograph shows the inside of the Hawaiian Volcano Observatory (HVO) Multi-Gas site at Sulfur Cone. Currently, HVO has 2 of these continuous gas monitoring sites in operation on the Island of Hawai‘i. This one at Sulfur Cone high on Mauna Loa's Southwest Rift Zone, along with one at the summit of Kīlauea.
This photograph shows the inside of the Hawaiian Volcano Observatory (HVO) Multi-Gas site at Sulfur Cone. Currently, HVO has 2 of these continuous gas monitoring sites in operation on the Island of Hawai‘i. This one at Sulfur Cone high on Mauna Loa's Southwest Rift Zone, along with one at the summit of Kīlauea.
Title: What on Earth is going on at Kilauea Volcano?
Title: What on Earth is going on at Kilauea Volcano?
USGS geologist Laura Clor (right) and Rachel Teasdale (California State University – Chico, left) collect gas samples from a thermal feature at Sulphur Works in Lassen Volcanic National Park.
USGS geologist Laura Clor (right) and Rachel Teasdale (California State University – Chico, left) collect gas samples from a thermal feature at Sulphur Works in Lassen Volcanic National Park.
USGS geologist Deborah Bergfeld collects a gas sample from a superheated (hotter than the boiling point) fumarole in Little Hot Springs Valley at Lassen Volcanic National Park.
USGS geologist Deborah Bergfeld collects a gas sample from a superheated (hotter than the boiling point) fumarole in Little Hot Springs Valley at Lassen Volcanic National Park.
Hawaiian Volcano Observatory Geochemist Jeff Sutton and CSAV international volcanology students visit a continuous gas monitoring site on Kilauea's east rift zone during field studies portion of the summer training course.
Hawaiian Volcano Observatory Geochemist Jeff Sutton and CSAV international volcanology students visit a continuous gas monitoring site on Kilauea's east rift zone during field studies portion of the summer training course.
Hawaiian Volcano Observatory Geochemist Jeff Sutton and CSAV international volcanology students visit a continuous gas monitoring site on Kilauea's east rift zone during field studies portion of the summer training course.
Hawaiian Volcano Observatory Geochemist Jeff Sutton and CSAV international volcanology students visit a continuous gas monitoring site on Kilauea's east rift zone during field studies portion of the summer training course.
Sulfur dioxide gas emissions from the crater of Pu‘u ‘Ō ‘ō on Kīlauea’s east rift zone and the vent within Halema‘uma‘u Crater at Kīlauea’s summit create volcanic pollution that affects the air quality of downwind communities. Here, a USGS Hawaiian Volcano Observatory gas geochemist measures Pu‘u ‘Ō‘ō gas emissions using an instrument that detects ga
Sulfur dioxide gas emissions from the crater of Pu‘u ‘Ō ‘ō on Kīlauea’s east rift zone and the vent within Halema‘uma‘u Crater at Kīlauea’s summit create volcanic pollution that affects the air quality of downwind communities. Here, a USGS Hawaiian Volcano Observatory gas geochemist measures Pu‘u ‘Ō‘ō gas emissions using an instrument that detects ga
This photo was taken from the lamp on the other side of the plume. The FTIR is the small dark silhouette on the rim across the crater gap.
This photo was taken from the lamp on the other side of the plume. The FTIR is the small dark silhouette on the rim across the crater gap.
HVO gas geochemists deployed a FTIR spectrometer on the east rim of Pu`u `Ō `ō crater. The FTIR measures the composition of the East Wall vent gases by "looking" through the plume at an infrared lamp (obscured by fume in this photo)
HVO gas geochemists deployed a FTIR spectrometer on the east rim of Pu`u `Ō `ō crater. The FTIR measures the composition of the East Wall vent gases by "looking" through the plume at an infrared lamp (obscured by fume in this photo)
A gas plume arising from Augustine Volcano during it's eruptive phase 2005-06. This photo was taken during a FLIR/maintenance flight on January 24, 2006.
A gas plume arising from Augustine Volcano during it's eruptive phase 2005-06. This photo was taken during a FLIR/maintenance flight on January 24, 2006.
USGS geochemist Bill Evans measures the temperature of a superheated (hotter than the boiling point) fumarole in Lassen Volcanic National Park.
USGS geochemist Bill Evans measures the temperature of a superheated (hotter than the boiling point) fumarole in Lassen Volcanic National Park.
USGS hydrologist Michael Sorey tries to locate the steam vent at a fumarole in the Bumpass Hell area in Lassen Volcanic National Park.
USGS hydrologist Michael Sorey tries to locate the steam vent at a fumarole in the Bumpass Hell area in Lassen Volcanic National Park.
USGS geochemist Cathy Janik (left) and Iceland Geosurvey chemist Jón Örn Bjarnason (right) collect a gas sample from a fumarole in Lassen Volcanic National Park.
USGS geochemist Cathy Janik (left) and Iceland Geosurvey chemist Jón Örn Bjarnason (right) collect a gas sample from a fumarole in Lassen Volcanic National Park.
USGS geologists gathered samples by hand from vents on the dome and crater floor. Additionally, sulfur dioxide gas was measured from a specially equipped airplane before, during, and after eruptions to determine "emission rates" for the volcano.
USGS geologists gathered samples by hand from vents on the dome and crater floor. Additionally, sulfur dioxide gas was measured from a specially equipped airplane before, during, and after eruptions to determine "emission rates" for the volcano.
ARRA-funded student Taryn Lopez (Univ. Alaska-Fairbanks) sampling gas emissions at fumarole next to dome at the summit of Augustine volcano.
ARRA-funded student Taryn Lopez (Univ. Alaska-Fairbanks) sampling gas emissions at fumarole next to dome at the summit of Augustine volcano.
Taryn Lopez measuring the temperature of gas emissions near summit of Augustine Volcano.
Taryn Lopez measuring the temperature of gas emissions near summit of Augustine Volcano.