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Frequently Asked Questions About Volcanic Smog (vog)

By Hawaiian Volcano Observatory
Two color photographs, one showing a clear-day rock-wall face and one showing the rock-wall face obscured by volcanic gases.
Sources/Usage: Public Domain. View Media Details
Side-by-side comparison of the northwest wall of Kīlauea Caldera on a clear day (left) and a day with thick vog (right). HVO observation tower and building can be seen near the center in each photo. Photos taken in 2010. (Public domain.)

1. What is "vog"? How is it related to SO2 emissions?

Vog is a hazy mixture of SO2 gas and aerosols (tiny particles or droplets) which are primarily sulfuric acid 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 minutes to days.

The exact composition of vog depends on how much time the volcanic plume has had to react in the atmosphere. In areas far (tens to hundreds of km or miles) from active vents, aerosols are the main component of vog. Closer to the volcano, vog contains both aerosols and unreacted SO2 gas.

2. What controls vog on the Island of Hawai‘i?

Color photograph showing the summit of a volcano above volcanic gas interacting with the atmosphere
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The summit of Mauna Loa (left) rises above the vog layer on the leeward side of the Island of Hawai‘i in November 2012. The flank of Hualālai is visible at right. (Public domain.)

Vog concentrations are primarily dependent on the amount of SO2 emitted from a volcano, the distance downwind, and the wind direction and speed on a given day. The main wind direction in the Hawaiian Islands is from the northeast (trade winds). From May through September, the trade winds blow 80–95 percent of the time.

When Kīlauea is erupting (as was the case from 1983–2018), areas to the southwest of the volcano are most frequently affected by vog. Under trade wind conditions, vog travels around the southern part of the island, and along the Kona coast, where it becomes trapped by daytime onshore and nighttime offshore sea breezes. Most vog generated by Kīlauea stays below 6,000–8,000 feet above sea level, the usual height of the trade wind inversion. This layer of the atmosphere increases in temperature with altitude, inhibiting the rise of cooler, vog-laden air. When trade winds are absent, most often during winter months, East Hawai‘i Island, the entire Island of Hawai‘i, or the even the entire State of Hawaii can be affected by vog. Eruptions of other Hawaiian volcanoes will create vog impact dependent on the altitude and location of the eruptive vent.

Communities in the path of SO2 emissions, particularly those nearest the source vent, can be subjected to a very acrid haze that contains both SO2 gas and acidic particles. The near-vent volcanic plume has had little time to disperse and dilute before reaching downwind communities tens of km or miles.

3. What health effects are associated with vog?

Volcanic activity in Hawaii creates the potential for airborne health hazards to residents and visitors. Currently, the smell of volcanic gas (sulfur dioxide and hydrogen sulfide) can be detected near Kīlauea's gas-emitting vents, particularly when the trade winds are absent. However, negative health symptoms generally occur at gas concentrations well above current levels. An eruption of Mauna Loa may contribute to local air quality issues, depending on the size and location of the eruption. 

It is very important to take measures to protect yourself if you feel your health is being affected by volcanic emissions or vog. "Sensitive groups" most likely to experience health impacts include:

  • people with asthma, lung or heart problems
  • older adults
  • infants and children
  • pregnant women


SO2: Physically active asthmatics are most likely to experience serious health effects when exposed to sulfur dioxide—SO2. Even short-term exposures can cause narrowing of the airways (bronchoconstriction), causing asthma symptoms. Potential health effects increase as SO2 levels and/or breathing rates increase. At SO2 levels considered 'unhealthy' for the general population, even non-asthmatics may experience breathing difficulties. Short-term SO2 exposure is connected to increased visits to emergency departments and hospital admissions for respiratory illnesses, particularly in "sensitive groups". The long-term health effects of persistent exposure to low concentrations of volcanic SO2 are still being evaluated. 

Short-term health symptoms include:

  • eye, nose, throat, and/or skin irritation
  • coughing and/or phlegm
  • chest tightness and/or shortness of breath
  • headache
  • aggravation of asthma
  • fatigue and/or dizziness
  • naseau


H2S: The temporary physical symptoms associated with exposure to hydrogen sulfide—H2S are the same as those noted above for SO2 exposure. Exposure to high concentrations (more than 100 ppm) of H2S, or long periods spent at low concentrations, can cause a person’s sense of smell to become fatigued so the ‘rotten egg’ aroma can no longer be detected.

PM2.5: Both long- and short-term particle exposures have been linked to various health problems. High levels of particle pollution are linked to increased hospital admissions and emergency room visits, and even to death from existing heart or lung disease. Low levels of PM2.5 are not considered as problematic for asthmatics as low levels of SO2 gas. 

Particle pollution can cause temporary health symptoms such as:

  • eyes, nose and/or throat irritation
  • coughing and/or phlegm
  • chest tightness and/or shortness of breath

An indirect health effect of vog is associated with the leaching of metals from building and plumbing materials. Sulfuric acid in vog creates acid rain, which can leach lead and copper from roofing and plumbing materials, such as nails, paint, solder, and metal flashings. Leached lead and copper can pose a health hazard when they contaminate drinking water in rooftop rainwater-catchment systems.

4. Where can I find results from scientific research or technical data about the effects of vog on human health?

Find a detailed list of scientific articles from the Hawaii Interagency Vog Information Dashboard.

5. Does vog impact plants and animals?

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 chemical burns, which can damage or kill the plants. Sulfur dioxide (SO2) gas can also diffuse through leaves and dissolve to form acidic conditions within plant tissue. Farmers on Hawai‘i Island, particularly in the Ka‘ū District, have reported losses to agricultural crops and flowers as a result of the recent high SO2 emissions from the gas vent at Kīlauea's summit.

6. Who monitors air quality and vog in Hawai‘i, and where can I find current information?

The agency primarily in charge of monitoring Hawai‘i's air quality, including vog and its effects on people, is the Hawai‘i State Department of Health (HDOH). Air Quality is monitored by measuring the concentration of SO2 gas and inhalable particulate matter smaller than 2.5 microns in diameter (PM2.5). Stationary air quality monitors that measure particulate levels are located in Hilo, Kea‘au, Mountain View, Pahala, Ocean View, Kona, and Waikoloa on the Island of Hawai‘i, and on Maui, O‘ahu, and Kaua‘i. DOH also has air monitoring stations for SO2 on the islands of Hawai‘i, O‘ahu, and Kaua‘i.

The National Park Service (NPS) monitors air quality within Hawai‘i Volcanoes National Park. You can find air quality monitoring data and links to additional resources from the Hawai‘i Interagency Vog Information Dashboard.

7. Why are some roads and trails in Hawai‘i Volcanoes National Park closed?

Hawai‘i Volcanoes National Park monitors SO2 within park boundaries to inform the public of hazardous conditions and to mandate closures if necessary. Roads and trails downwind of and leading to Halema‘uma‘u Crater were closed by park officials in February 2008, when SO2 emissions and ambient air concentrations reached levels that were potentially hazardous to human health. These areas remained closed because SO2 emissions and downwind concentrations remained elevated during the Overlook vent eruption, which ended in 2018. Following the 2018 Kīlauea summit collapse, hazardous areas of Kīlauea's summit remain closed. You can find updated information about closed areas on the National Park's website.

8. What can I do to protect myself from vog?

Color image of a booklet cover
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The "Frequently Asked Questions on Vog from Kīlauea Volcano" booklet is one of the many information products available from the Vog Information Dashboard. (Public domain.)

Vog is a hazy mixture of sulfur dioxide gas (SO2) and particles (PM2.5) originating from Kīlauea Volcano. Sensitivity to vog varies among people, so all people need to take responsibility to protect themselves and dependent family members, as necessary. Individuals in 'sensitive groups,' such as people with pre-existing respiratory or other medical conditions, are expected to be at highest risk for potential health effects, depending on the amount of emissions, distance away from the vent, and wind direction from day to day.

Below are suggested actions for limiting impacts of vog: 
 

Prepare for vog exposure:

  • Understand the hazard: Get familiar with key air monitoring websites and the SO2 and PM2.5 advisory codes/levels.
  • Learn about wind conditions: Be aware of winds that could carry vog to your area. This will help you to better keep track of, and predict, when you might be affected by vog.
  • 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.

Protective actions when vog is a problem:

  • Seek medical assistance as necessary: Assume that asthma could get worse during periods of high vog. If you are having asthma symptoms, such as trouble breathing, wheezing, increased coughing, or chest tightness, contact your doctor or seek other medical assistance.
  • Take care of yourself:
    • Do not smoke: Also, avoid secondhand smoke.
    • Stay hydrated: Drink plenty of liquids to help loosen congestion. Warm or hot liquids in particular may help some people.
    • Manage congestion or irritation: Over-the-counter nasal sprays or eye drops can help reduce upper respiratory symptoms for some people.
  • Reduce your exposure to vog:
    • Limit strenuous activities: Outdoor exercise or exertion increases your chances of being affected by SO2 gas and/or PM2.5. When vog levels are elevated, reduce strenuous activity, if possible.
    • Stay indoors: When vog levels rise, go indoors and close all doors and windows to the outside. Eliminate sources of indoor pollutants (e.g., smoking, candles/incense, and improperly vented fuel-burning stoves) and beware of becoming overheated as a result of closing up your house. If your house is not well-sealed, it may still offer some protection. Alternatively, consider visiting indoor areas that are better-sealed and/or have air conditioning (e.g., commercial buildings or businesses).
    • Reduce indoor vog with an air cleaner: If doors and windows in your house, or in one room of it, can be closed, the use of an appropriate air-cleaning device can help reduce the levels of both SO2 and PM2.5 (if you live near the source vents), or just PM2.5 (if you live farther from the vents).
    • Leave the area if appropriate: If indoor areas also have poor air quality, consider temporarily relocating to a less impacted part of the island.
    • Restrict vog from entering your vehicle: If driving through the dense volcanic plume near Hawai‘i Volcanoes National Park, to minimize air infiltration, temporarily close your windows and vents, and turn your fan and air conditioner off.
  • If you are visiting Hawai‘i Volcanoes National Park (HVNP): Follow the advice of Park rangers and posted signs. The National Park Service (NPS) HVNP advisory website provides information on air quality in the park. See links below.
  • Respirator or face mask use: The Hawaii Department of Health (HDOH) does not recommend the use of respirators or face masks for the general public at current vog levels. 'Sensitive groups' such as children or people with pre-existing respiratory conditions should be especially cautious because respirators/masks typically do not fit children and the breathing resistance caused by respirators/masks can worsen respiratory disorders. Respirators/masks may not protect people with beards or mustaches. Many commonly available paper dust masks, bandanas, or surgical masks may not provide any significant protection from either SO2 or PM2.5. Safe occupational use of respirators requires correct mask and/or filter cartridge selection, fit testing, physician screening, and training on correct use, maintenance and storage.

Additional information

9. Should I cancel my plans to visit Hawai‘i because of SO2 and vog?

During non-eruptive conditions (for example, late 2018-present) gas emissions from Hawaii’s volcanoes are very low, and the air is generally clear of volcanic pollution.

During eruptive periods, individuals with pre-existing respiratory conditions are the primary group at risk of experiencing health effects from vog exposures, but healthy people may also experience symptoms. If you have existing heart and/or respiratory ailments or other conditions that compromise your physical health, or if you are pregnant, check with your personal physician for advice about traveling to any location with poor air quality. While vog does contain acidic particles (as well as elevated levels of SO2 gas at near-vent locations) it lacks other contaminants typically present in urban or industrial smog, such as ozone, hydrocarbons, nitrogen oxides, and heavy metals.

Advice for visitors to Hawai‘i Island is a resource created for visitors to find information and data about air quality, including in Hawai‘i Volcanoes National Park.

During eruptive periods, predicting vog concentrations for a short stay in Hawaii is as difficult as predicting the weather. Once volcanic gas is in the atmosphere, it is distributed by prevailing winds. Trade, or northeasterly, winds are dominant 80-95% of the time from May through September and can produce poor air quality on the southern and western sides of the island. The east or windward side is more likely to experience poor air quality during trade wind interruptions, which occur more frequently during the winter months. Where and how bad the vog is ultimately depends on wind direction, wind speed, air temperature, humidity, and rainfall as well as the location of the source and amount of SO2 being emitted.

Because of the proximity to areas that cold erupt, SO2 and other acid gas concentrations can reach unhealthy levels quickly in some areas within Hawai‘i Volcanoes National Park. Elevated vog levels, and poor air quality at the coast, are most common when trade winds are absent. Current air quality and wind conditions can be checked online prior to visiting the park, and people with preexisting respiratory conditions should have their medications available. All visitors should pay attention to park warnings and follow park advisories.

In general, to protect your health, take heed of all advisories released by Civil Defense and other relevant emergency management agencies.

During periods of volcanic eruption, whether or not you should cancel your trip to the Island of Hawai‘i is a personal decision that only you can make. To help you make an informed decision, information about vog and air quality in Hawaii is provided in the Frequently Asked Questions above, as well as on the IVHHN vog dashboard at ivhhn.org/vog. You can also monitor the volcanic gas plumes and real-time air quality conditions for yourself on local Webcams and air quality advisory websites.

10. How do other communities near active volcanoes deal with SO2 emissions?

In any given year, there are 50-70 volcanic eruptions around the world, during which water vapor (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), and other gases are released. Many volcanoes also emit gases between eruptions. The impact of these gases depends on the vigor and duration of the eruption, the proximity of the volcano to populated areas, and wind speed and direction.

Examples of volcanoes that have adversely affected people due to persistent SO2 emissions include Nyiragongo (DR Congo); Masaya (Nicaragua); Poas (Costa Rica); and Miyakejima, Aso, and Sakurajima (Japan).