Vog is the haze that forms when sulfur dioxide from an erupting volcano reacts in the air and drifts downwind as sulfate aerosol. Kīlauea has been erupting episodically at Halemaʻumaʻu since December 23, 2024, and between fountaining episodes the summit still releases roughly 1,000 to 5,000 tonnes of sulfur dioxide a day. For most visitors this is a flattened view and a scratchy throat. For sensitive groups it is a health issue. It is predictable enough to plan around.
Vog is a hazy mixture of sulfur dioxide gas and aerosols, which are primarily sulfuric acid and other sulfate compounds. The aerosols form when volcanic gases interact chemically with oxygen, moisture, dust, and sunlight over minutes to days. Far from the vents, aerosols are the main component of vog; closer to the volcano it contains both aerosols and unreacted sulfur dioxide gas.
Kīlauea has been erupting episodically since December 23, 2024, from two vents in Halemaʻumaʻu, and more than fifty fountaining episodes have occurred. Episodes generally last less than 12 hours and are separated by pauses that can be longer than three weeks. Between episodes, the summit sulfur dioxide emission rate typically varies within a range of 1,000 to 5,000 tonnes per day, and emissions can remain locally hazardous immediately downwind of the vents even when the vents are not actively erupting.
The main wind direction in the Hawaiian Islands is from the northeast. From May through September the trade winds blow 80 to 95 percent of the time. 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. The leeward side people book for the sunshine is the side that collects the haze.
When trade winds are absent, most often during winter months, East Hawaiʻi Island, the entire island, or even the entire state can be affected. Most vog generated by Kīlauea stays below 6,000 to 8,000 feet above sea level, the usual height of the trade wind inversion, which is why the summits of Mauna Loa and Mauna Kea often sit above the vog layer.
USGS names the sensitive groups most likely to experience health impacts: people with asthma, lung or heart problems, older adults, infants and children, and pregnant women. The Hawaiʻi Department of Health adds outdoor workers and people exercising outdoors. Physically active asthmatics are most likely to experience serious effects from sulfur dioxide, and even short-term exposure can narrow the airways. If you have asthma, keep your medication available and use it as prescribed.
The short-term symptoms listed for vog exposure are eye, nose, throat, and skin irritation, coughing or phlegm, chest tightness or shortness of breath, headache, aggravation of asthma, fatigue or dizziness, and nausea. At sulfur dioxide levels considered unhealthy for the general population, even non-asthmatics may experience breathing difficulties.
Sulfur dioxide and PM2.5 are measured inside Hawaiʻi Volcanoes National Park and across Hawaiʻi Island by the state Department of Health Clean Air Branch, the National Park Service, and the USGS Hawaiian Volcano Observatory, with a public network of low-cost PurpleAir sensors filling gaps. The Hawaiʻi Interagency Vog Information Dashboard collects those readings alongside vog forecast tools. It is a collaboration of the International Volcanic Health Hazard Network, the state Department of Health, USGS Hawaiian Volcano Observatory, Hawaiʻi County Civil Defense, the National Park Service, the County of Hawaiʻi, the Center for the Study of Active Volcanoes at the University of Hawaiʻi, NOAA's National Weather Service, and the USDA Farm Service Agency.
The practical version: check the dashboard or AirNow before you commit to a day, look at wind direction, and put the hard outdoor thing on a day the trades are running.
Standard guidance for a high-vog day is to reduce outdoor activities, go indoors and close all doors and windows, seal gaps under doors and around windows, set air conditioners to recirculate, keep medications on hand and use them as prescribed, and drink plenty of liquids. An appropriate air-cleaning device reduces sulfur dioxide and PM2.5 in a sealed room or home.
What does not work is a mask. The Hawaiʻi Department of Health does not recommend the use of respirators by the general public to protect against volcanic gases. N95 masks can help protect against volcanic ash but not against gases such as sulfur dioxide, and most common masks and bandanas will not provide substantial protection from PM2.5 or sulfur dioxide. Facial hair prevents a tight seal, and N95s carry their own risks for children and people with respiratory conditions. The Department of Health does recommend N95 masks, heavy-duty gloves, long clothing, shoes, and eye protection for cleanup in ash-affected areas. If you drive near volcanic emissions, temporarily close your windows and turn the fan and air conditioner off.
The eruption is occurring within a closed area of the park. The park runs an air quality alert page and advises checking air quality frequently during your visit, because hazardous volcanic gases can present a danger to sensitive groups. Rangers close areas when conditions warrant; follow ranger instructions and posted closures.
Lava fountains create tephra: small glassy volcanic fragments including volcanic ash, pumice, scoria, Pele's hair, and reticulite. Larger particles fall near the vents while light particles can be carried greater distances, and they can be remobilized during windy conditions after a recent episode. A light fall of Pele's hair was reported at several sites in the park outside the closed area after episode 51, including Uēkahuna overlook and the Kaʻū Desert trailhead on Highway 11. USGS advises minimizing exposure to these fragments, which can cause skin, eye, and respiratory irritation.
The park is partway through a two-year construction project at the summit to repair or remove damaged buildings and infrastructure, so expect closures and delays. Entrance is $30 per private vehicle, valid seven days, and the park does not accept cash. Parking lots can be full by 10 a.m.