Vog is volcanic smog, a hazy air pollution created when sulfur dioxide gas and other volcanic emissions react with oxygen, moisture, dust, and sunlight in the atmosphere. It’s most closely associated with Kīlauea Volcano on Hawaiʻi Island, where ongoing eruptions can release large volumes of sulfur dioxide that drift across the Hawaiian Islands and create a visible, sometimes irritating haze.
How Vog Forms
Volcanic eruptions release sulfur dioxide (SO2) directly into the air. Once airborne, that gas doesn’t stay in its original form for long. It reacts chemically with oxygen, water vapor, and dust particles to produce fine aerosols, primarily sulfuric acid droplets and other sulfate compounds. This conversion has a half-life of roughly six hours, meaning the gas steadily transforms into a cloud of tiny, breathable particles as it travels downwind from the volcanic source.
The result is a mixture of residual SO2 gas and extremely small particles (classified as PM2.5, meaning they’re less than 2.5 micrometers across). These particles are so fine they can stay suspended in the air for days and travel hundreds of miles from the eruption site. Sunlight accelerates the chemical reactions, so vog tends to be most concentrated during clear, sunny conditions with light winds.
How Vog Differs From City Smog
Vog and urban smog look similar from a distance, but their chemistry is quite different. Industrial smog is a cocktail of pollutants: ozone, hydrocarbons, nitrogen oxides, and heavy metals, all generated by cars, factories, and power plants. Vog lacks most of those contaminants. Its primary components are sulfur dioxide gas and sulfuric acid aerosols, both originating from a single natural source.
That doesn’t make vog harmless. Sulfuric acid particles are corrosive and can penetrate deep into the lungs. But the narrower chemical profile means vog behaves differently in the body than the complex pollution found in major cities.
Health Effects of Breathing Vog
Sulfur dioxide gas irritates skin, eyes, nose, throat, and airways on contact. At the concentrations found near active vents, it can cause immediate respiratory distress. Farther from the source, the fine aerosol particles become the bigger concern. Because they’re small enough to reach the deepest parts of the lungs, they can trigger asthma symptoms and aggravate existing respiratory conditions like chronic bronchitis.
Common physical complaints during heavy vog episodes include headaches, breathing difficulties, watery eyes, sore throat, flu-like symptoms, and a persistent lack of energy. People with preexisting lung or heart conditions tend to experience the worst symptoms, as do young children, whose smaller airways are more easily irritated. Even healthy adults often report feeling generally unwell on high-vog days, particularly if they spend extended time outdoors.
Wind Patterns Control Where Vog Goes
On most days, Hawaiʻi’s prevailing trade winds blow from the northeast, pushing Kīlauea’s emissions to the southwest and out over the ocean. The vog then curves around the southern tip of the island and drifts up the leeward (western) coast, which is why communities like Kailua-Kona and the Kohala Coast frequently experience hazy skies even though they’re far from the volcano.
When trade winds weaken or reverse into what locals call Kona winds (blowing from the south or southwest), vog can blanket the entire island chain, reaching Oʻahu, Maui, and beyond. These Kona wind episodes are when air quality across the state drops most dramatically. Eruption intensity matters too: during Kīlauea’s 2018 lower East Rift Zone eruption, SO2 emissions surged to levels that made parts of the Big Island dangerous to breathe in without protection.
Impact on Agriculture and the Environment
Vog doesn’t just affect lungs. When volcanic gases mix with rain, they produce acid rain that damages crops, alters soil chemistry, and corrodes metal roofing and outdoor equipment. Acid falling on plant surfaces disrupts photosynthesis and water absorption. Over time, repeated acid rain shifts soil pH and strips nutrients, making it harder for crops to survive even between vog episodes.
Farmers on the Big Island’s Kona coffee belt and other agricultural regions have reported leaf burn, reduced yields, and soil degradation tied to prolonged vog exposure. Ornamental plants and native forests are also vulnerable, particularly species adapted to Hawaiʻi’s naturally low-sulfur environment.
How Vog Levels Are Tracked
The Hawaii Interagency Vog Information Dashboard (available at vog.ivhhn.org) provides real-time monitoring and forecasts. It combines data from air quality sensors across the islands with wind forecast models (known as VMAP) that predict where vog will travel over the next several days. The dashboard displays current SO2 and particulate levels, color-coded by health risk, so residents can decide whether to limit outdoor activity.
For context on what those numbers mean: the EPA’s primary air quality standard for sulfur dioxide is 75 parts per billion averaged over one hour. The agency also recently set a secondary annual standard of 10 ppb to protect ecosystems and visibility. During active eruptions, monitors near Kīlauea can record readings well above the hourly limit, while communities dozens of miles downwind may hover near or above the annual threshold for weeks at a time.
Protecting Yourself on High-Vog Days
Staying indoors with windows closed is the most effective way to reduce exposure during heavy vog. Running an air conditioner set to recirculate (rather than drawing in outside air) helps filter particles. Standalone air purifiers with HEPA filters capture the fine sulfate particles that make up most of vog’s particulate load.
If you need to be outside, N95 masks filter PM2.5 particles effectively, though they do little against SO2 gas itself. People with asthma should keep rescue inhalers accessible and monitor the vog dashboard before planning outdoor exercise. Contact lens wearers often find that switching to glasses on bad vog days reduces eye irritation significantly, since lenses can trap acidic particles against the surface of the eye.