Vog is volcanic smog created by gases from Hawaii’s active volcanoes, primarily Kilauea on the Big Island. It forms when sulfur dioxide gas reacts with oxygen, moisture, and sunlight in the atmosphere, producing a hazy mix of sulfuric acid droplets and other sulfate particles. These reactions happen over minutes to days, meaning vog can drift far from its volcanic source and blanket large portions of the Hawaiian Islands.
How Vog Forms and What It Contains
Kilauea Volcano continuously releases sulfur dioxide (SO2) into the air, even between eruptions. When that gas hits the atmosphere, it undergoes chemical reactions with moisture and sunlight to create tiny aerosol particles, primarily sulfuric acid. These particles are small enough to stay suspended in the air for days and travel hundreds of miles on trade winds.
The result is a visible haze that can range from a light film over the sky to a thick, smoggy blanket that reduces visibility dramatically. During Kilauea’s active fountaining phases, emissions can reach tens of thousands of tonnes of SO2 per day. Even during quieter periods, emission rates stay well above background levels. The December 2024 summit eruption, for example, produced highly variable emissions, with intense fountaining episodes separated by pauses of days to weeks.
Where Vog Travels Across the Islands
Trade winds from the northeast typically push vog to the southwest side of the Big Island first, making the Kona and Ka’u districts the most consistently affected areas. When trade winds weaken or reverse, vog can spread across the entire island chain, reaching Maui, Oahu, and beyond. On calm days with little wind, the haze settles into valleys and low-lying areas and can persist for days.
Health Effects Beyond Sore Throats
The most common symptoms from vog exposure are irritated eyes, a scratchy throat, headaches, and persistent coughing. A study of schoolchildren on Hawaii Island found that chronic exposure to acidic vog was associated with increased coughing and signs of reduced lung function, though not with higher rates of asthma or bronchitis diagnoses. For most healthy people, vog causes temporary discomfort that clears when air quality improves.
The more serious risks are for people with existing health conditions. Vog regularly worsens asthma, triggering flare-ups that can send people to the emergency room. During Kilauea’s major 2018 eruption, the Air Quality Index exceeded 150 on multiple occasions, a level classified as “unhealthy” even for people without pre-existing conditions.
What often goes unrecognized is the cardiovascular risk. Fine particulate matter like the kind found in vog accounts for over 60% of air-pollution-related deaths, and those deaths are predominantly cardiovascular. Short-term exposure to these particles can trigger heart attacks, strokes, arrhythmias, and heart failure. People with high blood pressure or diabetes face added concern, since fine particle pollution can worsen control of blood pressure and blood sugar. Native Hawaiian and Pacific Islander communities, who already experience higher rates of obesity and diabetes, bear a disproportionate burden of these risks.
Damage to Plants, Crops, and Property
Sulfuric acid droplets in vog have the corrosive properties of dilute battery acid. When vog settles directly on plant leaves, it causes chemical burns that can damage or kill vegetation. SO2 gas can also penetrate leaves and create acidic conditions inside plant tissue. Farmers in the Ka’u District on the Big Island have reported significant losses to agricultural crops and flowers during periods of high emissions.
The damage extends to buildings and infrastructure. Vog-driven acid rain corrodes metal roofing, gutters, car paint, and outdoor equipment. Over time, this corrosion creates a costly maintenance burden for residents on the most affected parts of the island.
The Drinking Water Problem
Many homes on the Big Island rely on rooftop rainwater catchment systems for their drinking water, and vog turns that rain acidic. The acid then leaches lead and copper from roofing materials, nails, paint, solder, and metal flashings into the water supply. A 1988 survey found that nearly 40% of homes using catchment systems in the Kona Districts had drinking water contaminated with lead. This remains an ongoing concern for households that depend on collected rainwater, particularly during periods of heavy volcanic emissions.
How to Protect Yourself
The most effective strategy during heavy vog is staying indoors with windows and doors closed. Setting air conditioners to recirculate (rather than drawing in outside air) helps keep indoor air cleaner. Hawaii’s Department of Health specifically advises reducing outdoor activities when volcanic gas levels are elevated.
Masks are a common question, and the answer is more nuanced than people expect. A damp cloth, surgical mask, or N95 respirator can filter out volcanic ash and larger particulates. However, none of these masks remove SO2 gas, which is one of the primary irritants in vog. An N95 will reduce your exposure to the fine sulfate particles, but it won’t protect you from the gaseous component. Hawaii’s Department of Health does not recommend respirator use by the general public for volcanic gases, partly because ill-fitting masks can actually make breathing harder without providing meaningful gas protection.
For people with asthma or other respiratory conditions, keeping rescue inhalers accessible and monitoring Hawaii’s air quality index (available through the state’s Department of Health website) provides the best day-to-day guidance. On days when the index climbs above 100, limiting time outdoors makes a meaningful difference. Households using rainwater catchment should consider filtration systems designed to remove heavy metals, especially during sustained periods of volcanic activity.