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Vog is a form of air pollution that results when sulfur dioxide and other gases and particles emitted by an erupting volcano react with oxygen and moisture in the presence of sunlight. The word is a portmanteau of the words "volcanic", "smog", and "fog". The term is in common use in the Hawaiian islands, where the Kīlauea volcano, on the Island of Hawaiʻi (aka "The Big Island"), has been erupting continuously since 1983. Based on June 2008 measurements, Kīlauea emits 2,000–4,000 tons of sulfur dioxide every day.
Vog is created when volcanic gases (primarily oxides of sulfur) react with sunlight, oxygen and moisture. The result includes sulphuric acid and other sulfates. Vog is made up of a mixture of gases and aerosols which makes it hard to study and potentially more dangerous than either on their own.
In Hawaii, the gas plumes of Kīlauea rise up from three locations: Halemaʻumaʻu Crater, Puʻu ʻŌʻō vent, and from along the coastline where lava flows from the East Rift zone enter the ocean. The plumes create a blanket of vog that can envelop the island. Vog mostly affects the Kona coast on the west side of the Island of Hawaiʻi, where the prevailing trade winds blow the vog to the southwest and southern winds then blow it north up the Kohala coast.
Prolonged periods of southerly Kona winds, however, can cause vog to affect the eastern side of the Island on rare occasions, and affect islands across the entire state as well. By the time the vog reaches other islands, the sulfur dioxide has largely dissipated, leaving behind ash, smoke, sulfates, and ammonia.
Vog and smog are different. Vog is formed when sulfur oxides emitted by a volcano react with moisture to form an aerosol. The aerosol scatters light and so makes the vog visible. Smog is formed largely from the incomplete combustion of fuel, reacting with nitrogen oxides and ozone produced from carbon monoxide by reactions with sunlight. The result is also a visible aerosol.
When smog levels are high the sky looks yellowish grey because nitrogen oxides are yellow. In contrast, sulfur oxides are colorless and vog looks grey. Once vog dissipates, grey spots in the sky may for a time remain trapped in the inversion layer.
It is also important to note that the same chemicals are emitted from volcanoes as from cars.
Most studies of vog have been in areas where vog is naturally present, and not in controlled conditions. Vog contains chemicals that can damage the environment, and the health of plants, humans and other animals. Most of the aerosols are acidic and of a size where they can remain in the lungs to damage the lungs and impair function. Headaches, watery eyes, sore throat, breathing difficulties (including inducing asthma attacks), flu-like symptoms, and general lethargy are commonly reported. These effects are especially pronounced in people with respiratory conditions and children. Vog generally reduces visibility, creating a hazard for drivers, and for air and ocean traffic.
The long-term health effects of vog are unknown. Prof. Bernadette Longo has been investigating health impact of vog on Big Island and confirmed cardiac issues, including increased pulse rates associated with thickened blood from PM 2.5 particles.
Several studies are underway to measure the air quality near volcanoes more carefully. Sulfur dioxide emissions increased on March 12, 2008, when a new vent opened. The increased vog level has caused evacuations and damaged crops. In the summer of 2008 and in 2012, the County of Hawaiʻi received a disaster designation due to the agricultural damage.