Dr. Melissa Anne Pfeffer, atmospheric volcanologist at the Icelandic Volcano Observatory, explains:
Volcanoes in Iceland emit different gases while they are erupting and in between eruptions. The gases can affect air and water, with impacts on human, animal, and plant health. The recent Bárðarbunga eruption, which lasted for six months during 2014-2015, released a tremendous amount of the pollutant SO2 (sulfur dioxide). To put this in context, this six-month eruption released more SO2 than the entire European Economic Area in 2011. Hourly averaged health limit standards were exceeded many times in the town closest to the eruption, about 100 km away, and also in Reykjavík, 250 km away.
SO2 can irritate the eyes, throat, and respiratory tract. Close to the erupting lava, gas concentrations were sometimes greater than even the gas masks worn by the scientists and other essential people near to the eruption could filter out, requiring these people to move away from the high gas levels. CO2 (carbon dioxide) is released by volcanoes in Iceland both during and in between eruptions. CO2 is heavier than air, and can accumulate in low-lying areas, replacing the oxygen we need to breathe.
During the 1973 Vestmannaeyjar eruption on Heimaey, a person died from CO2 which had accumulated inside a building. H2S (hydrogen sulfide) is released by Icelandic volcanoes, mainly in between eruptions. This gas has some health impacts due to long-term low concentration exposure, and in high concentrations, it can cause many different problems to the eyes and respiratory tract. H2S is often released during volcanogenic floods from beneath glaciers, or jökulhlaups, perhaps the most famous Icelandic word in international use. In high enough concentrations, H2S can cause blindness.