Carbon dioxide (CO2) is a colourless and odourless gas. It is non-flammable and chemically non-reactive (Sax and Lewis, 1989). CO2 is 1.5 times as heavy as air (its density is 1.80 g L-1 at 25°C and 1 atm (Lide, 2003)) and, if it is emitted slowly, flows down-slope and may accumulate at low elevations. Concentration ranges of CO2 in dilute volcanic plumes can range from 1 ppm to hundreds of ppm above the tropospheric background of ~360 ppm (T. Elias pers. comm.; Oppenheimer et al., 1998), and the gas has a residence times in the lower atmosphere of approximately 4 years (Brimblecombe, 1996).
Due to the high levels of CO2 required to cause harm, concentrations of CO2 are often expressed as a percentage of the gas in air by volume (1% = 10,000 ppmv). This is in contrast to other volcanic gases.
Carbon dioxide (CO2) is a toxic gas at high concentration, as well as an asphyxiant gas (due to reduction in oxygen). Irritation of the eyes, nose and throat occurs only at high concentrations. The concentration thresholds for health effects are outlined in the table.
Health effects of respiratory exposure to carbon dioxide
Gas masks may be of limited use in high CO2 concentrations due to the lack of oxygen. Hence it has been recommended that working or living areas should be immediately evacuated when concentrations exceed 1.5% by volume (the occupational short-term exposure limit value). Ambient guidelines for CO2 do not exist. Occupational guidelines for CO2 concentrations are given in the table.
Occupational guidelines for CO2
Carbon dioxide (CO2) release during eruptions and from vents, the ground, and lava flows can pose a hazard where concentrations are very high and the gas is trapped near the surface. Emissions are most dangerous where they can build up in confined spaces such as natural topographic depressions, excavations and pits, or building basements and cupboards, and Le Guern et al., (1982) proposed that areas located down-slope from old fissures are of particularly high risk. Deaths from increased CO2 concentrations have been reported at Vestmannaeyjar on Heimaey (during the 1973 Eldfell eruption); Vulcano, Italy; Mammoth Mountain, USA and Nyiragongo, DR Congo amongst others (see table). The three events that dominate the CO2 casualty list are the gas-outbursts of Lake Nyos and Lake Manoun, Cameroon and the gas cloud emission at Dieng. Although the two lake outbursts are frequently quoted in volcanic literature, their initiation is thought to be unrelated to volcanic activity at the time. The phreatic eruption on the Dieng Plateau, Indonesia in 1979, was the worst CO2 related tragedy not associated with lake-overturn. This eruption released a cloud of CO2 that overwhelmed ~142 villagers trying to escape from the area and claimed more lives when people tried to rescue the bodies. Gases sampled at the active fissure shortly afterward contained CO2 concentrations of 98-99% (Le Guern et al., 1982). The 18 April 1906 eruption of Vesuvius, Italy is also implicated in a CO2 related death: Perret (1924) recorded that CO2 from the eruption rendered the air "almost irrespirable" and, along with fine ash, blames it for the death of a 19 year old who had a recent history of bronchitis. Gas following major gullies on the west flank of Concepción, Costa Rica during its eruptive activity in 1986 resulted in sore throats and drowsiness and was tentatively thought to be CO2 (Smithsonian Institution, 1986). In Indonesia, CO2 clouds have been reported to flow down the slopes of Tangkubanparahu volcano, sometimes killing children (Le Guern et al., 1982).
Ground emissions of CO2 are particularly hazardous, as there is often little warning of high concentrations:
Soil gas emissions of CO2 pose a hazard to workers and residents in volcanic and geothermal areas, due to their diffusion and accumulation in confined locations:
Mortality and morbidity incidents associated with volcanic CO2 emissions
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