Eruption rule Avoid breathing potentially toxic fumes

The importance of carrying a gas mask on an active volcano has already been discussed. Volcanic gases are often just unpleasant; however, they can cause death. Visitors who suffer from asthma should be particularly careful. The only casualty of 1973 eruption of Heimaey, Iceland, was a man who reportedly was overcome by toxic fumes. The gases that come out of volcanoes include carbon dioxide, sulfur dioxide and hydrogen sulfide. The rotten-egg smell that I fondly associate with volcanoes is from hydrogen sulfide, a gas many times more lethal than hydrogen cyanide (the toxic of choice for gas chamber executions). Hydrogen sulfide can be particularly dangerous because one's sense of smell quickly becomes insensitive to dangerously high levels of the gas. Although the concentrations of gases are usually not high enough to cause death directly, they may cause dizziness that could have fatal consequences on an active volcano. A colleague from Hawaii tells of a time when he was observing the activity on Pu'u O'o when suddenly the direction of the wind changed. He found himself enveloped in the gas plume, frantically trying to get his gas mask out of his backpack. After this unpleasant experience he started carrying his mask outside his backpack.

A last resort for those who may find themselves in dangerous conditions without a gas mask is to tie a wet cloth over the nose and mouth. Volcanology folklore says that the cloth should be wetted with urine -personally I think it is best always to carry a gas mask.

In addition to gases, eruptions can produce smoke when, for example, lava flows over a paved road or vegetated area. The smoke can be overpowering, so stay at a safe distance from the flows. A different type of danger, though rare, can be caused by simple steam, as when still-hot lava flows in damp weather. The resulting steam can reduce visibility, which again can be a serious predicament to be in during an eruption. It is sensible to move away from any place that is getting steamed up.

Lava that is entering the sea, as is common in Hawaii, can produce large steam clouds that contain hydrochloric acid. Breathing these fumes can cause respiratory problems and unpleasant eye irritation. It can also lead to tragedy. Two hikers visiting the Pu'u O'o eruption in 2000 died after apparently being overcome by fumes near the shore, where a lava flow was entering the ocean. This was, however, a rare event and can be avoided by being sure to stay upwind and sufficiently far away in case the wind changes direction. Often, lava flows pour into the sea slowly without a significant steam cloud forming so they are quite safe to watch, as many visitors to Hawaii can attest. A last but essential warning about "lava shelves" - seashores being created by active lava flows: do not approach the shore. I stay at least 30 m (100 feet) back. The ocean undermines the lava crust (hence the name "shelf"), which suddenly, and often, collapses.

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