On its Web site, Hawai'i Volcanoes National Park issued this alert: "Four miles of Crater Rim Drive are closed — from Jaggar Museum to the Chain of Craters Road junction — due to extraordinarily high concentrations of sulfur dioxide gas issuing from Kilauea's summit caldera."

Sulfur dioxide at high levels can be fatal, and it is a particular threat to those with asthma and other respiratory conditions.

Hawaiian Volcano Observatory officials said gas levels are 10 times higher than the levels at which the observatory evacuates its offices and the Jaggar Museum.

Sulfur dioxide is always present around the volcano, but normally in much lower concentrations.

In a statement, Hawaiian Volcano Observatory officials said there have been concentrations of the gas measured at more than 10 parts per million in the Halema'uma'u Crater area within the caldera during the past day. Under normal conditions, concentrations in small areas around the crater can reach 2.5 parts per million. But levels as low as 1 part per million prompt evacuation of offices and public areas around the volcano, the observatory said.

"High sulfur dioxide concentrations have been blamed for several fatalities, many amongst those having a history of asthma. The International Volcanic Health Hazard Network has compiled several studies indicating that 10 to 15 parts per million concentrations of sulfur dioxide is a threshold of toxicity for prolonged exposure," the observatory said in its statement.

The observatory cited the health hazard network Web site, www.esc.cam.ac.uk/ivhhn/guidelines/gas/so2.html, which includes this list of impacts from the gas: "Sulphur dioxide is irritating to the eyes, throat and respiratory tract. Short-term overexposure causes inflammation and irritation, resulting in burning of the eyes, coughing, difficulty in breathing and a feeling of chest tightness."

In the presence of moisture — from the air or in the lungs — sulfur dioxide can turn into an acid. The U.S. Department of Health and Human Services, at its Agency for Toxic Substances & Disease Registry Web site, says, "Inhaled sulfur dioxide readily reacts with the moisture of mucous membranes to form sulfurous acid (H2SO3), which is a severe irritant."

For more information, see the observatory Web site at hvo.wr.usgs.gov or the national park site at www.nps.gov/havo.