When the SO2 reacts with other chemicals in the air, it creates a hazy pollution known locally as “vog,” short for volcanic smog. Press reports about vog first showed up in the 1950s, and mentioned eye irritation and allergy-like symptoms. But Big Island residents have long believed that vog is hazardous to health, and by the mid-1980s, Hawaii had the highest asthma death rate in the U.S., according to the Journal of Allergy and Clinical Immunology.

In 1996, a comparison between Big Island emergency visits and vog levels between 1981 and 1991 found that people in high vog exposure areas sought treatment for asthma and chronic obstructive pulmonary disease more often when vog levels were high. The U.S. Geological Survey and National Park Service have developed a sulfur dioxide monitoring and advisory plan to alert visitors and workers near the Kilauea caldera in Hawaii Volcanoes National Park. The American Lung Association recommends that people stay indoors during periods of heavy vog.