A chemically active dust
Sakurajima is one of Japan’s most active volcanoes, frequently dusting the surrounding Kagoshima Prefecture with ash. This is not inert soot; it is pulverized rock and volcanic glass whose surfaces are chemically reactive. As hot gases and aerosols stream from the volcano's vent during an eruption, compounds like hydrogen fluoride adsorb onto the surfaces of the fine ash particles. The result is a dust coated in a film of soluble fluoride. Studies of Sakurajima's ash show its fluorine content is higher than the average for Japanese volcanic rocks.
When this ash settles on fields and pastures, the danger begins. Even a layer of fine ash just one millimeter thick can carry enough fluorine to be toxic to grazing animals. Rainwater dissolves these surface compounds, creating a toxic runoff that contaminates soil and surface water. In Kagoshima City, the annual deposition of fluorine has been measured to be as much as eight times higher than in other major Japanese cities like Tokyo. Research indicates that between 7% and 57% of this deposited fluorine comes directly from Sakurajima's ashfall.
The mechanism of fluorosis
Livestock, particularly cattle and sheep, are vulnerable to this contamination. They ingest the fluoride-coated ash along with the grass and soil they consume while grazing. Inside the animal, the soluble fluoride is absorbed into the bloodstream. While some is excreted, the rest accumulates in the bones, leading to chronic fluoride poisoning, or fluorosis. This condition interferes with normal bone development, causing skeletal deformities.
The effects are severe. Bones can become thicker and more porous. Animals develop exostoses (bony growths) especially on the mandible and leg bones, which causes pain and intermittent lameness. In younger animals, the developing enamel of their teeth is damaged, leading to discoloration and excessive wear. This condition is called dental fluorosis. This makes chewing difficult, leading to reduced appetite, weight loss, and general poor health. The only effective mitigation is to remove livestock from contaminated pastures until rain can wash the soluble fluoride from the ash and soil.