Exposure to fly ash through skin contact, inhalation of fine particle dust and drinking water may well present health risks.

After taking into account fine particle levels, the association with coarse particles remained but was no longer statistically significant.

Sulfur dioxide, which effervesces from active volcanic vents and craters, reacts with sunlight, moisture, particulate and oxygen to produce clouds of fine particles.

The largest particles fell out in only a few months, but the fine particles left behind did plenty of damage.

Fine particles inhaled into the lungs migrated to the blood and damaged the inner lining of arteries, he said, causing them to narrow and stiffen.