From Mashable: Pele's hair, which is named after the Hawaiian goddess of fire, is created from the stretching or blowing-out of molten basaltic glass from lava, which spurts from fountains, cascades and flows, according to the US Geological Survey.
Residents have been "urged to minimize exposure to these volcanic particles, which can cause skin and eye irritation similar to volcanic ash." Drivers are also warned to not use their wipers if it falls on their windscreen, as the glass is abrasive.