The injured man was sitting on a balcony at his home when “lava spatter” – projectile molten rock – landed on him.
“It hit him on the shin and shattered everything there down on his leg,” a spokeswoman for the county mayor said.
The Kilauea volcano on Hawaii’s Big Island erupted at the beginning of May, and the situation for residents has steadily been worsening.
On Saturday, a key coastal road used as the main escape route for residents was in danger of being cut off, which could hamper evacuations.
The possibility of the lava flows reaching the ocean, meanwhile, threatens to release toxic gases in a plume called a “laze”.
When molten lava hits sea water, the chemical reaction can create “hazy and noxious conditions” laced with hydrochloric acid and tiny particles of glass, the US Geological Survey (USGS) says.
“Even the wispy edges of it can cause skin and eye irritation and breathing difficulties,” USGS warned.