The lava can kill people, but the ash and dust can cause breathing problems, and it stays in the air a long time. Volcanoes can also give off poisonous gases. The Kileaua Volcano in Hawaii gives off 2,000 pounds (the weight of a car) of sulfur dioxide each day. The sulfur dioxide can hurt people, especially the ones with breathing problems. It also harms crops and water. The Hawaiians call the dust “vog” - volcanic smog. the vog can get so thick, it’s hard for people to drive because they can't see through the vog. The only benefit of volcanoes is that the ash can be used to absorb smells.

source:

http://wrgis.wr.usgs.gov/fact-sheet/fs169-97/