Volcanoes release carbon dioxide, sulfur dioxide, and other gas molecules.
Poisonous clouds of gas will suffocate a person caught in them as they rush down the volcano side. The sulfur dioxide gets converted to sulfate particles. These particles reflect sunlight back to space which cools Earth's climate. The recent eruption in the Philippines cooled the Earth by about one degree F and may have been responsible for the cool summer of 1992. The particles spread all around the world so the effect is fairly uniform.