Gases such as sulphur dioxide (SO2) and hydrogen sulfide (H2S) are present in poisonous amounts close to the vent or dome of an erupting volcano. Further away from the vent, these gases can dissolve in the atmosphere's clouds to produce an acid rain or mist, which affects human and animal eyes and respiratory systems. These gases corrode metal-building materials as well. Carbon dioxide (CO2) is not poisonous, but it is perhaps the most dangerous volcanic gas. It is heavier than air and gathers in hollows in the ground (like valleys and basins). Since it is colourless, odourless and tasteless, people do not know it is there and can suffocate. In the past, several people have been killed in this way at the Boiling Lake in Dominica.