Some gases are corrosive. They chemically attack and irreversibly damage human tissues such as eyes, skin or mucous membranes. They also chemically attack and eat away metal, rubber and many other substances. Some gases are not corrosive in their pure form, but can become extremely destructive in the presence of substances such as moisture or other gases. A tiny leak of hydrogen sulfide, for example, can become a large leak as H2S reacts with oxygen in the air to eat away the container in which it is stored.