Both ethylene and acetylene, the compounds used to speed up the ripening, are gases. They completely disappear from the fruits and vegetables by the time they reach the shelves of the shops. There is, however, one particular concern with the treatments of fruits and vegetables by acetylene: the source of the gas. The problem is that the cheapest and most available source of this gas is calcium carbide. This simple inorganic compound easily releases acetylene upon chemical reaction with water. Even though calcium carbide does not represent any danger since the products are not exposed to it directly, it might cause problems if it is not chemically pure. Industrially used calcium carbide is often contaminated with various toxic ingredients, such as phosphorus and arsenic-containing chemicals, that can be released together with acetylene and become absorbed by the treated fruits and vegetables.