Every year tens of thousands of tons of lead are introduced into the sea as a result of air pollution caused by cars and industry. After rainfall, or as a result of the direct contact between the polluted air and the surface of sea, vast quantities of toxic matter are introduced into the sea.
One of the greatest threats to the waters and shores of the Mediterranean is petroleum pollution. Since it is a relatively small and enclosed sea, an incident such as the sinking or puncturing of a fuel tanker on the scale of the serious case of the Exxon Valdez in the 1990s, or the Prestige off the Atlantic coast of Spain at the end of 2002, would have a catastrophic effect. Petroleum pollution on a smaller scale occurs frequently in the Mediterranean. The result is the accumulation of oil slicks and pollution of the coast with tar. Petroleum and oil slicks disperse over time, but the toxic material in the petroleum remains in the marine system and accumulates in marine creatures. A further danger faces marine mammals when they come to the surface to breathe in seas covered with petroleum or oil slicks.