Steel, aluminum, titanium, and their alloys are among the metals utilized in the aircraft manufacturing business. Aluminium alloys are distinguished by having a reduced density (about one-third that of steel alloys) and strong corrosion resistance qualities. They're also easy to work with and relatively inexpensive. Most commercial aircraft are now made from this material because it allows the manufacturers to reduce weight without sacrificing strength or durability. Titanium alloys are almost twice as strong as steel but are about one-fifth the weight. Because of these properties, they're being investigated for use in aircraft construction. Some titanium alloys can be extremely resistant to corrosion because they contain small amounts of oxygen. Others require a special treatment to prevent them from rusting. Zinc is another metal being studied for use in aircraft construction. It has the advantage of being lightweight and very strong. The problem with zinc is that it's very brittle and can break easily under stress. This makes it not suitable for many aircraft components such as fuselage frames and wings. Mercury has been used in aircraft before World War II but is no longer permitted due to health concerns. The mercury in older planes would leak out over time and cause pollution problems if it was not removed from the vehicle. Newer planes must be specially designed to avoid containing any mercury.