Ozone is what is known as a trace gas, meaning there is very little of it in the atmosphere. In fact, out of every million molecules of gas in the atmosphere, less than ten are ozone. Ozone is measured in what are called Dobson units. A Dobson unit of gas is equal to a layer of gas, at the surface of the Earth, with a thickness of one hundredth of a millimeter. The ozone in the atmosphere is about 300 Dobsons. That means if you brought all of the ozone in the atmosphere down to a layer of pure ozone at the surface, it would be about three millimeters thick. It doesn't seem like much, but it's enough to protect you from the Sun's UV rays. Much of the ozone is located about twenty kilometers above the ground, in a part of the atmosphere called the stratosphere. This is why it's called the "ozone layer".