What height does the ozone layer start at?

Approximately 20 to 30 kilometers

The ozone layer is essentially located in the lower division of the stratosphere, from approximately 20 to 30 kilometers (12 to 19 mi) above Earth, although its density fluctuates seasonally and geographically. It is basically a protected layer which protects us from harmful rays

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.