**2. Overview of the environment and its various components (media)**

The environment can be defined as all that which is external to the individual host. It can be divided into physical, biological, social, and cultural factors, any or all of which can influence health status in populations [6]. In other words environment refers to those things that surrounds living beings and affect their lives. The environment is man's basic life support system. It provides the air we breathe, the water we drink, the food we eat and the land where we live.

The four major components of environment include lithosphere, hydrosphere, atmosphere and biosphere, corresponding to rocks, water, air and life respectively.

*Lithosphere* is the outermost layer of earth called crust, consisting of various minerals, it is where we have the rocks and soil. It is about 100 km in depth, and is found on both land and seas. The principal component of lithosphere is earth's tectonic plates.

*Hydrosphere*, covering 70% of earth's surface, involves all the varieties of water bodies on earth such as seas, oceans, rivers, lakes, lakes, streams, etc. 97.5% of water found on earth is in the seas as salt water. Just 2.5% of water on earth is freshwater. Out of this, 30.8% makes up the rivers, repositories, and lakes and is easily accessible to man.

*Atmosphere* is the gaseous, envelope-like material encasing the earth. It is unique to the earth because its abundance of oxygen (20.95%) which is vital in supporting life on earth. other constituents of the atmosphere includes 78.08% nitrogen, 0.93% argon, 0.038% carbon dioxide, and trace elements like, hydrogen, helium, noble gases and water vapor.

*Biosphere* refers to all the regions on Earth where life exists. The ecosystems that support life could be in soil, air, water, or land.
