Depression increases your risk of a number of diseases and other conditions by, for example, increasing levels of stress hormones such as cortisol or adrenaline. Depression can affect the immune system, making it harder for your body to fight infection. Vaccinations are even less effective in people with depression.
The total surface area of the Earth is about 7008510000000000000â 510 million km2 (197 million sq mi). Of this, 70.8%, or 7008361130000000000â 361.13 million km2 (139.43 million sq mi), is below sea level and covered by ocean water.
Radius of earth is about 6371 km. So total surface area is 5.1E8 km^2. There are 9.29E8 ft^2 in a km^2, so total area of earth's surface in square feet is 47.4E16 ft^2. About 29% of earth's surface is land, so land area is 13.74E16 ft^2. Steve Williams' answer gives 6.81E14 ft^2 habitable area. Which would imply only 0.49% of land surface is habitable.