Hypoxia (environmental)

Atmospheric hypoxia

Atmospheric hypoxia happens naturally at high altitudes. As you climb higher, the air pressure drops, so there is less oxygen. This is called hypobaric hypoxia. Even though the air still has about 20.9% oxygen, it feels different from another type called hypoxic hypoxia, where there is simply less oxygen in the air. Some animals, like blesmols that live underground, experience this too. Athletes often train at high altitudes to get used to lower oxygen levels, and some companies create similar conditions for training using special air mixtures.

Aquatic hypoxia

See also: Ocean deoxygenation

Water that lacks oxygen can be hard for many creatures, but some tiny water organisms need these conditions to survive. In clean water, oxygen levels are about 7 parts per million.

When oxygen levels drop between 1% and 30% of what’s normal, the water is called hypoxic. Most fish can’t survive below 30% oxygen because they need it to get energy from food. Healthy water should almost always have over 80% oxygen. Low oxygen can happen all through the water, from the surface to the bottom, depending on the depth and how the water layers.

Sometimes, low oxygen causes many fish and other water animals to die. This can happen in summer or winter. In summer, warm water and extra plant material can use up oxygen. In winter, ice and snow can block sunlight and air, also using up oxygen.

Low oxygen often happens because of pollution, when extra nutrients from land cause tiny plants called phytoplankton to grow too much. When these plants die, bacteria break them down, using up even more oxygen. This can create areas with very little oxygen, called “dead zones,” where many sea creatures cannot survive.