Features:
1) Oxygen level in atmosphere.
2) Barometer
3) Barometric altimeter

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App working only if device has an integrated pressure sensor

As the pressure decreases, the amount of oxygen available to breathe also decreases. At very high altitudes, atmospheric pressure and available oxygen get so low that people can become sick and even die.

The reduction of oxygen availability in the air thus reduces the oxygen saturation in the blood and brains of not acclimatized people introduced to the environment. This is why people traveling from sea-level often feel pretty lousy for at least the first week when they arrive at high elevation destinations. At its extreme, this rarefaction of oxygen is what leads people to experience Acute Mountain Sickness (AMS), which is an incredibly dangerous condition.

91% Some adverse physiological effects occur, but they may not be noticeable.

72%-91% Impaired thinking and attention. Increased pulse and breathing rate. Reduced coordination. Decreased ability to work strenuously. Reduced physical and intellectual performance without awareness.

58% - 72% Poor judgment. Faulty coordination. Abnormal fatigue upon exertion. Emotional upset.

48% - 58% Very poor judgment and coordination. Impaired respiration that may cause permanent heart damage. Possibility of fainting within a few minutes without warning. Nausea and vomiting.

Below 48% Inability to move. Fainting almost immediate. Loss of consciousness. Convulsions. Death.

Atmospheric pressure
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth.

Relative Altitude
Relative Altitude. An atmospheric pressure expressed in terms of altitude which corresponds to that pressure in the Standard Atmosphere.