This question is very similar to the type of question that asks you what the percentage of a gas in a mix is at a given depth in that it's asking for the physical amount, not the physiological effect if breathed by a diver.

The mix of air at altitude is the same as it is at sea level. The ambient pressure is reduced so there will be a physiological change if breathed at that new pressure even though the percentage remains the same. 

Learn more about Physics in diving on my upcoming PADI Divemaster and PADI IDC.

The PADI Skin Diver program may be conducted by a PADI Divemaster and is covered in the Skin Diver Workshop on the PADI Divemaster Course.

Details regarding the program can be found in both the course outline in the PADI Instructor Manual; and recommendations for its conduct in the PADI Guide To Teaching.

An astonishingly small amount of the water on Earth is freshwater and less than 1% of it is available for drinking the rest is in the form of glaciers, snowfields etc.

Be smart and conserve water and refuse items and produce from water-intensive industries and activities that pollute freshwater systems. After all, water is life!

A basic understanding of human anatomy is essential for an effective Emergency First Responder and an EFR Instructor should continue their education and learn more about the body and it's functions whenever possible.

Want to learn more? Join my next Emergency First Response Instructor Course and I'll give you some of my insights from being a Chamber Operator/Tender and a Diver Medic Technician. 

The reduction of high-pressure air to intermediate pressure at the first stage of the regulator can be broken down into two basic types; Piston and Diaphram. The latter being preferred in colder waters.