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# What is the Armstrong Limit?

The Armstrong Limit is an altitude beyond which nobody can survive. It is the altitude that produces an atmospheric pressure so low (6.3 kPa) that water boils at the normal temperature of the human body.

Sometimes it takes a popular event to discover something new about science. A skydive stunt led to the discovery of the now popular Armstrong Limit. The boiling point of water is 100 degrees Celsius only at sea level. However, the boiling point of water decreases as one goes higher due to a decrease in pressure to the point that the boiling point of water at the summit of Mount Everest drops to 71 degrees Celsius. Extending the height means that it reaches a certain altitude where water boils at body temperature. The point where water boils at body temperature is referred to as the Armstrong Limit.

## Definition Of The Armstrong Limit

The Armstrong Limit, also referred to as Armstrong’s line, is the height that has atmospheric pressure so low (6.3 kPa) that the boiling point for water equals the normal human body temperature which is normally 37 degrees Celsius. The Armstrong limit begins at an altitude of approximately 60,000 to 62,000 feet. It was discovered by Harry George Armstrong who was the first to realize this occurrence which takes place at an altitude above which no one can survive in the low or unpressurized environment. Armstrong was the founder of the US Air Force Department of Space Medicine.

## Effect On The Body Fluid

At or above the Armstrong’s line, the body fluid including saliva, tears, and blood will begin to boil away as no amount of inhaled oxygen administered by any means will support life at this point. At normal body temperature, the pressure of the water vapor is 6.3 kPa meaning, at ambient pressure of 6.3 kPa, the boiling point of water is about 37 degrees Celsius. Blood pressure is also measured relative to ambient pressure. The blood pressure also increases with the increase in altitude such that those with a diastolic low blood pressure (8.0 Kpa) are likely to experience higher blood pressure when they reach Armstrong’s line. The extra pressure is too much to prevent the blood from boiling at 62,000 feet while the heart is still functioning.

## What Happens Below the Armstrong Limit?

The human body requires supplementary oxygen while below the Armstrong limit to avoid hypoxia. Hypoxia is a condition in which the body or parts of the body is denied adequate oxygen supply at the tissue level. For most people, extra oxygen is needed at an altitude of 15,000 feet. The US aviation regulation requires pilots to be on supplementary oxygen if the plane takes above 30 minutes at a cabin altitude of 14,000 feet while passengers should be supplemented with oxygen at a cabin altitude of above 15,000.

## Significance Of The Armstrong Limit

The Armstrong Limit represented a new fundamental law that goes beyond the normal subjective observation of human body. Pressure suits were initially worn at altitudes that were below the Armstrong limit to prevent hypoxia. However, pressured suits are only required at an altitude of around 49,000 feet. For military aircraft operating above 59,000 feet, pilots wear counter pressure garments with high altitude capabilities. The Armstrong Limit has also opened the space for the study of effects of altitude on pressure and body fluids.