As well as the standard health and illness issues addressed as part of a Dive Medical, divers should have a reasonable level of physical fitness to cope with the environmental stresses of being underwater. Even a diver in good health can experience physiological strain from environmental factors that include:

• Exertion required for propulsion through the surrounding water
• Heat loss to water that is generally colder than body temperature
• Breathing gas of compressed density
• Changes in the cardiorespiratory system from using underwater breathing gear
• Changes in the gas volume and pressure within air spaces in the body eg; ears, stomach, etc
• Introduction into the body of gases that can have toxic, narcotic, stimulatory or gas solubility effects on bodily functions.