There are external factors that should be accounted for when training. Higher altitudes have lower O2 supply which increases exertion as air supply is limited, lending itself to increases in Heart Rate. Training in high altitudes can increase performance and levels for mitochondria, hemoglobin, and O2; however, the stress can sometimes be dangerous.
Health and temperature can affect training, and an excess of heat can cause dangerous thermogenic responses. Our bodies have the great ability to regulate and cool itself through radiation (heat can be gained or passed through sunlight or cold solids), convection (transferred to the air), conduction (transfer of heat to a colder object or through the tissues), and evaporation (water passing through the skin into the air) to return to homeostasis.
Heat acclimation is important to aerobic training as it helps performance, but there is a point where it is just too risky to workout. Proper dehydration, increase level of plasma, increase in sweat, better blood flow, and better thermogenic responses to homeostasis are all ways of acclimating to heat.