This is from my own personal experience and experiences of others
For those in the US right now you may be aware that the weather is one of the worst heatwaves in a long time. Not only is it extremely hot during the day (often 100+ degrees) but it is also extremely humid at night (humidity 90+ percent). If you like to work out this could put you in a dangerous situation where it becomes extremely easy to over train due to being literally overheated and dehydrated.
What is rhabdomyolysis?
Rhabdomyolysis is typically diagnosed when you end up at the ER or at the doctor and they give you a test called a CK blood test. This blood test combined with the color of your urine can indicate whether you are taking on an unusually high amount of muscle injury (under recovery). This essentially means your body is over trained but it is also potentially life threatening.
Rhabdomyolysis is dangerous in itself but when combined with dehydration it puts the patient at risk of kidney failure. Dehydration happens extremely easily in situations where it is hot and humid. In addition rhabdomyolysis itself threatens the kidneys because when muscles are breaking down (to be consumed as fuel) the body produces a toxin which the kidneys have a difficult time filtering out. The end result is this toxin from muscle breaking down can clog the kidney and injury the kidney (acute kidney injury). This is ultimately one of the reasons why rhabdomyolysis can quickly become life threatening.
What is the typical treatment for rhabdomyolysis?
Typically you'll end up in the ER and will be given an IV. They'll give you that IV if your CK levels are high and also if you are dehydrated. This usually stabilizes the patient enough to send them home and from there it's a matter of resting and getting lots of fluid. The point being if you want to reduce your risk of overtraining, of dehydration, of rhabdomyolysis, then when the weather is extremely hot (and humid) it is best not to do cardiovascular training unless you stay in a well air conditioned cool dry environment. If you train in the humidity then it is easy for the body to overheat because of inability to sweat and easy for the body to become dehydrated as well. If you do heavy weight lifting in a new exercise you don't usually do then it can cause excessive tears to the muscle fibers and in a normal situation you might recover but when it's also very hot this reduces your ability to recover.
For more about rhabdomyolysis: