Chinese scientists analyzing health data from more than 200 countries and territories discovered people are more likely to suffer fatal or disabling strokes when the climate changes. While the recent study in the journal Neurology does not directly attribute the strokes to human-caused climate change, it notes an increase in the number of strokes clinically diagnosed as at least partially caused by "sub-optimal temperatures." The total number of patients whose strokes fall into that category — 521,031 stroke deaths in 2019 alone — is higher than one would anticipate even when adjusting for other variables that could explain it (such as obesity, lifestyle and income).

There is important context for that figure. "Sub-optimal temperatures" refers to temperatures both warmer and colder than the range associated with lower death rates. Among the more than half-million 2019 stroke deaths in which sub-optimal temperatures were a factor, 474,002 were on the lower rather than higher end of the spectrum. At the same time, the number of strokes linked to heat is increasing rapidly, with the victims disproportionately coming from low income regions such as Africa and Central Asia.

"The current burden of stroke due to nonoptimal temperature is enormous," the authors write, adding that lower temperatures may increase the likelihood of suffering a stroke by stimulating the sympathetic nervous system and thereby elevating one's blood pressure. "Cold exposure is also related to thermogenesis and inflammation," the authors write. "In addition, high temperature may cause dehydration, elevated blood viscosity and dyslipidemia."

Elevated heat is linked not only to strokes, but to a wide range of cardiovascular diseases. Tony Wolf, a postdoctoral scholar at Pennsylvania State University's Noll Lab who studies thermoregulation and microvascular physiology, told Salon in 2022 that the cardiovascular system struggles to help the body regulate its internal temperature when an overheated person can no longer easily redistribute heat between internal organs. People sweat when this happens as the body's last resort effort to cool off. When that fails, the result can be fatal.

"Eventually, the temperature and/or humidity is too high for us to be able to compensate physiologically through sweating and that convective heat loss, and we're no longer able to maintain a stable body temperature, and we have a continuous rise and internal or core temperature," Wolf said.