It can happen in many places … even in Boston:

> Hyponatremia occurs in a substantial fraction of nonelite marathon runners and can be severe.

> As marathon running has surged in popularity during the past quarter-century, reports have emerged of serious illness and death from hyponatremia, as in the case of a 28-year-old woman who died after the 2002 Boston Marathon.

> Participants in the 2002 Boston Marathon were recruited … 488 runners (64 percent) provided a usable blood sample at the finish line. Thirteen percent had hyponatremia (a serum sodium concentration of 135 mmol per liter or less); 0.6 percent had critical hyponatremia (120 mmol per liter or less).

[1]: Hyponatremia among Runners in the Boston Marathon. https://www.nejm.org/doi/full/10.1056/NEJMoa043901