Yesterday's planned race had to be downgraded to a recreational run, with 87 runners ending up in medical tents and 39 taken to hospitals.
Sunday experienced the warmest low temperature of 75 °F on record for the District of Columbia for the month of October and was accompanied by very high humidity, with a 72 °F dew point. While the high temperature was only about 85 °F, the high humidity meant that runners could not cool themselves by sweating and were overheating.
The race is scheduled for October because conditions have historically been suitable for a long run.
Much of North America has been experiencing record high temperatures since August as cooler autumn temperatures have been delayed in many areas by more than a month. In some regions, temperatures have averaged more than 20 °F above normal for weeks.
Human CO2 emissions have peaked and may soon start to go down, but atmospheric greenhouse gas concentrations continue to rise and CO2 is currently over 403 ppm, compared to pre-industrial levels of 270 ppm.
Carbon Dioxide Levels to September 2017