Is 80 degrees too hot to race in?

Humidity, not dew point, affects how efficient your lungs can exchange gases. It's still an important metric no matter how much you don't want it to be.

Thats just dumb. Dew point is measured. Humidity is a derivative of dew point.

With no external changes in atmospheric conditions, meaning the same amout of water vapor in the air. RH will be highest at the coolest time of day, usually in the morning. And lowest in the hottest time of day. The amout of water vapor is exactly the same.

The dew point is not a measure of energy in the air. It’s the temperature to which air would have to be cooled to achieve a relative humidity of 100%.

When the atmosphere cannot contain any more moisture, it means that you cannot cool yourself through evaporation of sweat. That’s when you’re screwed.

I hate running in the heat more than just about anyone, and for that reason all of my training is done before breakfast and, in the summertime, mostly before sunrise. I would not run a long race (marathon or half) when it's 80 degrees but I have no problem running a 5K under those conditions. However, it may be tough to PR.