The concept is as air heats up, the particles move apart causing reduced oxygen in each breath. In addition, water vapor displaces more oxygen. thus a hot humid day is similar to running at a higher altitude.
In addition, warmer humid air is more viscous than cooler dryer air. So if it feels like you are breathing pea soup, you are because the air is harder to suck in.
Lastly, the body fatigues faster with constant temperature and rising humidity as well as with constant humidity and rising temperature. so you will fatigue faster on hot humid days.
So it makes sense that runing on hot humid days will somewht mimic altitude. It will also challenge your stamina and build your respiratory muscles. Practically speaking, there is nothing like running on that first cool day of autumn.