Humid air is difficult to breathe

The "heaviness" we feel is not due to the high water vapour content. The water vapour pushes out small amounts of nitrogen and oxygen, meaning there's less oxygen to breathe. Our bodies are already taxed by being overheated, so it feels like it takes more work to breathe.

But the good news is that our bodies can adapt to these changes. It takes on average nine to 14 days to fully acclimate, depending on one's fitness and body size.