You could begin with stale, then move on to oppressive, maybe muggy and always overpowering. It is suffocating, and sometimes brutal. Never helpful, often stifling.
It is blistering, boiling, tight, searing, roasting, baking. If it can happen in the oven in can happen under the sun. The heat abuses, it tramples, it smothers, and it tortures.
The English would say it's close, or very warm, but I would say it's heavy!