@raghav I react the same way. Heat and (especially) humidity really aggravate my ME/CFS symptoms. Although now I'm starting to realize that it may be the low barometric pressure that is the biggest part of the problem. I feel much more energetic on dry, crisp, cool days, when the humidity is low and barometric pressure is high. I think this is probably connected to the poor blood flow ("sticky blood") that many of us patients have - the higher atmospheric pressure on cool, dry days helps to counteract that. The fact that there is more oxygen and less water in the air, thereby increasing the oxygen in our blood, is probably helping too.I cannot tolerate summer heat in my city Chennai and I have to spend the daytimes in my airconditioned bedroom. I also get an immediate boost in energy when I take bath. I think it involves both cooling of body and removal of salt deposit from sweat on my skin. I get energy boost during winter when I take a hot bath as it is always above 90% humidity in my city throughout the year.
This also correlates to how I (and many people, even if they don't have ME/CFS) feel throughout the day. Each day around 10 am, barometric pressure is at its highest, and I feel more energetic. Conversely, it is at its lowest around 4pm, which is when I feel most sluggish.
Similarly, I feel better if I have a fan blowing on me. Not sure if this is because of all the oxygen the fan is sending my way, the air pressure that the fan creates, or both.
And I would argue the same phenomenon occurs when we take baths or showers - they cause a dramatic (although temporary) increase in blood flow (and therefore oxygen delivery), and we feel better.