I am not a doctor, but I can certainly understand how you feel during hot and humid days. I have a number of respiratory problems and am vent dependent, even with a vent breathing effort is much harder when the air is heavy. When it is dry, I do much much better and have more energy because it is easier to breathe, I am not using as much energy to get my breaths.
I know that those with lung disease work much harder to breathe than the average person...it is even difficult for people without pulmonary/heart problems to breathe in oppressive weather. Your body works hard to cool itself, it is harder when it is humid because the air is so saturated with water vapor (which is why I feel sticky and never dry on days like those) so the sweat just stays on your skin and doesn't evaporate as well as it would on a nice, dry day. With increased energy use comes more oxygen demand which is tougher to meet for those with lung disease and can increase the sensation of shortness of breath. Another reason why humid air is so hard to breathe in is because it is more dense (there are much more water molecules in the air in humid air than drier air)...once it hits the airways in our lungs, it is harder to move through them because of the increased resistance that occurs which in turn increases the work of breathing as well.
I hope this helps...Sunny
I also forgot to add that the heat is very draining on the body...I feel weak after a time probably because I become dehydrated as well. I get out of the heat and drink plenty of fluids and that weakness/ dizziness goes away.