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A similar question was asked in The Great Outdoors Stack Exchange. The short answer is
Long sleeve light colored thin fabric cotton is leagues better than any synthetic.
The short explanation is that cotton retains water (sweat in your case), so it dries slower. Cotton will keep you cool by slowing down the evaporation experienced at high heat and low humidity.
As to why use long sleeves, that came up too in another question and it gives 3 reasons to be covered from head to toe in a desert:
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Wear cotton clothes when traveling in Egypt during summer - because of the heat you will sweat a lot and cotton fabric absorbs this sweat and helps the body to cool down. Cotton clothes provide better air circulation leading to more absorption of sweat and thus gives a cooling effect.
During winter temperatures are more human and you can dress pretty much as you like.
For a great guide on what to pack for Egypt consider this article: https://www.egypttoursplus.com/what-to-pack-for-egypt/
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Your best is summer wool to wear. 40% wool & 60% cotton. So it don't itch. You would need buy that there. Have made there. As wool evaporates from the inside out Cotton wicks water to the surface. Not having that go with light cotton. Long sleeve. A good vented hat. Carry a sweat rag. Loose fitting cloths.