I also tend to feel sweaty after long flights.

> Canton to Sydney, which is quite a long trip.

On long journeys like the one you mentioned in that comment, there is a simple answer: You're going for quite a long time without showering.

Normally, most people shower at least once a day. Many people shower twice a day. There are good reasons for that. Sweat, salts, etc. accumulate on the body, causing various uncomfortable sensations. Long air journeys can take much longer than that, sometimes causing you to not shower for 40 hours. Perhaps, if you went that long without a shower unrelated to flying, you would notice some buildup of sweat too.

Also, I don't know if this applies to *you*, but a lot of people can get a slightly stressed while going on long journeys. They have to pack everything, remember everything, have their documents in order, keep track of their passport and money, be on time for everything, pass security, customs, immigration, guard against theft, wait in lines, etc. Most people (including me, certainly), can feel mildly stressed due to this. It is a well known fact that [stress increases perspiration][2].

Also, you're typically sitting in the same position for many hours. This means that there is little or no air circulation between your body and the seat. It also depends on the quality of the seat, but it's quite common for it to get clammy due to your backside being pressed against the seat for long periods of time.

Here's a final point that I expect to be (and welcome) being challenged on, but I'm including it anyway: The air in airplane cabins is [extremely dry.][3] This  may cause the body to compensate by sweating, etc. Of course, water also evaporates much faster in the dry air. However, only the water evaporates. The salts and other components of your sweat all remain on your skin/clothes. This may increase the feeling of being sweaty. (Again, I do feel that this last point is somewhat speculative, so I'm including it as a hypothesis more than a fact.)


  [1]: https://www.skyscanner.co.in/transport/flights/maa/syd/160917/160918/airfares-from-chennai-to-sydney-kingsford-smith-in-september-2016.html?adults=1&children=0&adultsv2=1&childrenv2=&infants=0&cabinclass=economy&rtn=1&preferdirects=false&outboundaltsenabled=false&inboundaltsenabled=false&ref=home#results
  [2]: http://www.wsj.com/articles/SB10001424127887323926104578278290520663794
  [3]: http://www.farecompare.com/travel-advice/how-to-keep-from-drying-out-on-a-plane/#/