Skycells as they were written are more likely to cause someone to freeze to death.
The answer to this question is largely dependant on exactly how high the Skycells are. And for this you'd need to be exact. There are at least 10 mountains on this earth above the height of 20,000 feet. It took them a long time to get up the mountain, so just going to say your estimate of a couple thousand feet seems...off. By a lot.
The book mainly talks about people going mad and falling out accidently, but, it's the cold at that altitude that is more likely to kill.
You will lose an average 3.5 degrees Fahrenheit for every 1000 feet of elevation you gain.
If you start out at 1000 feet, and climb to 6000 feet, that’s a 5000
foot difference (6000 – 1000 = 5000). So, since you’re gaining 5,000
degrees, you’ll use a 5 in your calculation. 5,000 feet, times 3.5
degrees. Just drop the (thousand). So, ( 5 x 3.5 = 17.5 degrees). So
roughly, you’ll expect to lose at least 17.5 degrees. I always round
up to the nearest 5 just to add some safety buffer, so a 20 degree
difference. Simply subtract this number form your expected low,
according to the forecast, and you have your expected temperature.
Ex: The weather man says it’s going to me 60 degrees today for the
high in your city. Your city is at 6,000 foot. If you’re climbing from
6,000 feet to 14,000 feet, That’s an 8,000 foot difference. 8 times
3.5 is 28 (8 x 3.5 = 28). You can assume a 30 degree difference after rounding, so it’s only going to be 30 degrees max at the top of the
mountain (A high of 60 minus your 30 difference)! Remember, that’s for
the high. Always consider your low temperatures too.
Besides the base height, there's also wind chill to consider.
Questions to ask:
It doesn't need to be as high as in the books to kill folk--could be as little as 400 feet, as long as the ground beneath is rocky and jagged. Do that, and pitch the floor a little more, and it's more likely that they would die from something other than hypothermia. But seriously, it's hypothermia.