Do airline pilots have more haemoglobin?

Chris - Airlines pressurise their airliners to about 7,000 feet worth of altitude, so slightly higher than ground level, and therefore, there will be a slight augmentation in haemoglobin, but not a huge one. Probably not a physiologically (in other words, bodily) significant effect.

If those planes weren't pressurised and they were flying at the kind of altitude they did, everyone onboard will be dead, of course. Most airliners are flying at more than 30,000 feet. That's the equivalent of the top of Mt. Everest where if you don't have supplemental oxygen there and you're not acclimatised, then you'd be dead very, very quickly.

So the answer is, when you go to high altitudes, you get a little bit more haemoglobin to compensate for the reduction in oxygen in the bloodstream, but it is proportional to how long you spend at altitude, and how high you go. And because those planes are not flying very high - equivalently speaking because of the pressure in the cabin - and the exposure is limited, there won't be a very dramatic effect, but there might be a small one.