If it's a short journey we need to undertake - say, a mile - then often it's not just healthy but more fun just to walk - especially if the weather is nice and the traffic is heavy. Cycling's also good too, as it's a healthy way for getting from A to B. So long as A and B aren't that far apart, of course. And the longer the journey, the less chance there is of finding an alternative means of transport.
Sure, you *could* travel from London to Sydney without the aid of an aeroplane. But it would be a bit of a trek. So, what can we do to make sure our flights are as healthy as they can be?
Make sure you know the facts about deep vein thrombosis - and how it can be avoided. This is something that is important to do if you intend on long distance travel. Things that help include doing leg exercises and if possible getting up out of your seat every so often for a short walk. This page on preventing DVT has some good info. And importantly, it recommends that you see your doctor in the time prior to embarking on your journey if you are at risk of DVT.
Cosmic radiation. Interestingly, radiation levels are higher at the altitudes a plane flies at than on the ground. Cosmic radiation (in other words, radiation that's from space) is occasionally mentioned with regards to flying and health but most people won't travel in the air enough for there to be enough exposure that it would have any effect. And even in the case of pilots and other airline staff who are the most frequent flyers out of all of us, there's no proven link between this and any ill health effects.