If you could redesign a body part:

Let’s redo the lungs, shall we? They are toxicologist’s nightmare. First, let’s point out what’s great about our airway systems: the filtration provided by our nose, nasopharynx, pharynx and larynx; the clearance of foreign substances by dual layer of mucous in the airways (a thin layer in underneath the thick so the cilia can beat in the thin and move the thick layer up and out); and the generally competent immune function of the lungs. Let’s see how things go wrong.

Gravity isn’t much help here, when you breathe particles that are around 5 ´m in diameter as it helps to keep them down in the lung (smaller than that and the major effector is Brownian motion, not gravity); consequently, there’s a lot of stuff that never gets out. And for the stuff that gets trapped it’s much harder to push it up than over or down. So that’s my first fix - turn ‘em upside down and give the cilia a break. Maybe they wouldn’t get so tired by the time we get old. (Interesting note: most mammals in the evolutionary tree aren’t working against their lungs since they aren’t upright for a lot of the time).

The one ended design of the lungs also leaves a lot to be desired. The lungs can hold 6 liters or so if you really wanted them to, but during a normal breath you only breathe in and out about 0.5 liters.