Ergonomics of a notebook computer are poor enough because the only way to get the screen at the right height is to use a notebook stand, but then the keyboard is at an awkward position and can cause carpal tunnel syndrome. Perhaps the worst thing you can do is use it directly on your lap because the display is way too low for comfortable viewing and your wrists have to be bent. Add to that the risk of at least temporary impotency in males when it gets hot...
Best compromise is to use it on the desk, or if on the lap at least use a computer pad to raise it and provide insolation. Best bet for long term comfort and avoiding injury is to attach a monitor, keyboard, and mouse and use on a desk with the lid closed. It is unfortunate that Apple doesn't make a docking station to make this easier.