I've been a frequent smoker for years and years, and I haven't any noticable experienced lung impairment yet. However, I smoke good, clean weed from an acrylic bong with a steel screen. Sucking any kind of smoke into your lungs isn't good for you, but anybody knows that there's a big difference between pot and tobacco.

And there's also a big difference between passing out from the smoke of a huge house fire and inhaling a joint.

I'm not saying there's no lung damage at all, I'm just saying it's pretty insubstantial.

I'm still saving up for a vaporizer, though.