Other than the sealing they come with - I've used Nikon lenses in Iraq and Afghanistan with no problems...I carry both my cameras on my shoulders at all times when on deployment - they only get put away at the end of a job - out on the ground they're exposed to all weathers...
If it's very wet, I try to use a rain-cover, but I usually forget to bring it with me...no problems with water-ingress so far...including a complete soaking when I fell over during a river-crossing!
Even in quite severe dusty conditions, as long as you take basic precautions (I try to wrap a cloth over the lenses when I'm travelling in the back of armoured vehicles as fine dust and sand blow everywhere), you'll be fine as long as you buy the more expensive models - the cheaper lenses are less well sealed...