I was surprised to find that, if I hold the Canon EF24-70mm f2.8L USM zoom lens to my ear and operate the zoom control ring, it blows air into my ear. This happens as the zoom is changed from wide angle to telephoto (i.e. from 24 to 70 focal length). The opposite happens when moving the other way: it sucks air.
With this type of lens it is obvious that some venting to the surrounding atmosphere is necessary, since the lens overall is 127 mm long at 70 mm focal length, and 160 mm long at 24 mm focal length. However, it hadn't occurred to me that Canon would vent the lens into the camera. That is very undesirable as it is bound to increase the chance of the sensor getting dusty. Not only will any dust already in the camera and/or lens be stirred up, but dust may actually be sucked into the camera.
Dust can be sucked into the camera because, if the lens is venting into the camera then there has to be somewhere for the air to go, and that means the camera itself must vent to atmosphere. This indeed seems to be the case: if you put your ear to the viewfinder of an EOS 5D and work the zoom control on the above lens, a slight air flow can be detected. (I was alerted to this by this thread on dpreview.)
The EF17-40mm f4L lens doesn't suffer from this problem because it doesn't change its overall length. It does move a group of lenses within the lens barrel, and that movement is directly exposed to the camera, but it doesn't seem to generate much air flow.
Likewise, my EF70-200mm f2.8L IS USM does not suffer from the problem at all.
I was first alerted to this kind of problem by a review on Michael Reichmann's Luminous Landscape web site. He reviewed the Canon EF28-300mm zoom here and said that as this lens was of the "push pull" design it tended to blow air into the camera and he therefore avoided this lens. I didn't believe this and got out my old FD100-200mm lens (which I bought in 1972 and is also a "push pull" design) and satisfied myself that its back end was properly sealed and it did not blow air into the camera. However, sadly, it seems that Canon has forgotten a few tricks over the last 30 years.
If a lens has to be vented to atmosphere, it ought to be vented forward and not into the camera. (Of course, I see there is a problem achieving this if the backmost group in the lens is the one that needs to move.)
A camera body ought not to be vented at all.
Peter Facey, Winchester, England