Indeed!

As a matter of fact, I'm finding that I can actually mitigate the teariness of the onions by turning on the hood while I'm cutting, even 6' away from the hood.

I'm reminded of someone's joke photo of monkeys flying across the room into the hood here recently. I'm envisioning all the little odoriferous and tear-inducing onion olfaction bits flying up the hood now. Could all be psychological, but who cares!

I couldn't agree more. It really is amazing how a well hood vented to the exterior of a home can pull fumes from across the kitchen. It puts some perspective on the notion that an over sized hood is a must have. The downside is that I get neighbors two doors down complaining about how good my house smells when we are cooking.

Beyond the aesthetics issues of effective hoods, this is an example of why an acceptable hood for one person is a disaster for another -- odor. People vary in sensitivity and tolerance for odor; they also have different food choices and cooking techniques.

When not cooking, diffusive oils like onion propagate without any bias caused by heat rising. The rate of diffusion in the air is fairly high (probably in the few feet per second range) so the oils spread out in all directions. Only a modest number of molecules are needed for humans to smell odors (only a few for dogs). So the odor-causing molecules quickly fill the volume available.

What the hood accomplishes in this case (with the MUA, however provided) is air replacement, along with a slight bias in local air velocity toward the hood. Small cfm hoods will take longer to dilute the odor for a given house volume than large cfm hoods. Well commented here hood features related to capture and containment are less important in this case, particularly when the onions are being prepped away from the hood.

kas

The thing about Aesthetics is that they are subject to interpretation. I think overall a little perspective is all that's usually needed. I've seen others take comments here so completely out of context that one would think that unless you put a 48" hood over a 36" range (or add at least 6" on every side) ....the hood simply will not function, or it will be so ineffective it would be virtually useless.
That's not been my experience but every kitchen is different. I've always been happy with the same size hood as a given range with appropriate CFM and proper installation. My hood will certainly pull odors from across the kitchen. In a perfect world I'd like to add 3" to each side of my hood but with my range and cabinet configuration that simply wouldn't work.
The old Idiom "Horses for courses" seems to apply.
When I put together my current kitchen there were very few commercial style home products available.
I guess that's a long winded way of saying I try not to make things more complicated than they need to be. Hot air rises, a vented draw pulls air out. If the hood can pull air across the kitchen the bias in air velocity to the hood is sufficient, at least for me. ;)

OK, so kas -- you're saying this isn't my imagination, that in fact turning on the hood while cutting onions at a distance from the hood is making the air change over more rapidly, thereby drawing away the volatiles that are distressing my eyes? So it's simultaneously drawing away the volatiles and replacing it with clean air (what's the difference?)

Another advantage: when the toast burns, fills the house with smoke and the smoke alarms go on - the high, non-wired ones that you can't reach to push the button back into easily? Suck the guck out quickly, peace resumes.... It even causes the red in ones face due to embarrassment to recede!