[schiffern]

Carbon monoxide comes from vehicle exhaust, so the levels are still lower outside cities and off high-traffic roads. I'd rather recirculate my "stored up" clean air vs. pull in CO from the line of cars stopped ahead of me. I just involves being aware of the surroundings.

Ideally a controller would monitor the outside CO/PM and inside CO2 to control the recirculate door.

[Retric]

Location is generally less important than wind direction here, which you aren’t tracking.

Having a few internal and external sensors for various gasses could work, but it’s a surprisingly difficult problem.

[schiffern]

> Location is generally less important than wind direction here

My experience (and my nose) strongly disagrees with this assertion. YMMV.

The urban-vs-rural divide in my area is shockingly detectable just by the smell of combustion byproducts alone.

> you aren’t tracking [wind direction]

Who says I'm not? Tall flagpoles make for nice, ubiquitous windsocks. I wouldn't use it to land a plane, but for this purpose it works just fine.

And yes, it's absolutely a hard problem. Just look at all the ink I've spilled...

The human nose (especially mine) is still a remarkable chemical sensor, with only a few blind spots. In practice the major blind spots, namely CO, correlate well with detectable combustion byproducts. I appreciate your concern, but you worry for no good reason!

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I'll share one final trick, which should be pretty obvious: avoid sucking in the packet of air a bunch of cars (or one diesel truck) just accelerated through. In practice, usually this means turning on recirculate when stopping behind a line of cars, and then waiting to turn off recirculate until a short distance after going through the light.

Just to preempt the seemingly-inevitable negativity reflex: if you don't believe such hyper-local variations in pollution make a big difference, I guess you've never cycled before. ;)

To good air and good health! Cheers

[Retric]

Your nose is really really bad chemical sensor because it caress more about novelty than absolute levels.

https://en.wikipedia.org/wiki/Olfactory_fatigue

Urban/suburban areas provide a different variety of smells which is easy to confuse with overall toxicity levels. Chemical sensors will sometimes line up with your expectations and other times be very different.

[schiffern]

Trust me, I know what vehicle exhaust smells like. ;)

Living outside the city, I'm not constantly fatigued to its odors. So that's not a factor.

But more to my original point, it disproves the "wind > location" idea (in my geography), since odor acts as a tracer for packets of air coming from the city.

I wish I could calm your anxiety, but on the bright side I am truly touched by your outpouring of concern for my respiratory well-being! Thank you, kind stranger.

[Retric]

You seem to misunderstand, Olfactory fatigue is very fast. When people fart for example the perception of smell goes away vastly sooner than the actual smell.

If you’re trying to judge air concentrations based on how intense the smell, it simply doesn’t work.

[schiffern]

I'm judging the presence of air pollutants. If I can smell it (ie above the lower detection limit), then I know I'm being exposed.

The converse is not necessarily true of course, but I can do this without hundreds of dollars in sensors. "Do what you can, where you are, with what you've got."

To account for sub-detectable pollution levels, I generally give myself a little extra buffer room. If I observe that detectable pollution odors begin at a certain point, I'll engage recirculate a half-mile before.

Generally nowadays I successfully avoid any detectable pollution/proxy odors, using the sort of preemptive planning I've described. You should try it!

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TL;DR this whole thread: recirculate gives better air quality, just flush CO2 periodically, ideally when in relatively cleaner air

https://www.latimes.com/science/sciencenow/la-xpm-2013-sep-1...

https://news.ucr.edu/articles/2020/01/13/clearing-air-inside...