Since humans don’t exhale methane, it’s weird of the article to use “¼ of a human exhalation” as a measure of volume.

I think they missed a great title opportunity. A "fart detector that works a mile away" seems to be great way to phrase it; it's concise, easy to imagine, relatable and comical. The article could then go into the specifics of what the detector actually picks up (e.g. CH4).

I find the diffusion of the smell of flatulence to be a better comparison, because it invokes a visceral reaction ("eww", "yuck", etc.). Imagining a device which does this seems easier. I have a worse idea of what "1/4 of a human exhalation" means.

Unfortunately for the fart detector (but fortunately for us!), human farts average around 100ml [1], whereas an adult male breath averages about 500ml [2]. That could still be sorted out for your title, but we also have to consider that a fart is only about 7% methane [3]. This suggests the one mile sensitivity is going to be at around 18 simultaneous average farts, or 5 at the record-breaking end of the scale -- conditions which might indeed justify further surveying the region at altitude.

If we're willing to get closer than a mile (questionable in the above scenario), we can apply an inverse law for gas diffusion [4] (I was surprised it wasn't inverse square!) and find that the apparatus can can detect a single average fart at a distance of around 30 feet -- easily covering most elementary school classrooms and automating away the "whom smelt it dealt it" dilemma.

[1] https://www.chemistryworld.com/news/explainer-the-chemistry-... [2] https://en.wikipedia.org/wiki/Tidal_volume [3] https://en.wikipedia.org/wiki/Flatulence [4] https://physics.stackexchange.com/questions/290059/inverse-s...

Methane doesn't smell of flatulence; that smell consists of much larger molecules, which diffuse correspondingly much more slowly, and whose distribution is therefore more affected by advection.

Sounds like a real-life version of Futurama's Smelloscope?

And the comment of the year award goes to...

They do, but at the other end.

They do, in very trace amounts. The exhalation of inflated amounts of methane during a breath test is actually often an indication of a malabsorption syndrome.

Tried to find a counter-citation for this, and the best I could do was " A total of 3481 different VOCs [volatile organic compounds] were observed: 1753 with positive alveolar gradients, 1728 with negative alveolar gradients."

https://www.ncbi.nlm.nih.gov/pubmed/10410929

It turns out that humans exhale all kinds of things.

The lungs are an excretory organ. Sort of by accident as any volatile substance will naturally waft away due to the nature of the gas exchange medium, but they do alleviate some of the burden of the liver > gallbladder > bowels and / or liver > kidney processes.

It's just overall a bad measure of volume, because people have no idea how big a breath is.

I guess a can of soda is something comparable that people are considerably more aware of.

The title makes it sound like Human Exhalation Mile is a unit of measure.

1/4HEM

We can create methane but I'm not sure if "exhale" is the right word.

I think it's fine. It's a measure of gas flow I can relate to.

Well, that's a terrifying device for finding humans.

It seems that you also need to install mirrors in the area you want to check. So it really is a system and not a device.

It also has a slightly cyberpunk look that helps:

https://cires.colorado.edu/sites/default/files/mathaneinstru...

Terminator: a thousand humans. Tracking.

Terminator: Acquiring target. Distance 200m.

150m

100m

50m

Cow: MOOOOOOOOOO!

Terminator struggles 30 seconds to eject head from cow butt.

headline should read "a mile away"

also i don't think that methane comes out when a human exhales.

Perhaps a tiny fraction, and that's detected (?)

I'm not entirely sure about it but I guess they used "human exhalation" as a somewhat relatable unit of volume for a gas but the actual system has nothing to do with detecting humans? That's the only way that makes sense to me, although if that's the case that's really needlessly confusing.