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Yesterday my wife and I were out walking. The weather was cloudy and humid (it had been raining a few hours before). Around 15 C.

Suddenly, with no apparent reason, both of us had smoke coming out of our mouth when exhaling. A bit like your breath on a cold day. We had this smoke for 2 or 3 breaths of air. A car passed us just prior to the event. But in a distance of 3 to 4 meters. And the car had no noticeable smoke og fumes coming out. I was baffled...

Later on the same walk, a moped passed us at close distance, and the exact same thing happened again. This time we could clearly smell the exhaust, but not see it. Both of us 'blowing smoke' for some moments...

Can anyone help explain this?

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Although I think an explanation that atmospheric pressure, temperature & humidity changed just enough, possibly as a result of pressure changes in the air as the vehicle passed, that moisture was able to condense from your breath, with the atmosphere having already been very close to that point, here is another possible mechanism by which they might be relevant.

If the conditions were already very close to allowing water vapour to condense (which I think they must have been for any plausible mechanism) in your breath, then the existence of something – condensation nuclei – onto which water vapour could condense could easily cause condensation to happen. Well, vehicle exhausts often contain an aerosol of fine particles of soot & other crud, and these particles could provide just such nuclei for condensation. So, just possibly, the existence of an aerosol of crud from the exhausts of the vehicles was the cause.

Note I'm not suggesting that this necessarily was the cause: just that it is a possible mechanism.

In any case what you were seeing is condensing water vapour, not smoke.

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  • $\begingroup$ This sounds like a very plausible explanation. $\endgroup$ May 19, 2019 at 11:20
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The air conditions are not constant - you walked through a volume where the humidity or temperature was just sufficiently different to make your warm exhaled breath visible.

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The human body burns fuel & produces CO2,but it doesn't produce smoke. The 'smoke' you thought you saw was obviously condensation,perhaps tainted by traces of any exhaust fumes you may have inhaled.You must have walked through a pocket of air where the temperature or humidity was slightly different from the surrounding air,& that was enough to make your moist breath condense.It is possible that the passing of a vehicle displaced enough air to alter the air you were walking through for a few seconds.Vehicle exhaust contains water vapour,so perhaps that had something to do with it.

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