When it rains after a prolonged period of dry weather, there is a smell created. It's called Petrichor. A wonderful word, and utterly useless trivia item. The smell is caused by something call Geosmin, which is produced by bacteria in the soil. Humans can smell it at 5 parts per TRILLION. In contrast Sharks can detect blood at concentrations of 1 part per million. On this one substance, we have a better sense of smell than sharks do for blood. Humans are amazing!

@quixoticgeek
I'd wager most humans can't smell Geosmin at anything like 0.000005 ppm due to being overwhelmed by laundry detergent and other perfumes at much higher concentrations.

@fragrancesensitive That would only be the case if the receptor for Geosmin is the same as that used by Laundry detergent perfume. I agree that we are exposed to a lot of pollutants that overwehelm our olfactory system, but I think laundry detergent is not in the top 10. Car exhausts would be near the top tho.

@quixoticgeek
Given that detergent (+ other product) perfumes can be quite overwhelming, I wonder how many receptors they target. but I'm not expert with the biological smell receptors. Can one absolutely say what the top ten olfactory contaminants are? It would depend very much on where you take samples. In a car dense city, or a small almost car free village where everybody hangs laundry out on the balcony. Empirically, domestic product perfumes are in some people's top 10 for sure.

@quixoticgeek
I would also suggest that car exhausts tend to be non constant. It does depend on local laws, but many places now have quite high controls on exhaust emissions. Even if a particularly contaminating vehicle were to pass one by, the resulted pollutant will disperse within a few minutes. This is not the case for people who walk around all day (and sleep all night) immersed in a cloud of off-gassing from their products.

@fragrancesensitive Erm. That's an interesting theory. It's nuts, but hey if you're happy.

I can smell the pollution from cars in the city. Even at night when there aren't many cars around. It lingers. Far more than the "off-gassing" of my bed sheets or t-shirts.