What’s that smell right before it rains? Plus: Will spinning around too much kill you?

November 18, 2011

Dear Cecil:

What’s that smell in the air (some people like it, others don't) right before it rains? I’ve lived all over the country and there’s no variation … when you smell it, you know rain is on its way.

Cecil replies:

The smell is called petrichor, the scent of rain falling on dry earth. It’s caused by a couple of compounds in the soil, one of them known as geosmin, “earth-smell,” a term I found wonderfully Tolkienesque. (O geosmin! O earth-smell! A Elbereth Gilthoniel!) However, the Straight Dope copy desk recoiled, noting that any such reading would necessitate a vulgar conjunction of Elvish, pseudo-Anglo-Saxon, and Greek. We’ll therefore refrain from literary commentary and proceed in strict accord with science.

The human nose, not normally considered a particularly acute instrument, is extraordinarily sensitive to geosmin; we can detect it at a level of just ten parts in a trillion. Today this is mostly an annoyance, since in our supercilious age many prefer the fragrance of machine oil and ozone to the sweet smell of the planet. But I’ll venture to suggest it was important in an era long past.

Geosmin is produced by several types of bacteria and algae, which manufacture a volatile compound that can be kicked up when soil is disturbed, such as by gardening, plowing, or a hard rain. When a storm threatens and a few molecules of geosmin waft your way, that signifies rain is falling to windward, and in the fullness of time will fall on you.

Because we’re so attuned to it, a little geosmin goes a long way, and a lot can be decidedly unpleasant. Geosmin and another fragrant soil-borne compound, 2-methylisoborneol or 2-MIB, can make wine taste earthy, water yucky, and fish foul. (Catfish are especially susceptible.) The scent of geosmin may tell farmers their soil is healthy, but this is one area where a lot of non-farmers would be content to leave their ignorance intact.

Repellent though some find it, geosmin seems to be harmless to most animals, and in itself doesn’t signify that anything toxic is brewing. In fact, nobody really knows what it does or why we’re so sensitive to it, and most scientists, by nature practical folk, decline to speculate. But a scarcity of facts has never bothered me, and in this case we’ve got a sliver of information to go on. A couple UK scientists, wondering how Bactrian camels in the Gobi desert were supposedly able to sniff out water from 50 miles away, proposed that the animals were actually smelling geosmin carried by the wind from oases.

A survival trait so obviously useful to camels would likewise be advantageous to us. Long ago we were mainly nomads wandering in arid regions. It’s easy to imagine a parched band trudging mapless in the desert looking for the next watering hole. Then the breeze picks up, and what do they detect? Had they lacked the appropriate olfactory adaptation, nothing, with possibly disastrous consequences. As it was, if they were fortunate, they might smell the faint odor of moist earth, and with it the promise that they'd live another day.

Dear Cecil:

I just recently finished spinning around a lot at my friend's house and feel very dizzy even after two hours. This got me thinking: Is it possible to die from spinning for too long? Or would you just have one of the worst hangovers ever and possibly splurge your inner contents? Science demands an answer!

That’s how you and your friends like to pass the time, Jay — spinning around a lot? Well, it beats snorting PCP and thinking you’re Jesus. However, the practice isn’t without its perils. As a general matter the worst that could happen is you fall down, hurl, or look like a putz. But if you’re not healthy to begin with, there’s a nonzero chance you could die.

We know this because a couple people have, although at an amusement park, not at home. Specifically, two people expired after going on a Disney World ride called Mission: Space, which subjects you to sustained centrifugal acceleration of just over 2 g. While this is less g-force than is generated by other rides, including numerous roller coasters, those rides typically produce their peak force for shorter intervals. In one case, a four-year-old boy passed out while riding Mission: Space and later died as a result of a pre-existing heart condition; in the other, a 49-year-old woman suffered a fatal stroke as a complication of high blood pressure.

Then again, astronauts and others undergoing centrifuge training as part of the (real) space program endured spins of up to 32 g with nothing worse than mild sinus pain. One scientist, knowing sustained acceleration would substantially quicken a Mars trip, successfully withstood a constant 2 g for 24 hours. So unless you’re adding some twist to your spinning that I really don’t want to know about, you should be OK.

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References

Bláha L., Sabater S., Babica P., Vilalta E., Maršálek B. “Geosmin occurrence in riverine cyanobacterial mats: is it causing a significant health hazard?” Water Science and Technology 49.9 (2004): 307-312.

Gerber, N.N. and Lechevalier, H.A. “Geosmin, an Earthy-Smelling Substance Isolated from Actinomycetes” Applied Microbiology 13.6 (1965): 935-938.

Grimm, Casey C. et al. “Instrumental versus sensory detection of off-flavors in farm-raised channel catfish” Aquaculture 236 (2004): 309–319.

Lloyd, Steven W. and Grimm, Casey C. “Analysis of 2-Methylisoborneol and Geosmin in Catfish by Microwave Distillation-Solid-Phase Microextraction” J. Agric. Food Chem. 47 (1999): 164-169.

Lovell, R.T. “Off-Flavors in Pond-Cultured Channel Catfish” Wat.Sci.Tech. 15 (1983): 67- 73.

Mettheis, J.P and Roberts, R.G. “Identification of Geosmin as a Volatile Metabolite of Penicillium expansum” Applied and Environmental Microbiology 58.9 (1992): 3170-3172.

Paterson, R.R.M et al. “Why do food and drink smell like earth?” from Mendez-Vilas, A. (ed.) Communicating Current Research and Educational Topics and Trends in Applied Microbiology FORMATEX, 2007: pg.120-128.

Rackis, J.J. et al. “Flavor Problems of Vegetable Food Proteins” J. Am. Oil Chemists Soc. 56 (1979): 262-271.

Ritter, Stephen K. “How Nature Makes Earth Aroma” Chemical and Engineering News 24 September, 2007, pg. 19.

Simons, Paul. “Camels Act on a Hump” The Guardian 5 March, 2003.

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