We might have an answer to the pungent question in Central Illinois of late: Just what is that awful smell?
Back in September, residents across a big chunk of southwest Bloomington started complaining about a horrific smell. Bloomington-Normal was not the only place for this big stink. Champaign, Peoria, Springfield all had their own odiferous events. One elementary school in Bloomington even kept kids inside for recess because of the stink.
This week, it wafted across the land again. People had a lot of terms for it: rancid, barf-like, and disgusting.
"The most interesting one — somebody referred to it as smelling like a teenaged boy's locker room," said state climatologist Trent Ford of the Illinois State Water Survey in Champaign.
A number of possible sources didn't fit the nauseating odor profile. The Bloomington-Normal Water Reclamation District said, sure, sometimes the treatment plant on West Oakland Avenue can throw off some bad smells. But the wind was in the wrong direction in September, and the vomitous miasma affected too large a swathe of town to blame the treatment plant.
Another hypothesis was an agricultural source. The Illinois Farm Bureau said it's the wrong time of year for it to be manure or ammonia spread on fields. Farmers have been harvesting and are not yet prepping fields for next year.
Ford said people are usually really good at identifying smells like manure or, say, sycamore leaves — so the mystery itself is a clue.
"Given that people were not only smelling this kind of en masse, but also questioning what it was and their description of earthiness, pungency, and that sort of thing, it is very likely that petrichor is what they were smelling," he said.
Petrichor? Ford said soil is alive. There's a mix of organics, bacteria, and in some cases viruses in dirt. And when the weather is really dry, air replaces water in the pores of the soil and those organics build up in the soil. Then, when a little bit of rain comes — not a lot but a little — Ford said those organics come out into the air.
"Like if anybody has potted plants, if you forget to water that plant like I usually do, it dries out. You put the water in, and you start to see bubbles at the top of the surface. That's air bubbling out as the water fills that pore space. Well, that air contains some of the small things that were in the soil," said Ford.
When there's a good soaking rain, Ford said the odor stops because water replaces the air pockets. It has been a very dry autumn. Ford said large parts of Central Illinois are 2 inches of precipitation below average for the period.
There is some scholarship that suggests evolution has made humans particularly sensitive to petrichor, Ford said. Rain really matters, said Ford. Food depends on it.