For many, springtime on the East Coast means the welcome of thunderstorms. We had our first big storm yesterday, with window-shakingly close lighting strikes. One lightning strike was so close my office was filled with a pungent smell for about 15 minutes afterward. I love the smell of lightning, or more accurately, ozone. So, naturally, I got to wondering about how we detect and process the smell of ozone. Turns out, it’s pretty strange and mysterious!

Ozone and thunderstorms

Storm clouds are tall, billowy structures that form, usually, at the meeting of two fronts (one warm, one cold). The two fronts are like two walls of air, with differing characteristics, that slide by one another. That sliding, plus the shape of the clouds, leads to a build up of static electricity, similar to when you scoot your socked feet across the carpet in the winter, or rub a balloon on your head. (What, these aren’t things you typically do?) In this case, negatively charged particles (and electrons) accumulate at the bottom of the cloud. Since like repels like, the negative-charged cloud bottoms push away negative particles in the earth below, and draw positive charged particles in the earth toward the storm. Ominous.

Given enough build up, the energy will discharge, sending a very strong current down from the cloud as another strong current comes up from the ground, and the two bolts typically meet somewhere in the air. Most lightning bolts carry about 30,000 amps of current. Starting a car–the most taxing job for a car battery–requires up to 200 amps of current. So a lightning bolt is effectively a very large spark, like what happens when you touch your brother after scooting your socks across the carpet. (Sorry, not sorry, Kherry.)

As the lightning bolt passes through the air, it reacts with nitrogen, creating a whole lot of nitrogen oxides (NO and NO2), which both interact with the stable two-oxygen- atom molecule (O2) to create a three-oxygen-atom molecule, ozone (O3). (If you wanna nerd out about the chemistry, which I strongly suggest you do, check out NASA’s Earth Observatory!)

Schematic diagram of static charge in a storm cloud causing to lightning strikes.

What’s critical here is that ozone often requires the existence of free radicals in order to form, or creates free radicals as it splits from three oxygen atoms back to two. Free radicals is not just the name of my all-women alt punk band, but also what happens when electrons, which love to be paired, split up and go solo. Like a human with a recently broken heart, an unpaired electron is super reactive, and can basically react with a whole lot of molecules it comes near. Sometimes, that unpaired electron will break up other electron pairs in the reaction process, making more free radicals. So you can probably guess why people freak out about a lot of free radicals in the human body. Free radicals are naturally occurring, and most of the time, do so in low, safe quantities. Given enough time (as things approach equilibrium), unpaired electrons will find another unpaired electron and stabilize. Phew.

OK, so how do we smell ozone?

First off, we humans are really sensitive to the smell of ozone. We’re able to detect as low as 8 ppb (parts per billion). It’s the smelling equivalent of picking out a group of 8 people in a room full of 1/7th of all the people on the planet! Impressive, right? Well there’s good reason for this, as even numerically small concentrations of ozone can be toxic. In rats, concentrations of as little as 1 ppm of ozone can start to damage and kill olfactory tissue!

Ozone also crowds out other smells. Once we smell ozone, we don’t really smell much else. So we’re sensitive to it, and then it’s all we can smell. That’s kind of weird, right? What’s up with this persistent odor experience?

In our noses and all throughout our nasal cavities, there is a very thin layer of mucus. This mucus layer catches particles from the air and helps transfer them to our olfactory epithelium, where all the smelling gets done. You can think of the olfactory epithelium as a bunch of hands reaching out, waiting for something awesome, like fresh-picked cherry tomatoes or a giant gold goblet. The “hands” are olfactory receptors, which vary in size and shape, and can only grab what can fit in them (for example, small “hands” can grab the cherry tomatoes, big “hands” the goblet). When a particle binds to a receptor, a series of reactions occur in the cell, leading to the experience of odor.

Schematic diagram of the nasal cavity and olfactory epithelium.

It turns out that our olfactory receptors might be able to bind free radicals, and, since free radicals and ozone frequently occur together, that’s what we’re actually smelling.

In 1991, a group of patients undergoing irradiation for cancer all complained about a strong, pungent odor they later identified as ozone. This lead to a study where researchers discovered that humans can actually detect the irradiation of tissue by detecting free radicals trapped in mucus.

Another, older theory, is that ozone acts as the signal for irradiation. Ozone binds to and reacts with olfactory receptors the just as other odorants do, and effectively acts as a warning of more damaging chemicals nearby.

We don’t know too much beyond this, and as far as I can tell, ozone olfaction is not a very active area of research. Bummer.

Since we are so sensitive to the smell of ozone, and we don’t smell much else when we do smell ozone, I suspect that ozone/free radicals have many potential binding sites (a lot of those “hands” can grab ozone). When the ozone/free radicals does bind, it does so with high affinity, meaning it easily binds to receptors. This persistent and overwhelming smell experience is similar to other smells that indicate something noxious, including smoke and rotting tissue.

Ozone and free radicals, my friends, are how we smell lightning!

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