Friday, May 06, 2016

The "Fresh Air Buddy": Understanding Ozone

Not too long ago, I went on a trip with my family. We were leaving the country, taking a ten-hour flight that left way too early in the morning for my taste. Sitting at the kitchen table, munching on some fruit and squinting at the light, I heard the low rumble of a suitcase being rolled down the hardwood floor of the hallway, accompanied by the tap tap tap of my mom's heels. She strode into the kitchen through the open doorway, beaming with excitement about the coming vacation.

Image Credit: Ecoquestair
"What is that?" I asked, gesturing to the strange-looking device hanging from a cord around her neck. It was a little smaller than a fist, black, with a brass honeycomb pattern at one end.

"It's my 'fresh air buddy'!" she explained. "It's a personal air filter-purifier thingy. I don't want to catch something on the flight and ruin our whole trip." She flicked it on and it began to whir quietly, an electric fan pumping air up through the honeycomb and into her face. "It uses ozone." she said with a seeming sense of pride.

My ears perked up. "It's ozone? Are you sure it's not just one of those charged-plate things that ionizes the dust and catches it?" My alarm must have been palpable.

"No, there's no parts to clean. Why, what's wrong with that?"

I sighed and rested my head in a hand. "You don't want to use that."

There's an old saying—"A little learning is a dangerous thing"—and this is one case where that definitely applies. Ozone-based air purifiers are common appliances, and they work spectacularly, but to understand why having a personal, portable version isn't the best idea, it's important to understand how they work.

Ozone is a fascinating molecule—it's simply O3, a bundle of three oxygen atoms. The oxygen we breathe ordinarily is O2. Since a single atom of oxygen is two electrons short of a full shell, it forms a double bond with another oxygen atom, providing them both with complete, stable electron shells.

This double bond is pretty strong, but it can be broken by things like a heavy electric current. When a Tesla coil or other high-voltage electronic device is turned on, air can become ionized as electrons are torn free of their parent atoms. This is how electrical arcs like lightning occur—an ion channel forms to carry electrons from the clouds to the ground. When that lightning strike happens, oxygen atoms in the air can exist—temporarily—as single, free O1 atoms, since the electrons that held them to one another have had so much energy imparted onto them that they can't be contained in nice, neat shells. Singlet oxygen never sticks around very long, though—instead, that O1 quickly sticks to an O2 molecule, forming ozone. Ozone is still very highly reactive, but not nearly as much as singlet oxygen.
A ball-and-stick model of an ozone molecule.
Image Credit: Ben Mills. Public Domain.
You've no doubt heard of the ozone layer at the top of the atmosphere, which protects us from solar radiation, but ozone can be found closer to ground level—any time you're around high voltages, like if you've turned on a Van de Graaff generator, you might notice a distinct "electric" smell. It's sharp, almost tangy—it's the smell of ozone.

So what does all of this have to do with cleaning air? I mentioned earlier that ozone is highly reactive. Since oxygen is "happiest" in pairs, the extra oxygen atom in an ozone molecule has room to share some electrons, meaning it forms bonds really easily. When this happens, the oxygen pair that it was attached to leaves behind its "third wheel", and a single oxygen atom ends up permanently bonded to whatever nearby object the molecule reacted with. Ozone-based air purifiers take advantage of this fact to clear the air in a room of suspended dust particles, odor molecules, and other undesirables. When you turn on an ozone generator, it creates a bunch of O3 molecules, which are then blown out into the room. These highly reactive molecules stick to dust particles by the hundreds, adding a small but significant amount of weight to them. This causes the particles to sink to the floor, where they can later be vacuumed or swept up, leaving the air in a room fresh and disinfected.

Ozone isn't particularly friendly to living things, though. The strong reactivity that makes it so good for cleaning air means that it also sticks to our tissues pretty readily, where it can cause damage. You've probably heard of antioxidants, which are supposed to protect your body's cells from reactive molecules. Ozone is a super-oxidant that's been shown to aggravate asthma, increase your risk of death from respiratory illness, and even cause brain damage in rats at low levels of exposure. The problem of ozone's toxicity is so well-recognized that many cities and states issue ozone advisories when levels in the air are particularly high.

Ozone is a useful tool, but like most things, if used improperly it can be dangerous. If you have an ozone-based air purifier, don't leave it on while you're in the room, and especially not while sleeping. Turn it on, leave and shut the door, and then come back in a while to turn it off. Once it's reacted with other molecules, the ozone is destroyed, so the dust left behind on your floor by the process isn't harmful. A good rule of thumb is that you don't want to be in a situation where you can smell it for extended periods of time, which means you CERTAINLY don't want to strap an ozone generator around your neck.

Fly safe, stay healthy, and "Drink deep, or taste not the Pierian spring"!

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