Air pollution is one of the leading environmental threats to human health. Not only does unhealthy air cause suffering, but it also imposes a variety of other costs: Patients can wind up with huge medical bills. Crowded emergency rooms mean longer waits for everyone. Employees who miss work might not receive pay. Productivity drops if key workers are out sick. Businesses lose money when low air quality keeps people at home. Some cities run free buses and trains when air quality is poor. That costs money.

"Ah, that's not Jekyll's voice—it's Hyde's!" cried Utterson. "Down with the door, Poole!"

Those stark sentences come from The Strange Case of Dr. Jekyll and Mr. Hyde, an 1886 novel by Robert Louis Stevenson. Even if you haven't read it, you probably know the story. Utterson (a lawyer turned detective) can't figure out how smart, moral Henry Jekyll got mixed up with an immoral thug like Edward Hyde.

Just what does this have to do with global warming? You might think of ozone as the Jekyll and Hyde of Earth's atmosphere. This molecule, a trio of oxygen atoms, can protect us from harmful ultraviolet rays. But it can also cause health problems and make us seriously ill. It all depends on where the ozone is—and how it got there.

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	A single oxygen atom (O) teams up with an oxygen molecule (O2). Source: NASA

Ozone also forms much closer to home. But this ozone mimics the evil Mr. Hyde. When cars and trucks burn gasoline, they release an array of chemicals—including a brownish gas called nitrogen dioxide (NO2). Sunlight causes nitrogen dioxide to shed one of its oxygen atoms. Alone and hyperactive, the single oxygen atom (O) swiftly teams up with an oxygen molecule (O2). Do the math: O + O2 = O3.

Nitrogen oxides are found in the atmosphere naturally. Burning fossil fuels (from cars and industry) transforms these elements into pollutants such as nitrogen dioxide and nitrogen trioxide. Source: EPA: Good Up High, Bad Nearby

NOT-SO-NICE NEIGHBORS

Here's the problem: Humans and ozone weren't meant to be neighbors. Mingled with other chemicals from car exhaust, human-made ozone creates photochemical smog. Imagine a younger, nastier relative of traditional smog. For that reason, scientists often refer to this second variety as smog ozone.

Smog ozone is the stuff you hear about in weather reports. More smog ozone means more danger. That's because O3 turns into Edward Hyde when it hits our bodies. It irritates our eyes, inflames our throats, and makes us cough. That's just for starters. Repeated exposure to smog ozone can actually injure lung cells. Public health officials calculate that 20% of hospital patients with breathing problems are probably victims of smog ozone.

Smog ozone can turn lung tissue from healthy (left) to unhealthy (right). Source: EPA: Ozone

How will global warming affect smog ozone? That depends. Rising temperatures could spur more clouds to form in our atmosphere. That would mean less sunlight—and less smog ozone. Then again, smog ozone thrives on hot, dry weather. And that could be just what global warming produces.

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