What happens when smog descends on cities?

The weather in the Po Valley in northern Italy for the past weeks has been unusually mild and calm. Under such conditions, pollution and car exhaust get trapped close to the ground, causing winter, or industrial smog. But there is also what's known as summer, or photochemical smog, which is caused by the reaction between air pollutants and sunlight. Both mean bad news for Italians right now.

In the case of summer smog, ozone (O3 - pictured above) plays the key role. This may seem at first rather paradoxical, because ozone is the very compound that protects us from the sun's ultraviolet rays. The rule for ozone goes as follows: If it's high up (stratospheric), it's good. If it's down low (tropospheric), it's bad.

Ironically, although smog can greatly hinder vision, most dangerous particles are actually invisible to the human eye. Air pollutants or particulate matter (PM), though of course can be any size, are grouped into PM2.5 and PM10 (0.0025 mm and 0.010 mm), and are far thinner than a human hair or grain of even the finest beach sand. In winter smog, theses particles often accompany fog...

On October 30, 2014, German astronaut Alexander Gerst tweeted a pertinent question regarding a shady cloud he saw from space hanging over the Po Valley. "Is that fog or smog?" Well, technically, it's both. Don't forget the word "smog" is a portmanteau comprising smoke and fog.

Strangely enough, smog is often harsher outside of the cities where the pollutants that create it are emitted. The reason for this is that the reactions that ultimately cause smog actually take place at a much higher elevation. The situation is so dire in San Vitaliano (located just outside Naples) that the village has banned making pizza in traditional wood-fired ovens.

It's well known that Italy is not the only country in the world dealing with smog problems. In India, the government has announced a plan set to begin on Monday that will keep drivers off the streets of Delhi. Cars with even-numbered plates can drive on even days, odd plates on odd days. Somewhat ironically, India's chief minister has called the plan a "people's movement."

The Chinese metropolis Beijing, in addition to a host of other cities in the People's Republic, has seen an unprecedented level of red alerts in recent weeks. This is the highest warning on a four-tiered system, and come amid the highest levels of smog ever recorded in Chinese history.