This is what Chinese cities usually look like this time of year.

Thick smog blanketing the skyline.

But this year, there's something different in the air.

You may have heard stories about how cities across the globe

shut down due to COVID-19 are experiencing less air pollution.

Beijing's residents have been enjoying some unusually fresh spring air.

The smog's gone.

The haze dispersed.

India's extended lockdown has turned skies over New Delhi, a rare shade of blue.

From the Italian city of Milan, an unusually clear view of the Alps.

This is the impact of six weeks of lockdown over Venice.

Dan Weiss has heard these stories and seen photos of the vivid blue skies in Beijing or Los Angeles.

He says it might be his imagination, but in the last several weeks, the Chicago air has looked

clearer and smelled better. It struck me that, you know, the last air has looked clearer and smelled better.

It struck me that, you know, the last time when the skies would smell as sweet as they do, could have been from way before the height of the industrial era of Chicago.

So, he wanted to know, is the air, in fact, cleaner due to reduce traffic and industrial activity under Illinois stay-at-home order?

And if so, how far back in history would you have to go to find air this clean?

We'll get to the second part of the question about history in a minute, but the first part, how clean is the air now, is extra urgent.

Because COVID-19 is a respiratory disease, and air pollution affects respiratory health.

So first off, scientists do confirm that Chicago, in general, is experiencing cleaner air due to the stay-at-home order.

Here's Scott Collis, an atmospheric scientist at Argonne National Laboratory.

The measurements which we are taking from space tell us that there is at the moment

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