The grip of an epic dome of fog. The air pollution so dire, it's like a respiratory nuclear winter. Bob woodruff is on the ground. And he showed us how hard it is to reach clean air through all that... See More
The grip of an epic dome of fog. The air pollution so dire, it's like a respiratory nuclear winter. Bob woodruff is on the ground. And he showed us how hard it is to reach clean air through all that smog. Reporter: This is what it was like to wake up in China's capital. Life inside a toxic cloud, seven-straight days. I'm looking at the sky. It's about as polluted as I've ever seen. This is an app. This indicates the air quality index. That number is 512. W.h.o. Says 20 is what is safe. I once lived in beijing. But in the past 26 years, the pollution was never this bad. The air leaves a metallic taste in your mouth. We decided to head south in search of breathable air. We're pulling out of the train station. We're going to see when the smog ends and the blue skies begin. Our visibility dropped as we left the city and traveled across China's industrial belt. You look outside, you see even worse than it was in beijing. Our pollution index peaked at 610. We traveled a full 450 miles. The same distance from Boston to D.C., before finally escaping the worst of the haze. The first time we got some sun poking through the clouds. But massive amount of pollution. The number here is 142. Right now in beijing, it's 533. China's smog is result of a booming industry, without regulations. If you think it's only China's problem, think again. A new study shows how China's smog makes its way all the way to America, traveling in a matter of five to seven days across the pacific, contributing up to 10% of the annual air pollution on the west coast. 5 1/2 hours later, we arrived at our December nation, as far from the beijing as the distance between Boston and Columbia, South Carolina. It's only 501. It's easier to breathe. But that number is four-times as high as the W.H.O. Says is safe. We finally breathe cleaner air. But we've not fully escaped this polluted sky. Bob woodruff, ABC news, China. Our thanks to bob woodruff.
This transcript has been automatically generated and may not be 100% accurate.