I can tell you two reasons. One is that nine or 10 years ago, air pollution wasn't as bad as now. That's a fact. I don't have pictures from nine years ago, but I have pictures from five years ago and I can still show the difference. And I can tell air pollution has really gotten worse in Beijing over the last two years.

Secondly, I did not have a reference point. I went to other countries for short visits but it doesn't really register in your mind when you stay there for a few days and then move onto the next city for business. You come back to China and the air is bad, but after a while you forget.

But I think living in the U.S. for a long time, especially the Bay Area where you just have the most pristine air in the world -- that really registers the difference for you. If you have the opportunity to go back and forth, it doesn't take long to say, holy crap, what did I breathe for nine years?

What about the rest of China? Why do you think there's been this real grassroots movement in the past year to monitor air quality?

There was a particular worsening trend over the last couple years. I can see with my eyes how much worse it's gotten. Right now I'm looking at a link that I collected of a few photos from March last year, 2011, and I can tell you it's a lot of blue-sky days, real blue-sky days. Now over the last 10 months, I can count the number with my hands. It's that serious.

With the emergence of Weibo and other Internet tools it just makes it a whole lot easier to share with friends this information. That's the outside factor that helps foster this movement, but also I think people are more and more concerned about health.

Also, the U.S. embassy has started to publish its own data. People have seen that U.S. measurements are sometimes much higher than the Chinese government's. That made people more critical or suspicious of Chinese government data.

Are Chinese people aware of what PM [particulate matter] is? In the United States for example, there was backlash against regulation of PM just last summer. It was spun as the regulation of "dust" -- can Chinese make the link between PM and public health?

Chinese people have heard the terms a lot from radio and Weibo but there is still a lot of awareness-building to do. They won't know, for example, that PM-2.5 is smaller than PM-10 and is more hazardous to your health. I think there's still some way to go.

We've reported on some of the recent controversies around air pollution regarding discrepancies between the number of blue-sky days reported by the government, and the number that people see. What are your thoughts about these discrepancies?

Well I think in general the Chinese government has been very hostile to the non-governmental publishing of [...] data of any sort, not just air pollution but other things as well. When the American consulate in Shanghai published their own version of data, Chinese foreign ministers got really flustered or mad and said, "Okay you guys cannot publish your own data, that's illegal."