Definitely pollution! I’m all for athletes wearing masks, whether it embarrasses the Chinese government or not. Some of the national Olympic committees have asked their athletes not to wear masks - they have got to be kidding!
On my first visit to China in 1979, when there were few cars or trucks on the road, pollution was terrible in winter due to the use of high-sulfur coal at power plants. Now it’s a hundred times worse. Add to that the dust and sand blowing in from the Gobi desert and you have a real environmental mess. I wore a mask even back then.
A local guy described his visit to China several years back in the newspaper. He wore one of those cheap white mask. Said the front of it was black after being out all day. Wow.