It's been a noted trend in recent years that Chinese visitors to the Western world often wear face masks when they're out walking the streets. Some people think it's an amusing curiosity, while others think the visitors are being rude about the quality of air in their host nation. It's actually neither - just a hobby they've picked up from home.
You could make an argument that no country has adapted to industrialization as enthusiastically as the Chinese. They're the global masters at mass production. The consequence of this is that they emit immense amounts of pollution. There are only two types of air quality in built-up areas of China; 'unhealthy' and 'very unhealthy.' Both levels are hazardous to human health, and locals have become accustomed to wearing the masks in order to limit the damage to their bodies.