I was in Beijing about 10 months ago, and there was a fair amount of haze/fog/pollution. Beijing sits on a flat plain surrounded by hills, so it acts as a bowl to collect heavy air particles.
The US Embassy in Beijing has a live air quality monitoring service, which the locals use rather than trusting their own govt's service
http://beijing.usembassy-china.org.cn/aqirecent3.html
There are lots of apps that monitor air quality as well.
Basically, be prepared. Pack a few good quality masks (the types with dual vents on the side), drink only bottled or boiled water and avoid street food if possible.
Assuming you are going to visit as a tourist Paul? It is a very westernised city now, so you can get good service (at a price) if you need help and lots of locals do speak English. Having said that, go with a tour company with a good reputation, have realistic expectation and chances are you will end up having a good time.
Good luck.
Bo