Smog is becoming more of a problem with urbanization, especially in developing countries with fewer restrictions on emissions control.
(Mexico City smog (from Wikipedia))
In cities like Beijing, where it's a serious problem, there's often a government or city board that monitors it and provides warnings. In Beijing, that's the The Beijing Municipal Environmental Monitoring Center. Usually when smog levels are high or dangerous, they'll release a statement. So if you're concerned, keep an eye or ear out for their announcements.
What to do about it? Well one government, at least, has provided some suggestions. The Ministry of Environment (MoE) in Canada, produced a website on Precautions to take during Smog Alerts. The key points:
- avoid exertion outdoors
- replan activities for when smog levels are lower
- stay away from high traffic areas (exhaust fumes make it worse)
- stay in a well-ventilated place
- drink plenty of fluids
- if you experience any symptoms (shortness of breath, etc) seek medical attention!
They also have information on how you can help to reduce levels (on the same page).