Imagine if there were so many solid particles in the air that we would either have to wear high quality masks in order to go outside, or be forced to stay inside because we wouldn't be able to breathe. Unfortunately, this is the situation in Beijing. Exhaust emissions from vehicles and the burning of coal in industrial plants are the major contributors. Thankfully, this is not the case in the United States, but it is just a matter of time before we start seeing issues with our air quality. The Clean Air Act was established in 1970 with a couple of revisions that is designed to eliminate problems such as smog, acid rain, and damage to the ozone layer, yet it still happens. You can see what the air quality is like in any state, such as Ohio, by visiting the EPA's map portals. Here, you can compare different cities like Cleveland and Cincinnati. For the most part, it looks like Cleveland's air is surprisingly a little bit cleaner than Cincinnati's right now (Northeastern Ohio has had some history of being one of the worst areas for particle pollution in the country, despite improvements). Cleveland and the surrounding area's particulate matter is low, but most of the ozone quality indexes are in the 30s. Cincinnati and the surrounding area's particulate matter is a little higher, as are the ozone quality indexes (Cincinnati alone having a reading of 43, 7 points away from moderate health concerns).
What do these index numbers tell us? The Air Quality Index (AQI) is a range that tells us how clean or polluted the air is (as seen above). The best air quality levels are in the green region, with an index below 50 and no health concerns, the worst being anywhere between 301 and 500 in the maroon region with serious health effects. "Hazardous" is where Beijing is now with little hope to return to smog-free levels within the next 20 years. Let us learn from our mistakes now so that we may not have to close all industrial plants and live a life indoors.
