Smog in major American cities before the passage of the Clean Air Act in 1970 is now the stuff of legend. In Los Angeles, not only were the mountains a blur shrouded in fog, people actually wore gas masks to cope with the out-of-control pollution. But now we have strict legislation and government regulation, so everything is fixed, right? Sadly, not.

According to Science Daily, the term “smog” is a combination of the words “smoke” and “fog,” used to refer generally to air pollution. Originally, smog described the mixture of smoke and sulfur dioxide caused by burning coal. With advances in technology, smog now refers to the toxic combination of ozone, sulfur dioxide, nitrogen dioxide, and carbon monoxide, which are found in industrial pollution, vehicle emissions, and incineration. By whatever name, it is a toxic mixture that harms everyone, especially children and senior citizens. It’s also particularly dangerous for people with lung infirmities such as asthma and COPD.

If you want to monitor smog levels, follow the EPA’s Air Quality Index which measures average ozone concentrations in the air over an eight-hour period. If the ozone rate is too high, the EPA will announce that the air is “unhealthy for sensitive groups” or, at a more elevated level, that the air is simply “unhealthy.” If you live in a big city or near a factory, smog is a danger you should be aware of. In addition, smog can be worse in the winter and in low lying areas surrounded by hills where the pollution gets trapped in the valley. Places like New Delhi and Mexico City are famous for terrible air quality, but if you live in places like New York City, Los Angeles, or Houston, you should also take steps to protect yourself.