World Health Organization data shows that 4.2 million people across the globe die as a result of exposure to outdoor air pollution every single year. Air pollution is behind 43 percent of deaths due to lung disease and lung cancer, along with 24 percent of deaths from stroke and 25 percent of heart disease deaths. A genuinely shocking 91 percent of people all over the world live in places where the air quality isn't as good as the WHO believes it should be  — chances are that this includes you.
Even short-term exposure to highly-polluted city air can lead to reduced lung function, aggravated asthma, and more frequent respiratory infections . Those hoping to shift the responsibility for this global health impact to its victims may suggest a simple answer — move somewhere with cleaner air. With fewer and fewer places with higher air quality around, this becomes ever more difficult, never mind the fact that most people can't just leave their families, jobs, and homes behind.
Check Air Pollution Forecasts
Whether you already have a respiratory disease — wondering if over-the-counter medications can treat chronic bronchitis, or if you'll need to use your asthma inhaler more often today, for instance — or still haven't noticed any of the possible tangible effects of exposure to outdoor air pollution, you need to keep an eye on your local air quality index. Like the more familiar allergy or UV index, this is another extremely useful part of the "weather forecast" that you can follow, and like both these guides, it is color coded :
- Green (0-50) means good air quality.
- Yellow (51-100) means moderately OK air quality.
- Orange (101-150) means the outdoor air quality is unhealthy for sensitive groups.
- It only goes up from there — red means unhealthy, purple very unhealthy, and maroon downright hazardous, with its air pollution levels of 301 to 500. (Events like forest fires might cause air pollution numbers to reach beyond 300, while numbers over 500 can be found certain areas of China.)
You can use this air quality index as a guide. If you already have some form of lung disease, asthma or bronchitis for example, avoid being active outdoors for too long or too much when your air quality is orange or above. If you don't, the same goes for red or above.
Do Air Pollution Masks Help?
People who live in highly polluted areas will probably be used to seeing people with air pollution masks out and about, and may even use one themselves. Do they really work, though? One study using subjects with coronary heart disease living in Beijing, a notoriously polluted city, found that they do . Another study holds that "facemasks can only partially reduce the negative health effects of air pollution",  but "partially" is clearly better than "not at all". If your outdoor air quality is currently in the orange range or above, you may consider using an air pollution mask.
If you want to buy one, take into account that not all air pollution masks are the same, and :
- It's important your mask fits well — and that means snugly around your face, without air gaps. If it doesn't, you're no better off wearing one, and perhaps worse because the mask gives you a false sense of safety.
- Surgical masks don't actually do anything against air pollution — your mask needs to contain an air filter.
- You can check the US National Institute for Occupational Safety and Health to find out more about how effective a particular mask is.
- The mask itself still needs to give you space to breathe, especially if you are considering exercising in it.
Toward Cleaner Indoor Air
When the air quality immediately outside of your home is terrible, you want to do everything you can to maximize the quality of the air you breathe inside it. This includes taking a good, hard, look at the volatile organic compounds within your home — even supposedly "harmless" household chemicals can cause occupational bronchitis! Switching to natural cleaning materials can go a long way. Besides that, you will want to consider investing in a good-quality, effective, air cleaner. 
Should You Exercise Outside?
If the air quality has reached dangerous levels in your vicinity (orange, red, or beyond), you are better off exercising indoors. A home gym, professional gym, or even your local shopping mall — where you can certainly take a brisk walk — can help you out. Stay away from heavy traffic if you do plan to exercise outdoors, and don't let any young children you have play outside either. 
Steps You Can Take To Contribute To Reducing Air Pollution
Consumer behavior — including buying products from halfway across the globe, which then have to be transported to you — has a significant impact on global air pollution . Although much of what contributes to outdoor air pollution is outside of the hands of individuals like you and me, there are still things we can do in the attempt to minimize the health hazards we're exposed to through the air we breathe. They include :
- Avoid driving a personal car whenever possible — walk, use a bicycle, carpool, or use public transportation.
- Use as little energy in your home as possible. Encourage companies and institutions that are part of your life, like your workplace and your child's school to do the same.
- Look into the green energy options available to you.
- Use electricity-free tools wherever feasible.
- Never burn garbage or wood.
- Buy local, both food and other goods, when you can.