One of my favorite bike paths parallels the freeway, which is quite unfortunate because air pollution doesn't play well with exercising lungs. Air pollutants, like car exhaust, can irritate the lungs and respiratory system, making breathing difficult. If you run, bike, or skate outdoors, you shouldn't let pollution deter you from your exercise goals, but there are few tips to keep in mind — especially if you suffer from asthma or bronchitis.

• Do not run or bike on or near roads with heavy trucks or bus traffic.
• Work out in the early morning or later in the evening when the ozone (resulting from the interaction of sunlight and chemicals found in car exhaust) levels are at their lowest.
• Exercise indoors if possible.
• If you experience any difficulty breathing, stop your exercise immediately and call your doctor.

You can use the Air Now website to check the air quality in your area. If your area's air is considered unhealthy, you may just want to pop in a fitness DVD and get your heart rate up in front of your TV rather than out on the streets.

Source