Apparently I'm on a kick this week about reducing car trips. I think we all realize that cars contribute a large portion of green house gas emissions. It has also become common knowledge that car exhaust is contributing to various respiratory disorders, especially asthma. One study concluded that there was a consistent association between car exhaust and coughing/asthma in children. ("Locally generated particulate pollution and respiratory symptoms in young children." Pierse N - Thorax - 01-MAR-2006; 61(3): 216-20)
The obvious answer to living greener and improving health is to avoid driving cars! Exercise can improve general physical fitness as well as cardiovascular and respiratory health. Going on a short trip? Try walking. Walking provides great opportunities to get to know you neighbors. You can bring an extra bag along and collect any trash you find. This helps to beautify your neighborhood as well as recycle discarded items.
Is your trip a bit longer? Ride a bike! Cycling can be an amazing experience. I've become an avid cyclist at this point, but I haven't been for long. I didn't even learn to ride a bike until I was in 8th grade. The embarrassment of training wheels gave me an incentive to learn quickly. My bikes in the past were always the cheapest ones I could find. This was okay to begin on, but I didn't like to ride very far because my bike was so heavy. Hills were a nightmare even when I geared all the way down. I was finally able to upgrade to a lighter bike, and now love to ride! It can be cheap and simple to commute by bike. Here are some steps to successful bike commuting.
Going on a longer trip where walking and biking aren't possible? Try taking public transit, carpooling, riding the bus (Greyhound, charter), or train (Amtrak). In many areas there are several options to arrange rideshares/carpools. Most local governments can provide help. In the Portland Metro region my favorite resources are Carpool Match NW and Craigslist.
It's great when you can benefit your own health and the environment. Changing your mode of transit just 1 day a week can have a beneficial effect.