Seventeen-year-old Maria Coryell-Martin lives in the Laurelhurst district of Seattle, WA. She got the cycling bug 3 years ago when she bought her first bike and now regularly cycles 18 miles a day.

'When I got my first proper bike I was ecstatic! It was so good to have my very own independent transportation! I could go anywhere I wanted and have fun at the same time, I could spend time with my friends and stay in shape too. I spent that whole summer biking everywhere.

'I use my bike to cycle to school, to go rollerblading (or running) around a nearby lake and to visit friends. My parents were always against my biking to school because they were scared of me having an accident. But I found some friends who were willing to bike with me. Safety is something I always contemplate &emdash; and try to never compromise. I like biking anywhere I can as long as it's not really dark or raining cats and dogs.

'Not many people at my school cycle. It's either "I'm too lazy" or "It's too far!" etc. My school starts at 7.40 am so it is difficult for some to make the extra effort to bike. And I think most people just don't even consider it. Many would regard it as being too much trouble and are unaware of the benefits. But cycling makes you feel good. It's also so great when I've had a stressful day (especially at school) to push myself, to race through the hills and just exhaust myself. Then I always feel a little better.

'I wish people would become less dependent on cars. I read in the news that America is becoming more and more overweight and that exercise is healthy. So why not bike? Besides lowering the number of cars on the road, bikes also help lower the levels of air pollution. (Smog is not only unhealthy, but gross.) I think the majority of the population would benefit from biking.

'We need to wean ourselves off environment-damaging habits. The earth is all we really have. This means being aware of our personal impact and acting. Recycling, practising a healthy lifestyle and protecting our environment should be things we do unconsciously. The ozone hole, smog and the depleting forests are all evidence that change needs to happen.

'Knowing that I cycle places that I could drive (and others do drive) makes me happy because I feel I am practising what I preach: staying healthy and being aware of the environment and our impact on it. The air we breathe in and the world we live in need to be a priority for all'

Seventeen-year-old Nicola Bunt lives in St Agnes, Cornwall on Britain's south west coast. She has been surfing since she was 14 and now surfs competitively. Nicola often spends up to four hours a day in the sea where she is at risk from high levels of pollution.

'I worry about pollution and getting ill. There is a sewage outfall just around the headland at St Agnes but fortunately the treatment plant now includes UV treatment, which significantly reduces the bacteria and virus levels in the water. Supposedly it's safe to surf here without the risk of infection, though in reality there have been one or two problems from time to time, which means the pathogens aren't removed. You can tell when this happens because the water is a dirtier brown colour and scummy.

'Sometimes I surf at nearby Porthtowan, where the waves are bigger and more powerful. I've seen raw sewage in the water there, and tampons, condoms, needles and plastic strips often get washed up on the beach. The sea smells worse and the water is browner. But it's the invisible viruses and bacteria that worry me most, because when I'm surfing I tend to swallow a lot of water. A lot of it goes up my nose and in my ears, too.

'My dad and I went surfing together once and we both came down with a stomach bug. Generally speaking I've been lucky, though, with my health. A friend of mine recently had an ear operation because of all the infections he'd picked up from the sea.

'I decided to join Surfers Against Sewage because I want to stop sewage being dumped into our seas. The problem is pretty bad right now and I don't want it to get worse. SAS are doing something about it and I'm proud to put their stickers all over my board.

'It's time we stopped using our seas as sewers and started protecting them for future generations. If I had a daughter I'd be worried about her surfing. I'm not sure if I would encourage her to spend as much time as I do in the sea, though I'm hopeful that things will change.'