I dont have a bike (I walk lots though), but a agree that biking is a good way to imporve the earth;
If you are complaining about the metal used to make a bike; cars use so much more, and then those parts break, and then you replace them with even more, and then those parts break, you you replace them again, and then eventually the car is crap, and it gets junked, and you buy another. Now include car insurance, add in gas costs; a waste of money if you drive places a bike can get you.
Carbon dioxide is one of the reasons why the asthma and respiratory disease occurring in adolescents within cities and suburbs is increasing. This might not come across as an issue, but these things can lead to death, and it effects taxes due to higher medical bills and more frequent doctor and hospital visits.
Economically speaking, this is all problematic. On behalf of the ecosystem, this is all problematic.
Its healthy not to breathe in fumes, and to instead be out biking and becoming healthy.