Perhaps the most pressing environmental problem in the world is indoor air pollution, which kills 2.8 million people each year, just behind HIV/AIDS. The pollution is caused by poor people cooking and heating their homes with dung and cardboard. The solution is not environmental (to certify dung) but rather economic, helping these people build enough wealth to afford kerosene.
That is by Bjorn Lomborg, in Foreign Policy, July/August issue.
Two caveats. First, the best figure I can find appears to be 1.6 million lives; here is a WHO statement on the phenomenon. Second, the people die because the smoke renders them more susceptible to pneumonia and other respiratory diseases. But their poverty makes them more susceptible for a number of reasons. I doubt if the marginal product of the smoke can be isolated clearly; see this study. Nonetheless this is a very very serious problem that does not receive much attention.