It goes against the grain of contemporary journalism to have a mystery without a villain -- but that is just what Stephen Smith offers readers. He succeeds -- not just in coming up with an interesting story, but in writing one that deepens the reader's understanding of the subject. There seems little doubt that an epidemic of asthma has grown in the United States in recent decades, and changes in diagnosis and case-counting don't seem nearly enough to account for it. Smith lays out half a dozen current theories about what might be the cause, most of them environmental. They include more airtight houses, kids' spending less time outdoors, cockroaches, paradoxical immune effects from better hygiene, diesel emissions, and increased use of acetaminophen, among others. Not only does Smith avoid the temptation of blaming it all on a single culprit, but he also avoids the "silver-bullet" story. Merely by allowing for complex problems and solutions, he has pushed back against the cable-news tendency to reduce environmental health issues to 30-second nuggets. The story is just the latest of a handful Smith has done on facets of the asthma issue in the last year. (See http://www.boston.com/news/globe/health_science/articles/2004/04/13/the_asthma_riddle/).