Posted on Sun, Apr. 21, 2002 BY JENNIFER BABSON jbabson@herald.com BOCA CHICA KEY - They worry about allergies and immune system difficulties and ailments yet to be diagnosed. A few bolted for points north; others shuttered windows and stayed inside. Word that the U.S. Army was conducting biological and chemical detection tests off Key West last week -- using a crop duster to spray what it says are benign substances over a small swath of the Gulf -- set alarm bells ringing for some on this island chain. ''Monday I had my house closed up all day and the air...