<font face="Verdana, Arial, Helvetica, sans-serif" size="2">As more than 65,000 people a day heft luggage into San Francisco International Airport to be scanned for guns and bombs, hidden machines occasionally sniff the air they breathe for lethal gases and germs.

Inside SFO, defense scientists are quietly testing a variety of chemical and bio-warfare sensors in a race to guard airports nationwide against terrorist attacks.</font>