Some airports use "puffer" machines which will blow air on you and then sniff it for trace amounts of specific chemicals. In practice, they seem to have issues

Some airports use "puffer" machines which will blow air on you and then sniff it for trace amounts of specific chemicals. In practice, they seem to have issues with becoming clogged as well as false alarms - http://seattletimes.nwsource.com/html/nationworld/2003744097_puff12.html http://seattletimes.nwsource.com/html/nationworld/2003744097...