With America on full alert for the foreseeable future, interest in biometric scanners -- devices that verify a person's identity by measuring unique features such as nasal curvature, iris patterns, or hand shape -- has been booming. But civil liberties watchdogs, privacy advocates, and even some security gurus warn that the devices are less effective than advertised and that the personal data they generate could easily be misused.

Currently, biometric tools are primarily marketed as "smart locks" to prevent trespassers from entering restricted areas such as military bases. But industry observers foresee much broader applications, from fingerprint readers at ATMs to iris scanners in office elevators; already, a number of airports have announced plans to install facial scanners designed to identify criminals. By 2005, biometrics is projected to be a $3.5-billion industry.

"There was no single moment when we were all given email addresses," Samir Nanavati of the consulting firm International Biometric Group told the journal Computing in October. "It will be the same with biometrics. You won't even notice adoption over the next two to three years. It will just happen."

One problem with biometric devices, many security experts warn, is that they can only compare the features of individuals scanned with profiles stored in a database