It is very common to classify data such as a Federal Identification Number as sensitive. It is considered such because it is unique to the individual and is often used to gain access to additional data. Biometrics, in many cases, also uniquely identifies an individual due to being based upon their physical aspects.

The use of biometric data in combination with other identity verification data is highly valuable. It presents a convenient method of authentication since it is something that we do not need to refer to a post-it note to recall or derive a cryptic combination of our pets names and our favorite celebrity.

While fingerprints are usually the first biometric method that comes to mind there are many others, including some that may come as a surprise. Below is a short list of biometric data types:
 
Fingerprint
Fingerprints have been used to verify identity for thousands of years. There are records that reference the Ancient Chinese and Babylonian cultures utilizing fingerprints as signatures. We leave this calling card everywhere we go. The fingerprint pattern is unique to each person and does not change through their lifetime, barring serious injury to the finger. When a fingerprint is gleaned from an object and compared to a list of fingerprints that are associated to other personal information, such as a name, then personal identification is achieved.

Hand Geometry
The distance from the tip of your thumb to the tip of your index finger might be 6.04 inches while another person's might be 5.75 inches. The layout of a person's fingers when a relaxed palm is placed upon a reader may result in a pattern that is identifiable to the person. Jerry Garcia used his unique hand print as a logo that represented his work. These are all examples of hand geometry. While this data may not be as unique as a fingerprint it can be utilized to augment other biometric data.

Iris Recognition
Look at anyone's eyes and you will see their iris. The iris consists of varying structures, patterns and hues that are unique to each person. The technology of capturing these tiny details of the eye is a fairly recent achievement compared to other biometric methods. Unlike fingerprints that can be left behind on any object the source of this data remains with the carrier; thus requiring the presence of the person in order to collect the identifying information.
 
Retina Geometry
The collection of blood vessels, or more specifically capillaries, that supply the retina the nutrients needed to maintain its viability produce a pattern that is unique to each person and is considered by many as the optimal biometric data source. The ability to scan the retina and compare the results with a stored library of retinal scans is dependent on the retina remaining in tact without obstruction. Severe injury or disease, such as glaucoma, can make this type of data difficult to obtain for comparison.

Facial Recognition
The combination of facial features and their relationship to each other presents a form of identification. This is something that we practice naturally in recognizing our friends. Las Vegas has employed this method in their casinos for quite sometime to identify gamblers who are barred from their establishments. Photographs often are a source of data for facial recognition methods.

Vocal Patterns
One of my favorite radio personalities is Casey Kasem. His voice is unique and he can be identified without any visual representation. The combination of accent, rate, pitch and, in some cases, vocal ticks such as "umm..." create a unique pattern that can identify us without visual representation. These patterns can be captured and converted into data that is then stored and compared to future vocal recordings for identification purposes.

Signature
Believe it or not a signature is considered biometric data. The unique pattern of almost illegible squiggles and jots that make up our signatures is considered a behavioral biometric. It is the result of the action on our hands and wrists that generate these miniature pieces of ourselves. Banks have used items called signature cards for years as a means of identity verification. Credit cards have an area on the back in which a signature is to be placed. This location is intended to be used as a verification device to the signature that is written upon the transaction receipt.

Keystroke Dynamics
The interface in which we communicate to computers and our Internet communities is achieved through the use of a keyboard. The tap dance that our finger do on these keyboards differ from person to person. The amount of time that we press down on a key. The amount of time between pressing one key and pressing another key. Whether we favor the left shift key or the right. Our capitalization and punctuation patterns. The frequency of the use of the backspace key. All of these together indicate that we are a different person than the other users of the keyboard. While there are many variables in this behavioral biometric this can be used as a means to verify identification.

While this list can go on ad nauseam it does represent that biometric data surrounds us on a daily basis and is used in even the most routine transaction. All of these items can be reduced to a binary array that can, and is often, stored within a database and utilized as a source of identity verification.

Disclosure of biometric data to unauthorized persons could result in an extremely difficult situation to rectify. A person's fingerprints cannot be changed, like credit card numbers, if their unauthorized disclosure results in fraud or misrepresentation. Therefore, careful consideration and understanding of this type of data and its sensitivity is important in any data classification effort.