Watch lists vary in size, scope

What does it take to be banned from air travel? And how do you get on that list?

U.S. officials are reviewing the watch-list and screening procedures that ensure safe air travel, but not all lists are equal.

Here is a look at the lists:

Terrorist Identities Datamart Environment database: This is the largest collection, with about 550,000 individuals. U.S. intelligence and law enforcement as well as trusted allies can nominate "known or suspected terrorists" for this database, which is maintained by the National Counterterrorism Center and was set up in the wake of the Sept. 11, 2001, terrorist attacks.

Umar Farouk Abdulmutallab, a 23-year-old Nigerian accused of trying to attack an airliner as it approached Detroit on Friday, was in this database.

The Consolidated Terrorist Watchlist: About 400,000 individuals. People are moved onto this list, which is maintained by the FBI's Terrorist Screening Center, after two requirements are met. First there must be sufficient biographical and identifying data so that the person being screened can be matched to the name, and second, there must be enough information to justify a "reasonable suspicion" that the person is known or suspected to be engaged or preparing to engage in terrorist activities.

Timothy J. Healy, director of the Terrorist Screening Center, said earlier this month that officials get 400 to 1,200 additions, deletions or changes to the nation's terrorist watch list every day.

Selectee List: A subset of the watch list, with about 14,000 individuals. People are placed on this list when there is more information about their terrorist activities that suggests they may pose a threat, including to aviation. These people require more stringent or what is called "secondary" screening when they fly.

No-fly List: Also a subset of the watch list, with about 3,400 individuals, including roughly 170 U.S. residents. In order to be placed on the no-fly list, a known or suspected terrorist must meet more specific criteria showing they present a threat to civil aviation or national security.