Threat levels are issued on a five-level scale, from green for a low threat to red for a severe threat, according to the Department of Homeland Security. Orange is a high threat, yellow an elevated threat and blue a guarded threat. The Transport Security Administration alters regulations regarding air travel depending on the threat level.
When the threat level reaches elevated, restrictions are placed on items that can be taken on an airplane, such as liquids and aerosols, according to Continental Airlines. Liquids, gels and aerosols in carry-on luggage must be in containers 3.4 oz. or smaller and be carried in a clear, 1-qt., plastic bag.
Elevated threat levels at U.S. airports result in extra screening by airport security staff. Carry-on luggage is searched at each security checkpoint, meaning passengers should arrive at airports earlier for flights. NPR recommends international travelers arrive three hours before their scheduled departure time, and domestic travelers two hours prior to departure.
