If an airline does not use an allocation of slots (typically 80% usage over six months), it can lose the rights. Airlines may operate ghost or empty flights to preserve slot allocations.[4] To avoid pollution and financial losses caused by an excessive number of empty flights, these rules have occasionally been waived during periods of temporary but widespread travel disruption, including after the September 11, 2001 attacks and during the SARS epidemic, the Great Recession, and the COVID-19 pandemic.[5] Maintenance of the rules forces airlines to schedule extra unnecessary flights to keep their slots, wasting fuel; a 2021 expiration of a waiver in the United States will cause 20,000 tons of carbon dioxide emissions per day.[6]