The Environmental Protection Agency issued a new rule for airlines: start cleaning the water.

The EPA is forcing 63 U.S. airlines to implement a regular system of checking, disinfecting and changing their on-board drinking water.

This new rule is more than four years in the making. In 2004, airline regulators released reports identifying some airlines were not adequately maintaining their drinking water systems. Because there were no rules governing the quality of water on board flights, the EPA was unable to enforce changes. However, the rules have changed the airlines are now being held accountable for dirty water.

The EPA has informed airlines of the proper cleaning and disinfecting procedures and said the airlines have 18 months to comply with the new standards. The airlines are estimating it will cost around $7 million per year to comply.

Prior to the EPA guidelines, airlines regularly checked the on-board water for bacteria. However, the EPA said these new cleaning procedures will continue to protect passenger health.