 In 2013, the European Committee for Standardization (CEN) started work on standards for protection from noise pollution caused by heat pump outdoor units.Although the CEN/TC 113 Business Plan outset was that "consumers increasingly require a low acoustic power of these units as the users and their neighbours now reject noisy installations", no standards for noise barriers or other means of noise protection had been developed by January 2016.In the United States, the allowed nighttime noise level was defined in 1974 as "an average 24-hr exposure limit of 55 A-weighted decibels (dBA) to protect the public from all adverse effects on health and welfare in residential areas (U.S. EPA 1974).