The concept of the ‘critical load’ is similar as the ‘total maximum daily
load’ (TMDL) developed by the US Environmental Protection Agency (EPA,
2010a). The TMDL calculates the maximum amount of a pollutant allowed to
enter a water body so that the water body will meet and continue to meet
water quality standards for that particular pollutant and allocates that load
to point and diffuse sources, which include both anthropogenic and natural
background sources of the pollutant. Another concept closely related to the
concept of ‘critical load’ is the concept of ‘maximum permissible addition’
(MPA), which is defined as the ‘maximum permissible concentration’ (MPC)
minus the background concentration and thus equivalent to cmax – cnat
(Crommentuijn et al, 2000).