(16)The derivation of EQS for priority hazardous substances usually involves higher levels of uncertainty than is the case for priority substances but such EQS still establish a benchmark to assess compliance with the objective of good surface water chemical status, as defined in Article 2(24) and points (ii) and (iii) of point (a) of Article 4(1) of Directive 2000/60/EC. However, in order to ensure an adequate level of protection for the environment and human health, the cessation or phasing-out of discharges, emissions and losses of priority hazardous substances should also be aimed at, in accordance with point (iv) of point (a) of Article 4(1) of Directive 2000/60/EC.