(18)In order to protect human health and the environment as a whole, it is particularly important to combat emissions of pollutants at source and to identify and implement the most effective emission reduction measures at local, national and Union level, in particular as regards emissions from agriculture, industries, transport, heating and cooling systems and energy generation. Therefore, emissions of harmful air pollutants should be avoided, prevented or reduced and appropriate air quality standards should be set on the basis of, inter alia, the most up-to-date scientific evidence, including WHO recommendations.