reg 4 Air quality objectives
(1) The air quality objective for each substance listed in the first column of the Table in the Schedule to these Regulations shall be to restrict, by no later than the end of the relevant period prescribed by regulation 3(2), the level at which that substance is present in the air to the level set out in the second column of that Table for that substance. (2) The achievement or likely achievement of an air quality objective prescribed by paragraph (1) shall be determined by reference to the quality of air at locations which are situated outside of buildings or other natural or man-made structures above or below ground and where members of the public are regularly present.